Title: Gas exchange and metabolism in the Sirenidae (Amphibia, Caudata)
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Title: Gas exchange and metabolism in the Sirenidae (Amphibia, Caudata)
Physical Description: x, 112 leaves. : illus. ; 28 cm.
Language: English
Creator: Ultsch, Gordon Richard, 1942-
Publication Date: 1972
Copyright Date: 1972
 Subjects
Subject: Salamanders   ( lcsh )
Zoology thesis Ph. D
Dissertations, Academic -- Zoology -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis -- University of Florida.
Additional Physical Form: Also available on World Wide Web
General Note: Typescript.
General Note: Vita.
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Bibliographic ID: UF00097645
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000577414
oclc - 13978064
notis - ADA5109

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etabo,ism In the Sirenidae (Amphtbta, Caudata)










By

CORDON RICHARD ULTSCH










ION PRFSENTED TO THE GF-ADUATE COUNCIL OF
UNIVERSITY OF FLORIDA IN PARTIAL
OF THE REQUIREMENTS FOR THE DR0EB OF
DOCTOR O'i PHILOSOPHY










U OF FLORIDA


44IJ
































This work is dedicated to my wife, Sandra, for the many years

of work to support a student husband, and to my daughter, Julie,

who must have wondered at times if she actually had a father.





ACNOLEmmNT
I grtefllyackowlege he anypeole, othstuent an

instuctos, wo hlpedwithvarous hase ofthisstnd. Te ls
is uc to on t gie er, uttoanywh my ea t iswr an
wh aecotiuedete mn rmsce o hv ywamsI hn ks
I midbtdt rs ra M~b rakNrmmHg Ppne












TABLE OF CONTENTS

Page

Acknowledgments . . . . . . . .... ..... iii

List of Tables. ... . . . . . . .. . v

List of Figures . . . . . . . . . . . vi

Abstract. . . . . . . . .... . . . ix

Introduction. . . . . . . . . .. . . 1

Materials and Methods . . . . . . . .. . 4

Field Work . . . . . . . .... . ... 4
Maintenance of Animals . . . . . . . . 5
Surface Area Determinations. . . . . . . . 5
Metabolic Rates of Submerged Animals . . . . . 6
Gas Exchange Partitioning Experiments with
S. lacertina. . . . . . . . . . 9

Results and Discussion of Field Studies . . . . .. 11

Results of Laboratory Studies . . . . . . . .. 28

Discussion of Laboratory Studies . . . . . . ... 42

Metabolic Rate, Gas Exchange and Body Weight . . . 42
Gas Exchange Partitioning . . . . . . .. 65

Summary . . . . . . . . . . . . . 74

Appendix. . . . . . . . . .. . . . . 76

Literature Cited . . . . . . . . . . .. 109

Biographical Sketch . . . . . . . .. . 112












LIST OF TABLES


Page

1. Comparisons of pH, temperature, dissolved 02 and
dissolved CO2 in hyacinth and "open" water areas. . 20

2. Survival of submerged S. lacertina at 23-24C in
air-equilibrated water (p02 approximately
155 mmHg) . . . . . . . . . . . 32

3. Conditions of gas exchange partitioning experiments
for two large and one small S. lacertina. . . .. .33

4. Gas exchange partitioning between air and water for
large and small S. lacertina under conditions of high
02 and low CO2 in the water . . . . . .... .34

5. Conditions of gas exchange partitioning experiments
in a single, large S. lacertina . . . . .... .38

6. Gas exchange partitioning between air and water for
an individual, large S. lacertina (1453-1489 g) as
a function of concentrations of dissolved 02 and CO2
in the water phase . . . . . . . . . 39

7. Relationships between metabolic rate and weight and
0 exchange capacity and weight for various groups
o? animals. . . . . . . . ... ..... .51

8. Skin vascularization and epidermal thickness in
ranid frogs (data from Czopek, 1965). . . . ... .55

9. Standard metabolic rate, O exchange capacity, and
critical oxygen tension of S. lacertina of various
body sizes (for submerged animals). . . . . ... 60













LIST OF FIGURES
Page
1. Comparison of aquatic V02 of S. lacertina submerged
for approximately two hours (enclosed area)
and for 145 hours . . . . . . . . ... 8

2. Temperature and pH at the surface and bottom in
"open" water and hyacinths. . . . . . . . 13

3. CO2 and 02 concentrations at the surface and
bottom in "open" water and hyacinths. . . . ... .15

4. CO2 and 02 profiles for different thicknesses
of hyacinth cover, times of day, and times
of year . . . . . . . . . . . . 17

5. Concentrations of dissolved oxygen and carbon
dioxide as a function of thickness of hyacinth
cover, time of day, and time of year. . . . ... .19

6. Location of S. lacertina and S. intermedia within
the hyacinth area of the pond during the hyacinth
growing season (March-October), as a function of
concentrations of dissolved respiratory gases . . 23

7. Location of P. striatus within the hyacinth area
of the pond during the growing season of Eichhornia
(March-October), as a function of concentrations of
dissolved respiratory gases .. . . . .... 25

8. Location of Siren and Pseudobranchus within the
hyacinth area of the pond during the non-growing
season of Eichhornia (November-February), as a
function of concentrations of dissolved respiratory
gases . . . . . . . . . . . .. .27

9. Surface area of the skin in the thr e species of
Sirenidae as a function of body weight. . . . ... 30

10. Aerial-aquatic gas exchange partitioning in a high
02-low C02 aquatic phase as a function of body
size . . . . . . . . . . . .. 36

11. Aerial-aquatic gas exchange partitioning in a large
S. lacertina (1453-1489 g) under various conditions
of dissolved 02 and CO2 . . . . . . . . 41




vi .




I -

12. Predicted maxlxtum siza attainable by a I g
organism whose sarface area of the gaa exchanger
if (f) 10.67 and whose Tap-tabolic rate is either
(f) Wl-O or a) 140-75 . . . . 47

13. Permeability of the skin to oxygen in the Sirenidae
as a function of body size . . . . . . . 58

14. Metabolic rate and 0 2 exchange capacity of submerged
S. lacerLina as a function of body waight . . . 62

15. Per cent utilization of oxygen exchange capacity as
a function of body weight in the Sirenidae . . . 64

-16. Metabolic rate and 0 2 exchange capacity as a function
of body weight for submerged P. striatus and
S. intermedia .. . . . . . . . . . . 67

17. Aquatic VO, of a 44 g S. lacertina breathing
air and wa er . . . . . . . . . . 71

IA. Letabolic rate of submerged P. striatus (0.51 and
1.58 g) as a function of oxygen tension . . . . 79

2A. Metabolic rate of submerged P. striatus (2.88 g) as
a function of oxygen tension . . . . . . 81

3A. Metabolic rate of submerged S. inter-media (3.3 and
7.0 g) as a function of oxygel tension . . . 83

4A. Metabolic rate of submerged S. intermedia (13.7 and
29.6 g) as a function of oxygen tension , 4 4 4 + I 85

5A. Metabolic rate of sub-merged S. lacertina (0,36 and
0.56 g) as a function of oxygen tension . . . . 87

6A. Metabolic rate of submerged S. lacertina (3.0 and
6.5 g) as a function of oxy?,en tension . . . . 89

7A. ML-tabolic rate of submerged S. lacertina (13.7 and
42.7 S) as a function of oxygen tension . . . . 91

SA. Metabolic rate of siibmerged S. lacertina (73 and 103 S)
as a futctioll of Dxygerl tension 0 * * * 93

9A. Metabolic rate of submexged S. lacertina (178 and
Z69 8) as a function of oxygen tension, 95

10A. Metabolic rate of submerged S. lacertiaa (357 and
541 a) as a fuaction of oxygen tension. * * 97

11A. Met f sub a,025 aA4
n 99





IB. Predicted relationship between total weight and
skeletal of mammals required to give M = 70 W 0.75
when metabolism of active tissues does not change
with body size (black circles). . . . . . ... 103

2B. Metabolic rate of mammals as a function of body
size . . . . . . . . . . . . .. 105

3B. Relationship between weight of active tissues
(WA) and metabolic rate in mammals. . . . . ... 108


viii












Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of
the Requirements for the Degree of Doctor of Philosophy


GAS EXCHANGE AND METABOLISM IN TEE SIRENIDAE (AMPHIBIA, CAUDATA)

By

Gordon Richard Ultsch

August, 1972

Chairman: B. K. McNab
Major Department: Zoology

All members of the family Sirenidae (Amphibia, Caudata) are

usually found in areas of aquatic vegetation. They are particularly

abundant in water hyacinth communities, which were found to have low

concentrations of dissolved oxygen and high concentrations of dissolved

carbon dioxide.

Field studies indicated that the Sirenidae do not select micro-

habitats on the basis of dissolved respiratory gases, and that they

choose hyacinth areas over open water within the same pond.

Sirenids above 800 g are obligate air-breathers at 25*C in air

saturated water. Theory and experiments had predicted this weight

closely when body size, metabolic rate, and permeability of the skin

to oxygen were considered.

Siren lacertina was found to adapt to the hyacinth community by

breathing air in response to low levels of dissol-ved oxygen and by

eliminatin- carbon dioxide to the air and tolerating a high blood pCO 2

in response to elevated levels of carbon dioxide in the water.







A model is

large size of a

creasing size.

the 02 exchange


presented that shows the advantage in attaining a

decreasing weight-specific metabolic rate with in-

It is suggested that metabolic rate is limited by

capacity of an organism.


ii:. ..................


.. . .. . . . . .












INTRODUCTION

Animals that utilize both aerial and aquatic gas exchange are

particularly interesting subjects for studies of respiratory adapta-

tions. Differences within the two fluids in gas concentrations,

medium density, types of exchange organs, and diffusion and solubility

constants of the respiratory gases contribute to a complex set of

alternatives for meeting various respiratory demands. If the organisms

exhibit a large range in body sizes and inhabit waters with varying

concentrations of dissolved oxygen and carbon dioxide, one can expect

to observe various types of adaptations for metabolism and gas exchange.

A family of aquatic salamanders, the Sirenidae, is such a group

of animals. It is comprised of three species, Siren lacertina, S.

intermedia, and Pseudobranchus striatus. The wide range of body sizes

is indicated by the maximum weights of adults collected: S. lacertina,

1698 g, S. intermedia, 45 g, and P. striatus, 5 g. Considerable

attention will be paid to S. lacertina in this paper, which was studied

over a weight range of greater than three orders of magnitude.

The Sirenidae are effective air and water breathers (Czopek, 1962;

Freeman, 1963; Coin, 1941; Guimond, 1970). They possess highly vascu-

larized lungs (Guimond, 1970), and surface to breath air regularly.

Although external gills are present, the aquatic mode of respiration

is almost entirely cutaneous (Czopek, 1962; Guimond, 1970). Therefore

no effort was made in this study to evaluate the role of the gills as

separate from that of the skin in aquatic gas exchange.




2



These salamanders are found in waters that offer some degree of

refuge, usually in the form of aquatic vegetation. Pseudobranchus

has become particularly abundant in the water hyacinth (Eichhornia

crassipes) community and is usually associated with some form of

dense vegetation. Both species of Siren are also usually associated

with relatively dense vegetation, although S. lacertina appears to

venture into more open waters frequently, perhaps because its large

size makes it less susceptible to predation.

Aquatic animals that live in such vegetated areas are usually

subject to unfavorable concentrations of dissolved oxygen and carbon

dioxide. Data from Lynch et al. (1947), for water samples from areas

with various types of aquatic vegetation, indicate that oxygen is

usually below saturation levels and carbon dioxide concentrations are

elevated. They found that this was especially true for water covered

by water hyacinths, which makes this community a particularly hostile

environment for aquatic gas exchange.

The effects of Eichhornia on dissolved gases was one of the factors

considered in the choice of a field study site. The pond used was on

the edge of Payne's Prairie, Gainesville, Alachua County, Florida.

Approximately 95% of the water surface was covered with water hyacinths,

leaving an area of about 1/2 acre of "open" water (meaning the surface

was not covered by Eichhornia, although the submerged aquatic Ceratophyllum

was often dense). All three of the species of Sirenidae were present in

large numbers, especially P. striatus. The two available aquatic micro-

habitats provided an excellent field laboratory for studies of behavioral

adaptations associated with habitat selection.





3


!he purpose of this study was to investigate some of the be-

havioral and physiological adaptations of the Sirenidae that enable

them to cope with the respiratory stress placed on them by the low

levels of dissolved oxygen and high levels of dissolved carbon dioxide

associated with water hyacinth communities. As the extent of the

effects of Eichhornia on the aquatic microe-nvironment were rather

poorly documented, it was also necessary to study certain environ.-

mental parameters in detail in order to properly plan and evaluate

the laboratory studies. Temperature, pH, dissolved oxygen and dissolved

carbon dioxide in both the "open" and hyacinth-covered portions of the

pond were the factors chosen for investigation.













MATERIALS AND METHODS

Field Work

Temperature, pH, dissolved 02 and dissolved C02 were measured

in the hyacinth mat and "open" water at the surface and bottom.

Temperature was measured to the nearest 1/20C. Water samples were

drawn through a water-saturated cloth, which excluded most of the

detritus, into a glass tube that was stoppered at both ends with

rubber stoppers. The samples were placed in an ice chest and re-

turned to the laboratory, where they were warmed to 25C. A Radio-

meter PHA 27 pH meter and Gas Monitor, calibrated at 25C, was used

to determine pH, pCO2 and p02 for each sample. The pCO2 scale could

only be read down to 7 mmHg; any readings below this value were

assigned a value of 3.5 mmHg (7 ppm). Partial pressure readings

were converted to parts per million by weight (ppm) by multiplying

the pC02 readings by 1.96 ppm/mmHg and the p02 readings by 0.053

ppm/mrmHg.

It was found that some gas was being exchanged between the

rubber stoppers and the water samples between the times of collect-

ing and measuring. This change was predictable and was corrected for

by a table of correction factors derived from observing changes in

water samples of known concentrations treated in the same manner as

the field samples.

The environmental parameters were measured throughout the year

as a function of time of day, depth of the water column, season, and








thickness of vegetation. All measurements intended for use in describing

annual cycles were taken on sunny days to prevent the effect of cloud

cover from masking the effect of season.

In order to determine the location of the animals in the pond,

extensive collections were made with a dredge in the hyacinths and with

seines in the "open" water. Water samples were taken prior to all

collections.

In addition to animals collected at the Payne's Prairie site,

experimental animals were also collected from the River Styx in Alachua

County and from a culvert passing under SR-121 near Biven's Arm in

Gainesville.

Maintenance of Animals

All animals used in the laboratory work were maintained at least

two weeks at 25 + 2C with a 12-12 light-dark photoperiod. Animals

were not fed within three days of the start of any experiment. If

fecal material was visible in an experimental chamber at the end of

an experiment, the results were discarded because of the possibility

of oxygen consumption by decomposition of the feces.

Surface Area Determinations

Surface area determinations were made initially by anesthetizing

animals with MS-222 (tricaine methanesulfonate) and wrapping them in

aluminum foil. The foil was cut to fit the body contours, unrolled
2
and the outline traced on graph paper ruled in rm Surface area was

determined by counting squares. It was later found that a good approx-

imation could be made by treating the animal as a cylinder from the head

to the vent and then considering the remainder as a triangle with a base







equal to the distance from the vent to the tip of the tail. Six com-

parisons of the techniques were made, with animals ot 21 690 g. Con-

sidering the square-counting method to represent the actual surface area,

the geometrical approximation averaged an error of only + 1.8%. Most of

the additional determinations utilized the geometrical technique.

Metabolic Rates of Submerged Animals

Animals used in these experiments were placed in the experimental

containers with access to air the night before use to become accustomed

to the chamber. The type of container used varied with the size of

the animal, but was usually a jar or Erlenmeyer flask with a volume

that would result in a drop of pO2 of about 10 mmHg/hr when the

animal was in a relatively inactive state. The water was changed the

next day to eliminate any accumulated fecal material, skin, etc.

Well water was used rather than distilled water, to alleviate osmo-

regulatory stresses. Controls indicated the biochemical oxygen demand

of the well water was insignificant (decreases in pO2 of 0.0 0.5

mmHg/hr).

At the start of an experiment the container was filled with

water supersaturated with 02 at 250C. The animal was then submerged

in the filled and sealed chamber and allowed to respire for about

two hours before an initial oxygen determination was made, in order

to deplete the lungs of oxygen. Figure I shows a test of this

assumption. The enclosed areas represent metabolic rates measured

during experiments that were preceded by the two-hour submergence

period. The scores indicate metabolic rates of submerged animals

after six days of submergence. Presumably, all available 02 in the


































Fiue1 oprsno qai T0 fS aetn umre o

















X= 13.3g S.lacerlina
=12.9g (X= 137g)
o= 1 1.2g


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lungs would have been utilized by this time. The metabolic rates

after 145 hours of submergence are similar to those after only two hours

of submergence, indicating that the animals are functioning only as

water breathers in the submerged metabolism experiments. Metabolic

rate was determined by allowing the animal to consume oxygen from

the water for at least one hour, and converting the decrease in p02

to oxygen consumption in il 0/g-hr. The animal was considered to

have exhibited that metabolic rate at an oxygen tension equal to

the average tension for the interval.

Determinations of metabolic rate as a function of oxygen tension

were made for S. lacertina of average weight groups from 0.36 g to

1310 g, for S. intermedia from 3.3 to 29.6 g, and for P. striatus

from 0.51 to 2.88 g.

Gas Exchange Partitioning Experiments With S. lacertina

Gas exchange partitioning experiments were conducted in two-

section plexiglas chambers. The lower portion of a chamber contained

the animal and was filled with water; the upper portion was filled

with air. A small opening connected the two sections to allow the

animal to air breathe. The surface of the water at the opening was

covered with a 1 cm layer of heavy duty paraffin oil to minimize gas

exchange between the air and water. The chamber was painted a

dull gray except for the area above the breathing hole. Observatice

showed that the animals located the breathing hole much more readily

when it was the only source of light. In practice, the animals

usually stayed near the opening, and air breathing merely necessitated

raising the upper third of the body to be able to protrude the mouth

into the aerial chamber.








Gas concentrations were set at the desired levels by bubbling

CO2, 02, or N2 through the water. Gas exchange between the air

and water in controls was nil (0.006 vol%/hr for CO2 and 0.022

vol%/hr for 02). The animals were acclimated to the experimental

conditions for at least 24 hours before the start of a series of

experiments by placing them in the chamber at 250C and with the

appropriate set of concentrations of dissolved gases and leaving

the aerial portion open to the atmosphere. Immediately prior to

an experiment, the water was changed and the desired gas concen-

trations re-established in the water. One to two hours were allowed

before the aerial portion was sealed from the atmosphere, after which

the tank was completely submerged in the water bath at 25C. Generally,

two experiments were run per day, after which the gas concentrations

would be reset to the desired levels, and the animals would be left

in the chamber overnight. The same procedure was followed for the

next day and thereafter until a series of experiments for a particular

set of conditions was complete. Any acclimation was apparently

complete at the end of 24 hours, since there was no significant change

in metabolic rates or partitioning between first and final days of

experimentation for a particular set of conditions.

Changes in the vol% of CO2 and 02 in the aerial phase were

measured with a Scholander 1/2 cc gas analyzer. Dissolved 02 was

measured with the Radiometer PPiA 27 and Gas Monitor. Dissolved C02

was determined by calculation from an RQ of 0.91 (SE = 0.03) reported

by Guimond (1970) for S. lacertina at 250C.

All gas volumes are reduced to STPD.














RESULTS AND DISCUSSION OF FIELD STUDIES

Curves were constructed for pH, CO2, 02 and temperature for a

24-hour sunny day of each month. Measurements in the hyacinth area

were made during September and October of 1969, and for all other

months in 1970. "Open" water measurements were made monthly from

February, 1970, to January, 1971. Each curve was averaged over the

24-hour period, and these average values were plotted as single points.

Figure 2 shows tre results for temperature and pH, and Figure 3 for

dissolved 02 and CO2. Figure 4 gives profiles for 02 and CO2 in

summer and winter, and as a function of time of day. The profiles

reveal that depth is not an important factor in selection of a favor-

able respiratory microenvironment for a sirenid in the hyacinths,

since once the depth reaches only 20 cm, dissolved gases remain rather

constant. Therefore, the bottom measurements in the hyacinth area of

the pond may be considered to be roughly equivalent to the general con-

ditions existing anywhere within the water column under a hyacinth mat.

However, Figure 5 clearly indicates that horizontal movements from

hyacinth areas to open water can have a marked effect on the respira-

tory microenvironment.

The growing season for water hyacinths in the Gainesville area is

March-October. It is during this period that conditions become partic-

ularly unfavorable for aquatic gas exchange in the water covered by

hyacinths. Table 1 shows the average 02 for this period to be only




:,i i iiiiiiiii ............. !!!!!!!!!!!!!!!!!!!! ....... ....... iii ... l






































Temperature and pH at the surface and bottom in "open"

water and hyacinths. Each point is the average value

for a 24-hr sunny day for a given month.


Figure 2.


















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CO2 and 02 concentrations at the surface and bottom in

"open" water and hyacinths. Each point is the average

value for a 24-hr sunny day for a given month.


Figure 3.











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Figure 5. Concentrations of dissolved oxygen and carbon dioxide as

a function of thickness of hyacinth cover, time of day,

and time of year. Station 1 is "open" (no hyacinths, but

considerable Ceratophyllum in summer), Station 2 is "thin"

hyacinths (no accumulation of detritus in root mass), and

Station 3 is "thick" hyacinths (larger plants character-

istic of a mature mat, with considerable accumulation of

detritus in the root mass).















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0 C L. 0 '- 0



u N 41 4-
0 0 0 0 0 0 0
p.M E p0 r H f
o a a a a a a








0.6 ppm under the hyacinths, while the CO is 63 ppm. For organism
2 0
depeadeat upon aquatic respiration, the "open" water is a -refuge during

this period. In fact, larger fish such as the c2ntrarchids are found

only in the open water during the sumer, while they are aiso in the

hyacinth are-as in winter. Some fish, therefore, migrate horizontally

due to respiratory stress.

Extensive collections were macle. in both the "open" water and

hyacinth areas to determine where sirenids were located. Regardless

of the time of year, no P. striatus and only one Siren (probably

lacertina) were collected in "open" watur. The hyacinth areas are

definitely the preferred microenvironizent.

To determine if the animals were selecting areas within the

hyacinth portion of th-, pond, successful collecting attempts were

paired with the corresponding gas concentrations at the site of

coll--ct-i-on and compared to the set of all available ,-as concentrations

within tLe hyacinth portion of the pond. These results are shown ilt

Figures 6-8 as a function of the size of the animal (for Siren and

the time of the year. In all cases, Lhe animals are not avoiding any

particular gas concentrations, regardless of species, size, or time of

year.

The coziclusion drawu from the field work is that aqiAatic. gas

e:chane consideration are- not limiting in habitat selection in the

Sireci,ae, This evidence is iii agreemeat wltb the results of a pre-

vious study for P. 5triatus (filtseb, 1971). A probable corollary is

that the Sirenidae mu,9t depend heavily upon aerial respiration as a

sonrce of oxygen, and perhaps as a method of carbon dioxide eliminatioqn.















r-
0

r4.
00
oc


t o






Q uu
co 0






4- 4-
ci















v 0
U "


E3 O
0C 0

.c 0











i
Si I
.0
0 U







0 4 U1

0 (U4


0



O c
c ,i
&a c

U .0
.-i
o o


H O0
U ,C




CU~
U .0



0 S




o
0 S


01
c c


a u
0 0
TO O




C4
N



(U
> H
s0

r-4 0
0 4-





0 -H


0 Uc
o Oj

0 09
S1 a)
a r-


01
0 0
0 ro
En o
S 0


0 0




at
0 0





co p


01 V


4 S4
I- C/


.0
CU U
^i cn

'-
CO
(U -^


.0


;r3

r4


01


u



ca


0
En

o




Si
a








ci
-d
0

01




O4
0


0

0)
>
*-1


r-4
U

0




*t r
iSi
cd





H o
a *

0a a


* o











0
0
0
0
co
0 -:3 0 (D
u) u) fn u) co




CL



tj
CLI

CD
U-)w m U)IO m 0 0 m a
Al fj




J) ;IM
i7l


ol
CD
(w d d) 0 W 10 N088 VO


































Figure 7. Location of P. striatus within the hyacinth area of the

pond during the growing season of Eichhornia (March-

October), as a function of concentrations of dissolved

respiratory gases. Enclosed areas are as in Figure 6.


















120 *1
0 p striaitus
oo Surface

too- 0 o 0

E 0 00
CL I
CL O
-U qo 'o
LC) 1 O o o
x 0o
00*
0 ~ 0 0o s
60 0r 0 *
0 o .* *. W


40- o80 0 %








0 .2 4 6 8 10 ;2


































Location of Siren and Pseudobranchus within the hyacinth

area of the pond during the non-growing season of Eichhornia

(November-February), as a function of concentrations of

dissolved respiratory gases. Enclosed areas are as in

Figure 6, except for determinations being for November-

February.


Figure 8.













60- 0 ~ S. Iace rtin a
50- A Surf.
Bo., 50-300g
40 Surf.

30a Bot.

20



EOFF

60- S. acertmo
e surf.
50o 0 Bot.
S. intermedia
-40- Surf. <50g

co Siren sp.
C) 13 Surf.
20 --- A BOL.

0 lo
coOFF *


50-o 0 striatus
40 0, & Surf,
40O .1 0o BOt.
0 0) 0
30- o\

100
0 0 *0o

OF
0 2 4 6 8 to 1 2
0 X YG EN (ppm)













RESULTS OF LABORATORY STUDIES

The results of determinations of skin area at various body sizes

comprise Figure 9.

The results of progressive hypoxia experiments on submerged

animals are given in Figures 1A-11A in Appendix A. The animals are

definitely oxygen conformers at low 02 tensions and regulators at high

tensions. Two methods of evaluating the metabolic rate data are pre-

sented. In both cases, a point R on the p02 axis was chosen such that

it was obvious that metabolic rate had no significant dependence upon

pO2 at tensions greater than R. For all metabolic rates at oxygen

tensions greater than or equal to R, two levels were determined: the

average value (R), and a level where 10% of the points fell below and

90% above (10/90 level). Once these levels of metabolism had been

determined, that level was extended graphically from the highest p02

through lower ones as a straight line. In the case of the mean value,

the line was continued until 75% or more of the values fell below the

line for a given interval of 10 mmHg and for all following intervals.

The point at which this occurred (to the nearest 5 mmHg) was deemed to

indicate that the animal had given up regulation of oxygen consumption

and is labeled P (critical oxygen tension). A line was then fitted
c
by eye for the values remaining below P The same approach was used

to fix a Pc for the 10/90 level, except 50% of the values falling

below the line was the arbitrary level picked to indicate the abandon-
































LO
co

a)





14
_,4
V)

'44



a)




















V4
.H n










4-4
0








4 L4
:lj










-T4
r=4









In





0




zm


0 0 O
La z 1 U.













\ (

0 X
\n o _





D- -; to -1-























".
- 0 0 O














(V3V 3o.l o 9 0
>\ 0






2 -J






















(2m) v/3UV BOvIuns 901




31
meto rgltin om opaaieus ihtb ervdfrmcn
sieigtemiia eaoicrtsgvn yte1/0lie u ic
th aial er eltveyinctv i hemtaois hab rsth
meanmetbolc rte w~il th anmal s rgultin is robblythemos
relsicetmaeo tesanadmeaoicrt wies bmegd n
isth ale ht il e sd nal frhe dsusinsL//aiu
lations
Psuoracu adsal ie cnsrvv ubegd/nar













Survival of submerged S. lacertina at 23-240C in air-
equilibrated water (pO, approximately 155 mmHg).
Animals removed at 336-hours were in no difficulty.
Other times indicate the first observance of death,
and are therefore maximum estimates of survival time.


Survival time (hr)
336 +

336 +

336 +

336 +

336 +

114

30


Table 2.


Weight
83

191

277

463

670

805

998


1046













Conditions of gas exchange partitioning experiments for
two large and one small S. lacertina. The duration of
the runs was 2.75-5.00 hrs (aerial) and 3.08-5.50 hrs
(aquatic) for large animals, and 3.25-6.25 hrs (aerial)
and 2.00-6.33 hrs (aquatic) for the small animal. All
experiments were at 25C. Values are given as 7 + 2 S.E.


Dates of
Experiment


2/26/72-
3/4/72


2/22/72-
3/2/72


3/28/72-
4/3/72


Weight
(g)

1489
+ 26


1182
+9


89.6
+ 0.7


Final Air
02 (vol%)


15.6
+ 1.0


15.2
+ 1.4


18.3
+ 0.8


Mean Water
pO2 (mmHg)


145
+8


144
+3


143
+4


Mean Water
pCO2 (mmHg)

less than 7



less than 7



less than 7


Table 3.
























4J (U

W =3 i0 W 1

WU 0 4-4 1-4


rI
pZi 0
ed oI--



T) CnU rS 1



4CJ aj






.-j0 -I
-1 010











nE a) 0
l1 Cf 0
4J MPJl



crLl 4 II
LIM


0 w w*

o- E 0
VI-a UOLI
.0 u .0 u



4-H wC
CU U Li n












- -r 00 ,3. --
Ed C0 t00

o CU 4p


0O 4 -1
lC -I aU
CU COm 0





O. C Q Cr 4
o3 C U
o- 4 a)

C *l *-i Ul 0

.*1- 01 0



n) a C 1 0o


a) 4J Z.*-



o00: CU




o c -4 -3


34


C.



+-I



0














Ni

I-.
rn





o o
r- Icf
r- 0


-0r



* %








+|N
-1









C) CM
o
CM






0
N










00
Ll'.















I
m






o I-i


Co
aN





















-

O-- ,-







00
NI






Co
I






C-o









In
Ln D


-Ti-
C)



I-I

0

r-C. o
NJ in












CO


+jr 1
C14
+ICM


0-.









0

C4 04
CM4



+I1N




N n


co

,-

+1



Co
0
-aI











-3i-



















+
CM







.)
CM



C4






0
CM


\D


+1
rU








.0












CO
-3i-











C4
+1

















0r
C)
c-n
N









N

+1
C-)
0
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*>


NC 0
0 U






,'--4

n)
*H

dl


NM
NC 0
0 U
> .>





U

cd

3:




























Figure 10.


Aerial-aquatic gas exchange partitioning in a high

02-low CO2 aquatic phase (see Table 3) as a function

of body size. For each partition the upper line is for

W = 1489 g, the middle line for W = 1182 g, and the lowei

line for W = 90 g. Results are shown as i and the 95%

confidence interval. Non-overlap of confidence interval,

yields statistical difference in the mean values at the

95% level. Numbers in parentheses are sample sizes.













,- I (3)
AERIAL I I 7(7)
V02 (9
12 IG 20 24 28

I-f (8)
AQUATIC -i (6)
02 (5)I
6 10 14 IB 22 26 30

------ 1(8)
TOTAL II i(G)
V0 I I (9)
13 24 30 36 42 48 54 60
____ I ___. I_ _I _I _ _I I ,,.,I I I_
I------------I-------- ----,(8 )

AERIAL 1 ] (7)
COz 2 (9)
0 2 4 6 8

'-1- (8)
AQUATIC -------- (6)
Vc02 I (3)
12 18 24 30 3o 42 48
I I I I I I 1 A


TOTAL -- I(6)
vC02
Vc --- ---- (9)
1; 22 28 .4 40
.._____ ___________ I I '_ _-I--I Il-- I-

Oz CONSUMPTION OR COZ ELIMINATION (p/I /i.hr)




37



Since the field data determined that Siren could be found in a

nunber of different aquatic situations with various combinations of

dissolved respiratory gases, the final study dealt with the partition-

ing of aerial and aquatic gas exchange as a function of the le-vels of

dissolved 0 2 and CO 2' The largest animal available ww chosen for

this study, since the submergence experiments had determined that

large animals are most affected by the levels of dissolved respiratory

gases (Table 2). The various conditions used are given in Table 5.

The low 0 2-high CO 2 condition is most similar to the actual environ-

ment of vegetated waters. Animals are occasionally found in high 0 2-

low CO 2 situations, such as when some move into streams to breed in

February and March. The other conditions were investigated to

deter-mine the relative importance of low oxygen and high carbon dioxide

separately. The results are presented in terms of percentages iia

Table 6 and absolute amounts in Figure 11.














Table 5. Conditions of gas exchange partitioning experi-
ments in a single, large S. lacertina. The
duration of the runs were 2.75-4.50 hrs (aerial)
and 3.00-5.50 hrs (aquatic). All experiments
were at 250C. Values are given as x + 2 S.E.


Dates of
Experiment


Weight
(g)


Final Air
02 (vol%)


Mean Water
p02 (mmHg)


less than 7



40
+2


39
+2


less than 7


Mean
pCO2


Water
(mmHg)


2/26/72-
3/4/72


3/17/72-
3/20-72


3/8/72-
3/11/72


3/13/72-
3/15/72


1489
+ 26


1454
+ 10


1464
+4


1453
+2


15.6
+ 1.0


15.7
+ 0.5


15.3
+ 1.2


15.5
+ 0.9


145
+8


145
+2


24
+5


22
+4



















CD
co

(D cl) CD 11
CD -:r
+ + + +

-Z
r CD r
to -rq u 0 C-) 'If
41


-0 0
cu fl- 0 C)
7$ 171; I'D CIO C14
lt fl- 00 r- r-_ Ln
-ri w 0) .
> C) -H I-- -A 0 CD
.74 CD C)
ZI 0 + ],-? + IC-A + 1 +100
+ +
"A > -i D cn cn CD C'4
i C'j -4 --T r, D cli Ln Ln cl I'D (D cl

T 41 r- -i cc C14 -T CrN
w 0 4 -4
4 -H 0
0
44

4 0


co \JD

CA -4

+I cc + -Zp + CA + ],D + +
4 I I I
,H (u E--i W co cn - 0 -A 0
r, co C114 cc I I-) Ln 10

r, 0 ,-1 I'D
ci u C11 r-- CA L) OA
m 0
L-4 O (D
-Li 0 S--
0 -0 b.3

Lo
C-; 4-1 (L) 4-1 Ln
-r-i U 4 CIJ Cq C) 00 C14 co CIA -zr
C' C4 CD co
:J N C14
-l 4-4 Q 0
.0 .> 4J CD CO Ln
-4 W (Ij + r-I + I cq C14 + I co +1 + I
I-j 4 L) I I 1 1
4 CO C -H Q) :3: C,-) Cl) %0 cli C11 -o U") cli cr)
(i cr)
w 71 CrN Ln U,) I'D .o U-) r- Ln cq
Q 7 ul 1-1 r-- 14 CIA --I r-

VC1 cc > A
-It 0 Gj
-1 t4-4 F
I H
cq 0) r- A CA CIA 0 <'A 0
U") (A 0 u CD u
co ti
cd
u Cl. 0


QD

Ij

W:
0
F-I










AM




i,,iiiirwiiii iii im,,i .... im iiiiiiiii, ,


Figure 11.


Aerial-aquatic gas exchange partitioning in a large

S. lacertina (1453-1489 g) under various conditions

of dissolved 02 and CO2 (see Table 5). Results are

shown as 7 and the 95% confidence interval. Non-

overlap of confidence intervals yields statistical

difference in the mean values at the 95% level.

Numbers in parentheses are sample sizes.

















o" A I i '(8)
> B -(6)
4j C i--(-)
, D '(6)
< 114 16 1B 20 22 24




0 I 3 5 ,
A I I (3)
B> B (7))
-(_ c C.----. 16(6)


SCD --I-(6)





oA (5)
o A (3I (S)



- C I (6)
SD I ((6)
-- 15 19 23 27




I- I 1
4 D ---(6)(6)


0 C l--1--1(6)
04 6 1 15 12 21
o" A I I
B I 6(6)
-C --- (6)
D ------- (6)
6 9 12 15 18 21

C7 A W)I I ( _

c i t (6)
D I I (6)
13 17 21 25

A=HIGH 02,LOW CO. 02 CONSUMPTION OR CO2 ELIMINATION
B =HIGH Oz,HIGH COz
C = LOVW Oz,HIGH CO, (/I /g. hr)
D=LOW 02,LOW COz













DISCUSSION OF LABORATORY STUDIES

Metabolic Rate, Gas Exchange and Body Weight

Hemmingsen (1960) has shown that the metabolic rate as a function

of body size for a large number of organisms can be expressed by

(1) M = k WO.75

where M is the rate of oxygen consumption and W is the body weight.

This equation was the result of interspecific comparisons, and Kleiber

(1961) has argued that the relationship also holds for large intra-

specific ranges of body weight, although supporting data are scarce.

Why is the power function of weight in Eq. (1) 0.75 rather than

1.0? One possibility often mentioned is a disproportionate increase

of non-metabolizing (or low-level metabolizing) tissues with an in-

crease in body size. The W in Eq. (1) is actually W the whole body

weight, which is composed of WA (active tissue) and W (relatively

inactive tissue). Even without any actual change in the metabolic

rate of the active tissues, a disproportionate increase in W with

increasing WT would result in an apparent decrease in weight-sepcific

metabolic rate. Appendix B shows calculations that indicate that only

a minor percentage (at least for mammals) of the decrease in weight-

specific metabolic rate with size can be accounted for as apparent

and that the power of 1A in this relationship is close to 0.75 (about

0.77), even when disproportionate increases in W are taken into

account.




43



Another explanation has been offered by Kleiber (1961) which

considers heat exchange. lie points out that a 60 g mouse wit-h the

same weight-specific metabolic rate as a steer would need a 20 cm

thick fur coat to maintain its body temperature if the air tempera-

ture were 3'C. He uses this example to point out why it is advantageous

for mammals to have an increasing weight-specific metabolic rate with

decreasing size. However, this cannot be considered the cause in the

general case, since poikilotherms, which do not regulate their body

temperature, and small aquatic organisms, which would obviously have

no problem in dissipating heat, also have metabolic rates proportional

to W 0.75. Therefore it would appear that insulation -is adapted to

007,rather than vice versa, and some other factor is responsible

for M =k W07

Hemmingsen also showed that unicellular organisms, poikilotherms

and homeatherms ha-ve rates of metabolism that are quite different for

a given weight. Poikilotherms had metabolic rates about eight times

those that would be predicted from considerations of the metabolic

rates of unicellular organisms, and homeotherms had metabolic rates

about 28.6 times those of poikilotherms of the same size. However,

since the poikilot-herm data wqas for 20'C, and the homeathelrni data

for 39%C, we can say that the poikilotherm values (considering 01, = 2

and the temperature correction to be about 20'C) should have been some

four times higher than shown. This means that homeothierms generally

have metabolic rates about seven times greater than poikilotherms of

the same size. Hemmingsen then argued at considerable length that









surface area per unit weight that occurs as one follows the phylo-

genetic sequence from unicellular organisms to homeotherms.

This viewpoint has been supported by several workers. Anderson

(1970) showed that for two species of spiders of similar body weight,

there was a close correlation between metabolic rate and book lung

surface area. Tenney and Remmers (1963) compared the manatee with the

porpoise at approximately the same body weights. The porpoise had the

greater metabolic rate and the greater lung diffusing area. Whitford

and Hutchison (1967) demonstrated that lungless salamanders have lower

metabolic rates than salamanders of the same size with lungs (see

Table 7 for a re-interpretation of their data).

These observations suggest that the metabolic rate of an organism

is correlated with its life style, and that the surface area of the

gas exchanger will be adapted to help meet the metabolic demands for

oxygen. I suggest that there is a limitation to the ability of an

organism to obtain oxygen, and that this limitation is a function of

body size and is responsible for organisms showing a decrease in weight-

specific metabolic rate with increasing body size. The surface area

of the gas exchanger is one of the two factors determining this 02

exchange capacity; permeability to 02 is the other.

A model will illustrate this point. Assume that the surface area

(SA) of the external gas exchanger is the only factor determining the

rate of oxygen delivery to the tissues. The equation relating surface

area to volume for objects of similar shape but varying sizes is

(2) SA = k W0.67

where W has been substituted for the volume by assuming the average




*RW
111f 45



density of an orgaaism to be 1.0, andvhete k I -depends on the shape of

the object. The q#poaent of W in Fq. (2), -which relates weight to SA

ar-d therefore to the ability to supply 02' is less than the exponent

of W in Eq. (1), which relates weight to oxygen dE!mand. Therefore,

as an animal increases in size, the dam-aad for 0 2 will increase mcre

than the ability to supply it. At the theoretical maximum size, the

demand and supply of oxygen would be equal.

The model is illustrated in Figure 12. For an i=nature (small)

animal, the ability to supply 02 under the conditions set above must

be greater than the demand expressed by the standard metabolic -rate,

since it does grow to a larger size, and must also maintain some

scope for activity regardless of size. For some given differential in

PO aCros$ the external gas ey.chancer, assume the permeability to
2 0
oxygen to be 1 cc 0 2 /hr-cm 2 and constant with 'body size, This makes

the 02 exchange capacity (E c ) a function of surface area of the gas

exchanger only. By choosing a starting weight of 1 g, the values of

k and k- I in Eq. (1) and (2) will be the antilogs of the y-intercepts

of these equations when they are plotted in logarithmic form. For

convenience, points A and B are chosen to give values of 1.0 cc 0 2 /hrg

for L- and 1.74 cm 2 /g for k I' If the power of W an M in Eq. 00 were

1.0, then for a I g animal, M ='1.0 cc 02/hr and SA = 1.74 cm2, The

;5-ama values ari-- obtained for a I g animal if M is proportional to

W .75

The capacity of the external gas exchanger to take up 0 for a

given in PO 2 is

(3) E SA P)




















ci *-r4 44-
U O
m lu
4-4 41 ;PI
4 Cd 4-1

W0 H
EJ -4 ,.
cU
0101

41 QJ
o al p
ui B



0 0
E .


-r
m!
to C







0 0) 0


*00 0
33


Cd
*H 0 Lp
C *H r-
441
P*

to a 3
to
N c' t




x me


E U
j; -%
CC



a a p

p p r4
P (1, C




























51
.C
X x


II


II

U)


4-
w


("LI/IO 00)


C~j

N






0o


901


39NVHOX3I








where E is the oxygen exchange capacity, i.e., the oxygen uptake

that would occur across the gas exchanger if the animal consumed

all of the oxygen that crossed the exchanger. For a 1 g animal
2 2
Ec = (1.74 cm ) (1.0 cc 02/hr-cm2) = 1.74 cc 02/hr which is 74%

greater than his resting metabolic rate. This difference AX may

be termed "reserve 02 exchange capacity" and would be related to

the scope for activity.

The maximum weight that can be achieved can be calculated for
1.0 0.75
power functions of W and W by setting E = M. These weights
c
are 5.3 and 1000 g respectively, and represent a large difference in

maximal body size, associated with having a decreasing weight-specific

metabolic rate with increasing body size. Actually, the maximal

attainable weight would be less than these values since some reserve

02 exchange capacity must be maintained by the organism for those

times when its metabolic rate exceeds the average resting value.

This analysis points out an advantage of having a metabolic rate

proportional to a power less than 1.0. Figure 12 can also be used to

point out other strategies which would enable an animal to reach a

large size. Generally, any adaptation that will move the intersection

of the E and M curves to the right will permit a larger body size.
c
A decrease in the exponent of weight on metabolism is one strategy;

others deal with increasing the slope of the E curve. Since
2 2
(4) E = SA(cm2) P(cc 02/hr-cm .-mHg) ApO2(mmHg)

where SA is the surface area of the gas exchanger, P the permeability,

and Ap02 the difference in partial pressure of oxygen across the gas

exchanger, changes in any of these factors will affect the slope of

the E curve.
c






One adaptation would be microhabitat selection resulting in

increasing the differential in partial pressure across the exchanger.

The field results showed that Sirenidae do not do this.

A morphological adaptation which would affect 02 exchange capacity

would be to maintain a constant ratio of the surface area of the gas

exchanger to metabolic demands for oxygen. For example, consider a
0.75
cylindrical organism whose metabolic rate is a function of W

Ignoring the area of the ends of the cylinder which are small in

relation to the area of the curved surface, the ratio of surface area

to metabolism can be held constant if

(5) SA 2nrrL
M k W0.75 o,
or combining constants,

(6) rL = k W75

This means that if the increase in the product of radius and length is

greater than the increase in metabolic rate, a cylindrical animal can

grow indefinitely large.

When a sirenid is submerged, it is essentially a cutaneous

breather. Figure 9 indicates that the larger sirenids do not change

their skin surface area with increasing body weight in a manner

different from the relation predicted in Eq. 2.

A more general statement regarding the relationship between the

power functions of weight on surface area and on metabolism can be

made: there will be no limitation to body size due to limitations

of oxygen supply if the power of weight against 02 exchange capacity

is greater than the power of weight against metabolic rate. Since

the surface area of the gas exchanger is an important determinant of

02 exchange capacity, one might expect the relationship to hold for








it also. Table / presents a number of values for these two exponents

from the literature for various groups of animals. In all cases except

fresh-water turtles, the exponent of weight against surface area is

equal to or greater than that of weight against metabolism. While

this does not prove a cause and effect relationship between Ec and H,

it certainly supports the conclusion that the two are closely correlated.

One might ask why the exponent for weight vs. the exponent for the

E -related factor (usually surface area of the gas exchanger) is
c
usually greater than that of weight vs. metabolism. It is possible

that the permeability of the external gas exchanger to oxygen de-

creases with body size in many organisms, especially those using the

skin as a respiratory organ. Table 8 indicates that this is the case

for large ranid frogs, due to an increase in the thickness of the

epidermis without a compensatory increase in the vascularization of

the skin. Since E is both a function of surface area and permeability,
c

a decreasing permeability with increasing body size could be partly

compensated for by the increase in 02 exchange area being greater than

the increase in metabolic rate as body size increases.

The permeability to oxygen of the gas exchanger in a living animal

is difficult to measure directly, but it can be inferred for the skin

of submerged sirenids from the data of Figures 1A-11A. When 02

tensions are high and an animal is regulating oxygen consumption,

permeability will vary according to oxygen needs. But as the 02

tension in the water falls toward the critical 0, tension, whatever

adjustments can be made to increase the effective permeability of

the skin to 02 will come into play. Such changes could include






























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Table 8. Skin vascularization and epidermal thickness in
ranid frogs (data from Czopek, 1965).


Epidermal
Thickness


i meshes capillary
net per mm skin


Rana
escuelenta

Rana
escuelenta

Rana
escuelenta


Rana
grylio

Rana
grylio

Rana
grylio


Rana
pipiens
sphenocephala

Rana
pipiens
sphenocephala


1.7


53.0


250.0



4.5


24.0


291.0




0.6


35.0


39.1


62.3



24.5


204


220


225



103


32.3


107


12.7


132


127.0 37.0


Species


11 (g)


100








vasodilation of the peripheral arterioles and shunting of blood to

the skin. At P the permeability of the skin to oxygen should be

maximal, and at this time exchange capacity (Ec) will equal metabolic

rate. Therefore the 02 consumption at P is equal to (maximal

permeability)(surface area), and permeability can be solved for
2
directly in terms of p1 02/cm *hr. To express permeability in terms
2
of pl 02/cm -hr-mmHg, one need only to know the pO, of the blood,

since the pO2 of the water is known (i.e., P ). A reasonable estimate
2 c
can be made by assuming that the 02 tension of the blood is close to

the p50 when the blood returns to the skin to pick up oxygen. The

p50 values for the large aquatic salamanders Necturus, Amphiuma and

Cryptobranchus range from 14.5 29 (Lenfant and Johansen, 1967; McCutchec

and Hall, 1937; Scott, 1931). Even small (0.36 0.56 g) submerged

S. lacertina die at p02 tensions between 10 and 20, indicating that

such tensions are not enough to saturate the blood to a degree per-

mitting survival and are probably below the p50. Therefore, the

loading tension of the blood of a Siren will be considered to be

25 mmHg for purposes of calculation. For example, for S. lacertina
2
with W = 0.36 g, M = 35.3 il 0 /hr, SA = 4.886 cm and P = 80 mmHg,
2
permeability would have units of pI 09/hr-cm *mmHg or M/SA(P -25).

We are calculating maximal permeability (P ) and have Pmax = 35.3
max max
2 2
pl 02/hr-4.886 cm -55 mmHg or 0.1313 pl 02/hr-cm -mmHg. Utilizing

this method, permeabilities were calculated for each size of sirenid

for which submerged metabolism data were taken, and the results are

shown in Figure 13.



















4-1



4-4




V4






F-4
4-1
(Ii A
N a)




4-1 4-4
.H 0 44
'A
V4
0
U
44




tz
+A












Lim




















'01



3 0 o

-De
00
S*

ro
< < ( 0 *



rLO







d


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t





,0 ro w I
S".0 0
0 ci
o /o










d d Cd
PHo/ F A








Oxygen exchange capacity can now be calculated as

(7) E (I 0 2/hr) = P (p1 02/cm .hr-mmHg) SA (cm2) p0 2(nmHg)
c 2 max 2 2
where ApO is the differential between whatever the oxygen tension

of the water happens to be and the 25 mmHg loading tension assumed

for the blood. All S. lacertina were compared at 155 mmHg of 02 in

the water, which makes pO02 = 130 mmHg. In the example this means

that E = (0.1313 pl 0 /hr-cm *mmnHg) (4.886 cm ) (130 mmHg) or 83.4
c 2
I1 02/hr. This is the E for a 0.36 g S. lacertina submerged in air-

equilibrated water (p02 = 155 mmHg of oxygen) at 25C. Table 9

gives E M and P values for 13 sizes of S. lacertina. No P could
c c c
be determined for the 17S g size class, therefore no E could be

calculated. Note that 02 exchange capacity fell behind metabolic

demand for the largest size class. Regression lines are fitted to

log-log plots of the data in Figure 14. The slope of the metabolism

plot is 0.65, which is less than the 0.67 found for the surface area

of the skin in Figure 9, but the decrease in permeability with size

caused the slope of the E line to become 0.54, and, as predicted by

the model in Figure 12, there is a theoretical maximum size, which is

2908 g. This would mean that a S. lacertina of up to 2908 g could

survive submerged in water at 250C and p02 = 155 if it continually

maintained only his standard metabolic rate. However, some scope

for activity must be maintained by retaining a reserve 02 exchange

capacity. Thus we would expect death to occur in submerged animals

at a size smaller than 2908 g. Table 9 indicates that large animals

have standard metabolic rates close to their 02 exchange capacities.

Using the data from that table, a plot is shown in Figure 15 of the














Table 9. Standard metabolic rate, 02 exchange capacity, and critical
oxygen tension of S. lacercina of various body sizes(for
submerged animals).


Body weight
(T and range)


0.36(0.32-0.42)

0.56(0.49-0.63)

3.0 (2.7-3.6)

6.5 (5.0-8.5)

13.7(11.0-17.1)

42.7(40-45)

73(67-76)

103(93-112)

178(170-191)

269(233-280)

357(346-375)

541(472-590)

825(770-857)

1310(1090-1451)


Average
Metabolic
Rate
(Pi 02/hr)


35.3

47.6

153

280

438

769

1030

1710

2083

3040

3034

5031

5692

7467


02 exchange
capacity at
25C, 155 mmHg
(pl 02/hr)

83.4

137

331

485

759

1334

1756

2616


4652

3432

6543

6165

6696


Pc (mmHg)
C


80

70

85

100

100

100

105

110



110

140

125

145

170
























CO

r1






lu




to



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4-1






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>1







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ci
.ci








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ri











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ra




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0





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u
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to














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0 E .I





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0\ \ I-0













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o L
0














o in
ro O 0O
c' -
(JY4/01n) 39NIVHOX3 z0 901





























ll


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r4
-14





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o
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s-










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4- C
o -




























D


E 3

I -


(1l 0-


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\ a


I 1 I I
0 0 0 0
\J 0 CO

AiIOVdVO 39NVHOX3


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.^1 ('J


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per cent utilization of E for each body weight. It is obvious

that there is a more rapid increase in the per cent utilization at

larger body weights. This approach predicts a maximum body weight

of 955 g for submerged survival, and shows that animals of 825 g

are using 92% of their 02 exchange capacity just to maintain their

standard metabolic rate.

Actual survival was tested by submerging animals and checking

them periodically for up to 336 hours. If they were still alive at

that time, they were removed and considered to be able to survive in-

definitely. The results were given in Table 2, and it is clear that

animals of 800 g were in trouble, and that larger ones could not

survive.

The smaller size ranges available for P. striatus and S. intermedia

make similar calculations more tenuous. The results are given in

Figure 16, and qualitatively agree with the observation that all

sizes of these sirenids can survive in air-saturated water at 250C.

It is interesting that although the predicted maximum size for S.

intermedia is essentially identical to that of S. lacertina, S.

intermedia reaches a much smaller maximum size than S. lacertina

where the two are sympatric than it does where S. lacertina does not

occur. For the few sizes of P. striatus studied, the 02 exchange

capacity is increasing faster than the metabolic rate with increasing

body size.

Gas Exchange Partitioning

The previous discussion has centered on S. lacertina as a water

breather, but it must be borne in mind that this organism also uses





























4U
Oi
0 0

o w


cu 4-

4-4 *H


0
ri




ol ~

C
od a







u L
)o 0
44




to
C
0 P0 4



-.c
(N
o e

-t





co

U 0
*H *4-4

o



0 :3



-r
60








'1


to


















Ln
r6
co
Kj-
co CT)
N
C)
if it 0

r9 t-Ij

>
al CD
G) co
cq.
W CO
it
Ur u





b d


0
co


n m
0 IT (7)







Ln
cj













Ln


(Jtl/201'd) 30NVHOX3 0 DO-1








aerial gas exchange. Even small individuals in an aerated tank

will breathe air if allowed to surface, and certainly large in-

dividuals must do so to survive. Therefore to gain a more complete

understanding of what the animal is actually doing in his natural

habitat, the partitioning of gas exchange between the air and water

must be considered. This will, however, still be a function of body

size, regardless of the environmental conditions.

The results of experiments dealing with the effect of body size

on gas partitioning were given in Table 4 and Figure 10. Large

animals are obligate air breathers, due mainly to their inability to

obtain sufficient 02 from the water. Most aquatic amphibians eliminate

the majority of their CO2 into the water, and Siren is no exception.

The increase from 6% to 26% in aerial CO2 elimination associated with

large size is probably caused largely by the increase in aerial 02

consumption from 42% to 75%. Assumably, Siren smaller than the 90 g

individual studied obtain even less than 42% of their 02 from the

air. Guimond (1970) studied S. lacertina at 25C and found the per cent

aerial VO2 for three individuals to be 56% (1200 g), 53% (907 g), and

42% (482 g). For 11 S. lacertina averaging 628 g he found the

aerial 2 to be 50%, and the per cent aerial V to be 78%. This
0 CO
2 2
body size lies between the 90 g and 1489 g used here, and the percent-

ages for the aerial partition of each gas lie between the correspond-

ing results for this study, even though the absolute values given by

Guimond are much lower.

The total V for animals allowed to breathe air was about 3-5

times greater in S. lacertina than the average metabolic rate for the




II
samesiz anmalssubnered t th sae auatc pO Thi ispar
tiual rlvn n h aeofte9 Srn hchcnlv
sumrgdi~eiie.Smeo hi nces i xge osupin I

ca b sscatd ih ninres i ctviyconctd ih i

breahin, bt ithasalrady eenmenione Lht-.th ncrase in























(U
'C
H


0-0
JJ


cd o








ul
-c Ei
C-



Cd Oa
.4 N








to a)
Cd CU
ca 3






44
0 cd
'r U 44
L 4i 0


0 t 0





l 0 p
4 ca


o 0
















) 0





P0
rd


2 (U U








r-4
*i-l


-'-4
E24

















0



x X


x x




\x~



0
x O










E
0 0
oN


\X -0
\xx O 0

X










(\ i I O
\>< -0









0 0 0


(JqB/2Ol-) NOl.idl/AnSNOO NQ39AXO







before it can definitely be stated that the metabolic rate of Siren

depends upon the surface area for gas exchange, rather than being

fixed by some other variable.

Finally, to combine the effect of body size with the effect of

the concentrations of dissolved respiratory gases in the water, par-

titioning experiments were carried out for a large S. lacertina at

various partial pressures of 02 and CO2 in the water phase. The

results were given in Table 6 and Figurell. There was no difference in

the level of aerial V2 for any set of aquatic conditions. Oxygen

consumption from the water was significantly higher for conditions

of high 02 tensions than for low, and the aquatic VO2 was lower for

high levels of CO2 (for a given level 02 tension) than for low levels

of CO2. Although the latter differences were not significant, they

suggest the presence of a Bohr shift in Siren blood.

The results for CO2 were somewhat surprising. The absolute levels

of CO2 elimination to the water were highest when the CO2 in the water

was low, but at high 02 tensions there was no difference in the percent-

age of CO2 eliminated to the water whether the CO2 tension was high or

low. Even though there was a significant decrease in the absolute

amount eliminated to the water, 60% of the total VCO2 was aquatic

at tensions of 145 mmHg for 02 and 40 mmHg for CO2. For conditions of

low 02 tension, CO2 elimination to the water was significantly less

in terms of both absolute amounts and percentages for high CO, as com-

pared to low. However, even for those conditions most favoring aerial

respiration (low 02-high CO2), resulting in 96% aerial V02, the CO2

eliminated to the water was 46%. This means that a Siren must be able




73




to tlerte pC 2 i th bl od reatr t an 0 m~g, hic ishig

for n auati anmal.Thi adatatcn, ouped wch he ue o














SUMMARY

1. All three species of the family Sirenidae were commonly found

under water hyacinths.

2. The physical conditions under water hyacinths in terms of pH,

temperature, dissolved carbon dioxide and dissolved oxygen were

determined and compared to those of water not covered by hyacinths

in the same pond. Generally, dissolved 02 was lover, dissolved CO2

higher, p1l lower, and temperature lower under the hyacinths as com-

pared to the "open" (no hyacinths, but often with submerged Cerato-

pEyllum) water.

3. There is no avoidance of areas of low 02 and/or high CO2 by

any of the three species.

4. Siren, and probably Pseudobranchus, adapt to conditions of

low 02 by air breathing. They adapt to conditions of high CO2

partially by air breathing and partially by tolerating high levels

of blood pCO,.

5. Siren lacertina above 800 g cannot survive as water breathers at

250C in air-equilibrated water. Smaller Siren, and Pseudobranchus,

can.

6. All three species were found to have little or no change in

body proportions with increasing body weight.

7. All three species were found to be 02 regulators at high 02

tensions and conformers at low 02 tensions. Large sirenids have

higher critical oxygen tensions than smaller ones.









8. The permeability of the skin to oxygen decreases as a function

of body size in S. lacertina.

9. The oxygen exchange capacity of submerged S. lacertina in-

creases at a slower rate than does the metabolic rate, both as

functions of increasing body weight.

10. A model is presented that shows the advantage in attaining

large size of a decreasing weight-specific metabolic rate with in-

creasing size. It is suggested that metabolic rate is limited by

the 02 exchange capacity of an organism.








































APPENDIX




1w1


































Figure 1A. Metabolic rate ot submerged P. striatus (0.51 and 1.58 g)

as a function of oxygen tension.





















'--.------- --------
*. ..--* * .


-- -

MR 75 66
SP 100 70
'* a' R


P striotus

(0.47-0.57g, X=0.51)


80




60


c"
c 40

o"

20




820-
2o








0
O


80-
W
LiJ -
(D
S60-

0

40-




20-


a *a
a -
--


S--------
,----------------------*----------A-----
a a
0


* *


c6



. r x I 0/90
MR 59 46 P. striatus
PC 65 60 (.0-
R (1.40-1.82g, X=1.58)


0 20 40 601
0 20 40 60


I I I I I ~ ~ I


80 100 120 140 160
80 100 120 140


pO, (mm Hg)


* *


I


l D I I


i N I | i


l


*





































co
Eo

00






C,









0
14








to 0
SU








-4 Oi
P 4
01 t





0 0
(U















44
PH




yrcL
0 -
.0 *-

n) u











r-T-1





0!



































Figure 3A. Metabolic rate of submerged S. intermedia (3.3 and 7.0 g)

as a function of oxygen tension.


























-s

,




S -


. _*-------------------- -
*
*
S _


arl in


R MR 56 40
R Pc 100 75


(2.9 3.7 g, = 3.3)


1 C ~1- -T I- I -I --1- r 1 i -I --S


**. *


-- .. . .
S--^ --------- -;- --
S/' *
," ** *
*- S S '
S *. --S- ' --<-------
S S *


R 40 30

S. S. intermedia i L 0 .0
_I f t-I7'; t7fi


40 60 80 100 120 140 160
40 60 80 100 120 140 160


pO2 (mm Hg)


75-




60-


45-




30-


-c



0


0-Y



75-




60-




45-




30-



15-


n-L/


. *


20


I .


or #


/-


(5.0-8.5g, X" 7.0)






































Figure 4A. Metabolic rate of submerged S. intermedia (13.7 and 29.6 g)

as a function of oxygen tension.




IRE





60-


50- S. intermedin
I 4-17. 1 X=117) 0

40-


30-

0
20-
CL

10-
R i-3 21
PC 110 6515
R
0 -YA
Z


x S. intermedia
0
30- (26 33 R 29'e)



20-




Pit



































Figure 5A. Metabolic rate of submerged S. lacertina (0.36 and 0.56 g)

as a function of oxygen tension.


















** .
100 *
100 -------------------------------------
,* .


80


c' *
\ 60- ,-
0 *-
S. lacertina
40- '.
0 R (0.32- 0.42g, X 0.36)

0
20-
CL0. R 98 82
S1 PC 80 60





=? ----^----^---- ,'------ ---"---,------ --------,-----,------,
:* ,
0 120-
L> 100


---- - --- .










PC 70 70
0 800 40 60 80 1 1 1


o '"' S. locerina

S. | (0.49-0.63g, X=0.56)

0 R i_
40 R 85 -71
PC 70 70

0 20 40 60o 80 100 120 140 160o 180


p02(mm Hg)































60

tri

.LD





c1
vi



















0 0
Qj

El .
-o

cu
4-1
























uo
.- -
0 CO
00 0


L3 0




U




LDi





00




.r4























I

r I *
I
Io
S .
!



I *




*



*
I








o
i

\



4


Ix
I N


* i



0 X







.


- 0 I 0 ---1
0 0 00 00 00
~msN D i) r)C


I
S I

*
I
I


*o


in)
(D
II
o IX


0
O
o
I.!


I S
I *

Ce I
Col










55

L. 1-
"1 11






U'
S' *


U "I
-- 1


i o
I* I
*O N











a \


0 0 0 0 0 0 0
r- Q LO C p-o Nj -


-O


0
-O




0





0
-M





N



.E


E





CL

0





0



r

-


V-8/ ra) \Oldj0 lldnSNO N.50AXO



































Figure 7A. Metabolic rate of submerged S. lacertina (13.7 and 42.7 g)

as a function of oxygen tension.




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