The influence of body weight on gas exchange in the air-breathing fish, Clarias batrachus

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Title:
The influence of body weight on gas exchange in the air-breathing fish, Clarias batrachus
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viii, 63 leaves. : illus. ; 28 cm.
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English
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Jordan, Jill Ann, 1944-
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Catfishes   ( lcsh )
Respiration   ( lcsh )
Body weight   ( lcsh )
Fishes -- Physiology   ( lcsh )
Zoology thesis Ph. D
Dissertations, Academic -- Zoology -- UF
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bibliography   ( marcgt )
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Bibliography:
Bibliography: leaves 60-62
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Typescript.
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Thesis -- University of Florida.
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Vita.

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Full Text





















THE INFLUENCE OF BODY WEIGHT ON GAS EXCHANGE IN
THE AIR-BREATHING FISH, Clarias batrachus










By





JILL ANN JORDAN


A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY







UNIVERSITY OF FLORIDA
1973


_I














ACKNOWLEDGEMENTS

I gratefully acknowledge the help of the many people who

contributed to this study. Special thanks go to Vernon Ogilvie

of the Florida Game and Fresh Water Fish Commission who assisted

in collecting specimens and provided much useful information on

the biology of Clarias in Florida, and to Don Goodman who assisted

in collecting, setting up experiments, and proof-reading the manu-

script.

I am indebted -to Dr. Brian McNab for his assistance and criticisms

and for enduring the clutter and floor-to-ceiling mud in his laboratory,

and to Drs. Frank Nordlie, John Kaufmann, Pierce Brodkorb, Hugh

Popenoe and Stephen Zam for criticism of the manuscript and use of

space and materials. Dr. Gordon Ultsch provided much valuable in-

formation on the techniques used in the study.

Mr. Paul Laessle provided valuable counsel on the preparation of

the figures, and Ms. Donna Gillis typed the manuscript and aided in

the completion of the final details of preparation.















TABLE OF CONTENTS

Page

Acknowledgements. . . ii

List of Tables. . . iv

List of Figures. . .. .. v

Abstract. . . ... . vii

Introduction. ... . . 1

Materials and Methods. . . 7

Results . . . 10

Discussion .. .... ... .. 37

Summary . . 58

Bibliography. . . . 60

Biographical Sketch . ........ ............ 63















LIST OF TABLES


Page

1. The Metabolic Rate of Various Air-Breathing Fishes
(and Ictalurus nebulosus) Under Varying
Experimental Conditions. ................ 39

2. Oxygen Capacities of the Blood of Representative
Air-Breathing Fishes .................... 43















LIST OF FIGURES


Page

1. Metabolic rate of forcibly submerged C. batrachus
as a function of body weight. . .. 12

2. Metabolic rate of air exposed C. batrachus as a
function of body weight . .... 14

3. Metabolic rate of C. batrachus, in water with access
to air, as a function of body weight. . ... 16

4. Metabolic rates of C. batrachus (forcibly submerged,
air exposed, water with access to air) and poikilotherms
(from Hemmingsen, 1960) as a function of body weight. 18

5. Metabolic rate of C. batrachus weighing 42 g as a
function of oxygen tension. . ... 21

6. Metabolic rate of C. batrachus weighing 77 and 83 g
as a function of oxygen tension . ... 23

7. Metabolic rate of C. batrachus weighing 172, 195 and 206 g
as a function of oxygen tension . 25

8. Relationship between oxygen uptake from air and oxygen
tension of the water in C. batrachus weighing
30-71 g . . . ... 28

9. Relationship between oxygen uptake from air and oxygen
tension of the water in C. batrachus weighing
141-210 g . . . 30

10. Diurnal and nocturnal surfacing frequency as a
function of body weight . .. 33

11. Average number of surfacings per hour as a function
of time of day in air-saturated water and nitrogen-
saturated water . .... ..... 35

12. Gill surface area as a function of body weight for
various aquatic respiring teleosts (from Muir, 1969)
and C. batrachus (from Saxena, 1966). . ... 41









13. Critical oxygen tension of C. batrachus and two
salamanders, Siren (from Ultsch, 1972) and
Desmognathus (from Beckenbach, 1969), as a
function of body weight ............ .. 47

14. Gill surface area of C. batrachus as a function
of body weight (from Saxena, 1966). . 51

15. Buffering capacity of blood of selected vertebrates
at 25*C (from Howell, 1970,.and Rahn, 1967) ..... 55









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

THE INFLUENCE OF BODY WEIGHT ON GAS EXCHANGE IN
THE AIR-BREATHING FISH, Clarias batrachus

By

Jill Ann Jordan

June, 1973

Chairman: Dr. B. K. McNab
Major Department: Zoology

Aerial respiration and its capacity for terrestrial locomotion

have enabled Clarias batrachus, a clariid catfish from southern

Asia, to establish a large breeding population in southeastern

Florida. Clarias batrachus is a facultative air breather, and at

aquatic oxygen tensions exceeding 40 mmHg, an oxygen regulator.

The rate of metabolism of Clarias varies as a function of

weight, and access to air. Aerobic metabolism is primarily utilized,

even during forcible submergence when the metabolic rate declines.

Clarias, a terrestrially active fish under certain environmental

conditions, shows no metabolic reduction upon emersion.

The aquatic oxygen tension below which Clarias becomes an

oxygen conformer is directly proportional to body weight, and about

1/3 the value for bimodally breathing salamanders; the disparity is

presumably the result of the greater surface area for gaseous exchange

in Clarias.

The number of air breaths per unit time doubles at night and

is inversely proportional to body weight.









The sensitivity of its naked skin to desiccation and the low

bicarbonate buffering capacity of its blood limit the terrestrial

activity of Clarias to rainy nights when a cutaneous film of water

can be maintained.


viii















INTRODUCTION

The transition from aquatic to aerial breathing was one of the

most important events in vertebrate evolution. Air breathing has

evolved in numerous taxonomically unrelated groups of teleosts, and

has produced a remarkable degree of convergence in these fishes.

Bimodal breathers run the respiratory gamut from facultative air

breathers that can survive indefinitely on the oxygen dissolved in

water (except in times of severe oxygen stress), to obligate air

breathers (such as Electrophorus, Lepidosiren, and Protopterus)

that will drown when denied access to air; hence this is an excellent

group in which to study the physiological modifications imposed by

bimodal respiration. One such bimodally breathing teleost, Clarias

batrachus, the "walking catfish," is the subject of this study.

Air breathing has enabled this exotic introduction from southern

Asia to become one of the dominant fishes in southeastern Florida;

hence this species is an interesting form in which to study some

of the aspects of bimodal respiration that have contributed to

its success.

The two respiratory media, water and air, differ greatly in

their physical properties, imposing very different morphological

and physiological adaptations on the structure and function of

respiratory surfaces. Water is a medium that often offers marginal

conditions for gas exchange. It is frequently oxygen deficient.

At 25C, oxygen attains only 1/35 the concentration in water that









it does in air, but carbon dioxide is 27 times more soluble in water

than oxygen, and may reach concentrations greatly in excess of

aerial values. Consequently water breathers must ventilate much

larger volumes of a denser medium than air breathers for a similar

oxygen uptake. Air, In contrast, with its high partial pressure

of oxygen offers a favorable diffusion gradient but requires

significant structural support for respiratory surfaces. Furthermore,

an atmospheric humidity below saturation will cause evaporation from

the epithelial surfaces that must remain moist for gas exchange.

Therefore a premium is placed upon structures protected within the

body for aerial gas exchange. Obligate air breathers also must have

an increased efficiency of the internal buffering system because of

the elevated internal carbon dioxide tensions.

Since the differences in gaseous composition between air

and water increase with temperature, it is not surprising that

the majority of extant air-breathing fishes live in tropical fresh-

waters. These relatively warm waters usually have low oxygen and

high carbon dioxide concentrations, due to the lack of turnover

(because there is little wind disturbance and night temperatures

are not cool enough), the reduction of aquatic photosynthetic

activity because of shading, and the high B.O.D. of such waters.

Only a few temperate fresh-water species exhibit air breathing

(Lepisosteus, Amia, and Umbra). Johansen (1970) demonstrated that

air breathing in Amia calva increases at higher temperatures. The

few marine air-breathing fish [some members of the families

Gobiidae (including the mud skippers) and Blennidae, the clingfish


_ 1(








Sicyases (Gordop et ai., 1970), the eel, Angullla (Berg and Steen,

1965), and tarpons (Elopidae)] are mostly estuarine q~ecies and

probably often encounter deoxygenated water in areas of organic

decomposition such as mud flats, mangroves and vegetation mats.

Air breathing has enabled many of these fish to not only

inhabit water with unfavorable gas tensions, but even to leave the

water and spend part of their time on land. Certain fish, such as

* Anguilla (Berg and Steen, 1965), Symbranchus (Johansen, 1966),

Clarias batrachus (Smith, 1945; Das, 1927), Saccobranchus -

Heteropneustes, Anabas and Ophiocephalus (Das, 1927), frequently

make night sojourns from pond to pond through moist grass. Some

of these fishes are thought to feed while making these excursions.

The Chilean clingfish, Sicyases sanguinensis (Gordon et al., 1970)

rests on rocks in the inter-tidal splash zone, while the more

active mudskipper, Periophthalmus, actually leaves the water to

escape predators and to feed (Johansen, 1970). Other fish, such

as African species of Clarias may move from drying pools to permanent

bodies of water. Desiccation is a problem faced by most of these

fish when they leave the water, hence the extent of these land

excursions is a function of weather conditions, time of day,

and physical conditions of the area.

In India (Das, 1927) and in south Florida (Ogilvie, personal

communication), great numbers of Clarias batrachus migrate in the

rainy season, presumably for feeding purposes. Mookerjee and

Mazumdar (1950) reported that Clarias batrachus rarely migrates


C II)_


:









by a land route like Anabas, but frequently migrates in shallow

running water. This account corroborates descriptions of its

behavior in south Florida, and some of the conclusions reached in

this study.

If they are unable to escape a drying pool, certain air-breath-

ing fish burrow into the mud and become dormant. Estivation has been

reported for Clarias (Das, 1927; Sterba, 1963), Saccobranchus

(Das, 1927; Hughes and Singh, 1971), Ophiocephalus (Das, 1927),

Amia (Neill, 1950), Symbranchus (Johansen, 1966), and the African

and South American lungfishes, Protopterus and Lepidosiren. Clarias

and Saccobranchus, another clariid, bury themselves in moist mud

five to six inches below the surface (Das, 1927). But with the

exception of Janssens' (1964) experiments on Protopterus, little is

known of this state.

Most air-breathing fish are facultative air-breathers, employ-

ing aquatic breathing unless the water is deoxygenated. Willmer

(1934) observed that high carbon dioxide concentrations also

stimulate air breathing, even when the oxygen content is adequate

for water breathing.

All air-breathing fish utilize aquatic gas exchange to some

extent, because the air-breathing organs show a low gas exchange

ratio. Since carbon dioxide is highly soluble in water, its

elimination takes place primarily through the gills or the skin in

naked forms. Rates of oxygen uptake vary, depending on the

respiratory medium and dominant mode of respiration. Most air

breathers show a reduction in total oxygen consumption when


i r









breathing air alone, although this is not the case with obligate

air breathers and forms that are active upon emersion, such as

Periophthalmus.

The morphological and physiological modifications for bimodal

breathing in part account for the ecological success of many of

these species when introduced into new areas. Clarias batrachus, a

clariid catfish that is widely distributed from Ceylon through

S eastern India to the Malay Archipelago (Sterba, 1963), was introduced

into the United States for the aquarium trade and escaped from a

"fish farm" near Fort Lauderdale, Florida, probably in 1966 or 1967.

Because of its capacity for air breathing and mobility, C. batrachus

has spread via drainage ditches and canals throughout much of

southeastern Florida.

This study attempts to examine some of the aspects of respiratory

physiology of C. batrachus that have contributed to the success of this

species in south Florida. The breathing response of the animals as

a function of forcible submergence, oxygen tension, time of day,

and body weight was examined. Measurements were made of the rate

of oxygen uptake (V02) under various experimental conditions

(forcible submergence, air exposure, and water with access to air)

and related to the ecology and behavior of this species. An attempt

was made to determine if C. batrachus utilized anaerobic glycolysis

when the oxygen consumption was reduced during forcible submergence.

Electrocardiograms were taken to compare the cardiac response of

C. batrachus to those of other air breathing fish, and an attempt

was made to induce Clarias to estivate.










While this study was in progress, Singh and Hughes (1971)

published results of a similar study of C. batrachus that were

imported from India. Although there was some overlap in the scope

of the two studies, the findings of Singh and Hughes provide valuable

comparative data that augment this study.


I















MATERIALS AND METHODS

An electro-shocking apparatus and dip nets were used to collect

specimens of C. batrachus in the vicinity of Fort Lauderdale, Florida.

Fish were maintained in aquaria at 25 + 20C on a twelve light/twelve

dark photoperiod.

All metabolic rates were determined at 25 + 1C. Experiments

were conducted during the day to obtain values for resting metabolism,

as Clarias is nocturnal. In a few cases, as during partitioning of

aquatic and aerial phases of respiratory exchange, experiments

continued into the evening. All values for oxygen consumption

were converted to ccO2/kg-hr at STP.

Oxygen uptake of submerged animals was determined through the

use of a chamber (Erlenmeyer flasks or sealed battery jars) from

which samples of water were withdrawn through polyethylene tubing.

A Radiometer PHM 71 Acid-Base Analyzer was used to measure the PO2

of the water sample. Animals were acclimated to the chamber at

least eight hours prior to a determination.

Aerial respiration was determined by manometry. The apparatus

consisted of two equal volume battery jars, serving as the metabolic

and compensation chambers, respectively. A container of Soda Sorb

was placed in the fish chamber to absorb the expired carbon

dioxide. Oxygen used in the metabolic chamber was replaced by

means of a syringe. As long periods of air exposure produced









skin necrosis, animals were introduced into the chamber for only an

hour before metabolic determinations were made. To determine

cutaneous oxygen uptake in air, a latex condom was placed over the

head of the fish, preventing gas exchange through the air breath-

ing organ and gills.

Rates of oxygen uptake in water where the fish had free access

to air were measured in a two section chamber. A small opening

separated the aquatic from the aerial phase. A 1/4 inch mesh

funnel guided the surfacing fish to this opening. Water samples

were drawn from the aquatic chamber for PO2 determinations, and

aerial respiration was determined manometrically. In a control

using water equilibrated with nitrogen there was no significant

amount of diffusion of oxygen into the aquatic phase.

The frequency of aerial breathing as a function of body weight

and time of day and/or amount of incident radiation was determined

with a Hunter Model 3355 photorelay photo cell coupled to an

Esterline Angus event recorder. Air-equilibrated water was

circulated to the recording chamber to keep the PO2 and PCO2

levels constant.

Experiments were conducted to determine whether anaerobic

pathways made a significant contribution to the total energy

expenditure of forcibly submerged animals. Studies of fish

intermediary metabolism (Hochachka, 1969) indicate that the

Embden-Meyerhof pathway is the principal pathway of glucose

catabolism during aerobic metabolism, with the hexose monophosphate

shunt providing a minor contribution, but during anoxia the EMP is









operative exclusively. In order to prevent nearly all anaerobic

energy production without greatly interfering with aerobic processes,

fish were injected withiodoacetic acid, which irreversibly inhibits

phosphoglyceraldehyde dehydrogenase and thus blocks the Embden-

Meyerhof pathway prior to any energy release. In one experiment,

fish were injected intraperitoneally with a dosage of 15 mg per kg

of IAA after Belkin (1961). Another group was poisoned with a dosage

' of 20 mg per kg, after Rose and Drotman (1967). Half the poisoned fish

(and unaltered controls) were then forcibly submerged in air-equilibrated

water and half were allowed access to air to determine whether such

poisoning was detrimental to submerged fish.

Electrocardiograms were recorded on a Grass Model 79 polygraph.

Two leads were inserted into the fish's body on either side of the

heart and.tied dorsally. Fish were anesthetized with MS-222 to

facilitate electrode implantation, and allowed to recover for

.several hours before recordings were made.

Estivation chambers were constructed from a nylon mesh bag

placed inside a hardware cloth frame that was placed inside a

battery jar or refrigerator liner. Mud was placed in the bag, and

the container filled with water. The water was drained off at the

rate of an inch every.other day. Several different mud substrates

were used, and in some cases a bright light was left on the fish

during the day.


L














RESULTS

Clarias batrachus weighing 69 to 178 g survived at least two

weeks when forcibly submerged in air-saturated water. This species

can also survive out of water for extended periods, provided the

humidity is high and the skin is kept moist. A 54 g individual

survived 3 days and a 124 g individual survived 4 days at 21C when

kept in plastic boxes with wet cheesecloth. Individuals exposed to

room air succumbed in about six to eight hours. The skin of these

individuals rapidly dried during this period.

The rates of oxygen consumption varied greatly as a function

of body weight and access to air. The metabolic rate of forcibly

submerged Clarias can be described by the relationship Log M =

0.78 Log W 0.82 (r = 0.96; see Fig. 1). Their metabolic rates in

air are described by the relationship Log M = 0.72 Log W 0.54

(r = 0.87; see Fig. 2). When the fish were placed in water and

given access to air their rates of metabolism are described by

the relationship Log M = 0.63 Log W 0.36 (r = 0.93; see Fig. 3).

In Fig. 4 the three curves (forcibly submerged, aerial and water

with access to air) are compared. These curves are essentially

identical in their power of weight, but the curve for forcibly

submerged animals is lower than for individuals that have air

available for gas exchange. The relation of standard energy

metabolism to weight for poikilotherms, corrected to 250C (Log M =

0.75 Log W 0.69, from Hemmingsen, 1960) is also shown in Fig. 4.























































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Figure 2. Metabolic rate of air exposed C. batrachus
as a function of body weight.




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access to air, as a function of body weight.




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Active metabolism of a few air exposed individuals was obtained;

values for a 220 g and a 113 g individual were 148.2 ccO2/kg-hr and

290.2 cc02/kg-hr respectively, as compared to resting rates of

52.5 ccO2/kg-hr and 78.5 ccO2/kg-hr, respectively (Fig. 2). Activity

then may raise metabolism at least by a factor of three. Cutaneous

respiration accounted for up to 50% of the total oxygen uptake of

two large (200 and 220 g) air exposed fish.

The effects of gradual hypoxia on metabolism of submerged

Clarias weighing 42 to 206 g is shown graphically in Figs. 5, 6, and

7. The metabolic rate appears to be independent of oxygen concen-

tration over the range of PO2's from saturation to about 40 mmHg.

At oxygen tensions less than 40 mmHg these fish appear to be oxygen

conformers.

As submerged fish exhibited the lowest rates of oxygen uptake,

the question arose whether these fish might depend upon anaerobiosis.

Four fish were poisoned with a 15 mg lodacetate per kg body weight;

two fish were forcibly submerged in air-saturated water, and the

other two were placed in air-saturated water with access to air. One

unpoisoned fish was placed with each group. Weights of the individuals

used are given below:

Submerged Access to Air

113 g poisoned 156 g poisoned
153 g poisoned 190 g poisoned
178 g normal 163 g normal

The 153 g individual died after two days, all others survived for

14 days, when the experiment was terminated. This experiment was



































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repeated using seven fish and a dosage of 20 mg/kg. The weights

of these fish are given below:

Submerged Access to Air

77 g poisoned 99 g poisoned
132 g poisoned 181 g poisoned
164 g poisoned 139 g poisoned
180 g poisoned 140 g normal
69 g normal
224 g normal

The 181 g and 139 g individuals, which had access to air went into

tetany when placed in the tank and died in a few hours. The 77 g

individual died during the night. All others survived for two days

and the experiment was terminated. The fact that most of the sub-

merged animals that were poisoned survived would tend to preclude

a significant dependence on anaerobiosis when this species is denied

access to air in normoxic water.

The percentage of oxygen uptake from air when the fish were in

water of various 02 concentrations with access to air is shown in

Figs. 8 and 9. Aquatic breathing accounted for 80% to 90% of the

total oxygen uptake at higher oxygen tensions. At lower oxygen

tensions aerial respiration increases. Smaller fish appear to

increase the amount of aerial respiration at higher oxygen tensions

than larger fish, as the decrease in percentage of aquatic respiration

for smaller fish occurs when oxygen tensions are about 60 mmHg

(Fig. 8) as opposed to 40 mmHg for larger individuals (Fig. 9).

The total oxygen uptake is reduced when Clarias is kept in

deoxygenated water (PO2 = 6 mmHg) with free access to air. The mean

V02 of a 141 g individual was 39.1 ccO2/kg-hr compared to a level of




































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53.5 ccO2/kg-hr in normoxic water. A 71 g individual had a rate

equal to 51.4 ccO2/kg-hr, compared to a normal level of 94.2 ccO2/

kg-hr. Clarias proved capable of raising the V02 under these

conditions; the 71 g fish had an aerial VO2 of 93.8 ccO2/kg-hr

when active in the dark at a water PO2 that averaged 28 mmHg.

This increase was due to increased aerial gas exchange only.

Clarias can survive for weeks living in oxygen-deficient water if

it has access to air.

The number of air breaths taken per hour is inversely proportional

to the weight of the fish raised to a power less than one and is

influenced by the time of day (and amount of incident radiation)

(Fig. 10). The mean number of daytime surfacings for a 30 g

individual was 19, while that of a 244 g individual was 6. The

surfacing frequency increased by 60% to 100% at night. Lowest

values were between 1100 and 1400 hours EST. On cloudy or rainy

days, surfacings were similar to night values. Any disturbance

(turning on the light at night, standing near the apparatus) caused

surfacing frequency to decline. An 80 g fish plac ein nitrogen-

saturated water surfaced about the same number of times during the

day as it did in air-saturated water, but at night the number of

surfacings doubled (see Fig. 11).

Several attempts were made to induce estivation in Clarias.

Many constructed and used burrows that extended three or four

inches into the mud. Several individuals survived for a period

of three to four days in the estivation chambers, but invariably








































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with the fall in the water level, they emerged, desiccated and

died.

Electrocardiographic measurements of heartbeats demonstrated

some physiological changes upon emersion. Heartbeat frequencies

of a water-breathing fish were around 30 beats/minute and rose to

39 beats/minute following an air breath. When the water was drained

from the tank, bradycardia (15 beats/minute) ensued for about three

minutes, until the fish moved and gulped air. This was followed

by a tachycardia of 78 beats/minute.















DISCUSSION

Clarias batrachus appears to be a facultative air breather,

as forcibly submerged animals survived at least two weeks. Saxena

(1960) also showed that small C. batrachus and S. fossilis could

survive forcible submergence for at least two weeks. Clarias

batrachus has been described by Singh and Hughes (1971) and Das

(1927) as an obligate air breather. Singh and Hughes (1971)

provided no data to support this supposition, and Das' fish

succumbed after two hours when a copper screen was used to prevent

access to the surface. It is possible that copper poisoning, rather

than air deprivation, caused the demise of the fish used in Das'

experiments.

The energy requirements of a fish, which are reflected by the

oxygen consumption, depend on several factors, including physical

properties of the water (amount of oxygen, carbon dioxide, tempera-

ture) and the physiological, anatomical, and behavioral modifications

of the fish (ventilation rate, area for gas exchange, activity).

High ventilation rates, large areas for gas exchange, and an oxygen

rich environment all contribute to high rates of oxygen extraction.

Air-breathing fishes, which frequently inhabit waters of low oxygen

tensions, and often show reduced areas for aquatic gas exchange,

might be expected to have low metabolic rates. In Fig. 4, metabolic

curves for C. batrachus (forcibly submerged, aerial and water with

access to air) are compared to the relation of standard energy









metabolism to weight for poikilotherms, corrected to 250C (from

Hemmingsen, 1960). The curve for forcibly submerged Clarias is

slightly below the standard poikilotherm curve, while those for

Clarias breathing air or water with access to air are slightly

above.

The metabolic rates of C. batrachus and several other air-

breathing fishes are shown in Table 1. The rates of oxygen

consumption of air-breathing forms are usually labile, depending

upon experimental conditions and are often lowered when the animals

are submerged. In the case of Protopterus and Electrophorus, both

obligate air breathers, the low metabolic rates in submerged animals

are related to their small gill surface area. These fish will

drown if denied access to air.

The lowering of V02 in submerged Clarias is presumably not due

to the utilization of any anaerobic pathways, as submerged IAA

poisoned fish fared as well as those with access to air. Singh and

Hughes (1971) state that the lowering of the total bV2 in submerged

Clarias may be related to the reduced surface area of the gills and

the thickness of the gill epithelium. Saxena (1966) found the gill

area values of C. batrachus (295 mm2/gm) and of S. fossilis (395

mm2/gm) were small compared to values of open water forms (Rita rita

= 1,000 mm2/gm), but when values for Clarias are compared to data

on additional species (Fig. 12, from Muir, 1969), its gill surface

area surpasses those of many purely aquatic breathers-the carp,

Cyrinus carpio; the white sucker, Catostomus commersoni; and the

brown bullhead, Ictalurus nebulosus) Munshi and Singh (1968)













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determined that the respiratory epithelium is very thick in C.

batrachus (8 y-15 p as compared to 0.8 p-3.2 p for water breathers).

In addition, they estimated that the thickness of the basement

membrane in C. batrachus precluded all but 52% of the surface area

available for aquatic exchange. Even if Saxena did not account for

the reduction in exposed secondary lamellar area, the proportional

extent of the gill surface of Clarias still exceeds the values of

carp, bullhead, and sucker.

The thickness of the epithelium in Clarias undoubtedly reduces

the rate of diffusion of oxygen into the blood stream. But this is

probably at least partially offset by the high oxygen capacity

(18.0 vol.%, Singh and Hughes, 1971) of the blood. High oxygen

capacity values probably enhance the ability of the blood to pick

up more oxygen while passing through the gills. Johansen et al.

(1968) hypothesized that the high oxygen capacity of Electrophorus

blood was an adaptation to mixing of oxygenated and deoxygenated

blood that results from the shunting of respiratory efferent blood

to the systemic veins. Yet the blood of Symbranchus, Clarias, and

Saccobranchus show the greatest oxygen capacity values (Table 2),

and these species are among the few air-breathing fishes that have

the ideal perfusion pattern (no mixing of arterial and venous

blood) existing in purely aquatic-breathing fishes. All are

facultative air breathers. Symbranchus has a highly reduced gill

surface area (40 mm2/gm, Junqueim, Steen and Tinoco, 1967) and the

gill lamellae are thickened so they do not collapse in air. Thus

it may be that the high oxygen capacity enables these primarily



















TABLE 2

Oxygen Capacities of the Blood of Representative
Air-Breathing Fishes


Oxygen
Capacitv (vol. %)


Reference


Clarias batrachus

Saccobranchus fossils

Electrophorus electricus

Lepidosiren paradoxa

Amia calva

Protopterus aethipicus

Symbranchus marmoratus


18.0

17.5

13.9

8.25

7.8

9.50

17.30


Singh and Hughes, 1971

Singh and Hughes, 1971

Johansen et al., 1968

Johansen, 1970.

Johansen et al., 1970

Lenfant and Johansen, 1968

Johansen, 1970


Species


__









aquatic breathers to pick up more oxygen than would normally be

possible through their reduced or thickened gill surfaces. A high

oxygen capacity would also increase the interval between air breaths

when oxygen tensions in water were low, by increasing the oxygen

store of the body.

It appears that the low metabolic rates of submerged animals

areat least partially due to a decrease in activity. Hughes and

Singh (1970) demonstrated that the climbing perch, Anabas testudineus

showed a metabolic rate of 113.4 ccO2/kg-hr in water with access to

air, 75.5 ccO2/kg-hr while forcibly submerged, but in water with

access to nitrogen, where no additional aerial surface was available

for gas exchange, the VO2 was 127.5 cc02/kg-hr. Thus it appears

that surface area per se is not limiting these animals metabolically

while in the aquatic phase. The surface area available for aquatic

gas exchange certainly must set a limit to the extent of the animal's

activity, but this will be considered later.

Singh and Hughes (1971) report that the mean V02 of Clarias

in a continuous flow of 50% air saturated water drops to almost

half the value for fully saturated water. If subjected to hypoxia

in a closed chamber, V02 was reduced from 95.6 cc02/kg-hr at

151 mmHg to 44 cc02/kg-hr at 70 mmHg. These workers further state

that Anabas and Clarias resemble I. nebulosus and the toadfish

(Opsanus tau) which show oxygen dependent aquatic respiration

(oxygen conformance). The data from the present study demonstrate

that Clarias regulates its oxygen consumption at a constant rate

above water oxygen tensions to about 40 mmHg, suggesting this


. .









species is an oxygen regulator. Similarly, Farber and Rahn (1970)

found a linear decrease in water P02, indicating oxygen regulation,

even at partial pressures as low as 30 mmHg, in the electric eel,

an obligate air breather with reduced gill surface area. Perhaps

Singh and Hughes (1971) were observing a decrease in the active

metabolism of their animals with decreasing PO2 of the water. They

also stated there was no good correlation between weight and weight-

specific oxygen consumption of forcibly submerged fish, which

suggests that some of their animals may have been active.

The critical oxygen tension (Pc), or partial pressure of

oxygen below which an animal cannot regulate its oxygen consumption,

as a function of body weight is plotted in Fig. 13 for Clarias, a

transitionally breathing aquatic salamander (Siren), and lungless

terrestrial salamanders (Desmognathus). The Pc increases with

increasing body weight for salamanders, and appears to follow the

same relationship in Clarias. Job (1955) found that the effect of

reduced oxygen content on active metabolism was independent of size

for Salvelinus fontinalis, while Beamish (1964) makes no mention

of any effect of size on the level of Pc in the carp and goldfish,

even though he used a weight range of from 17 g-600 g.

Beckenbach (1969) states that the level of P depends upon the

weight-specific surface area (and therefore, size) of the animal,

the metabolic rate, and the temperature. The differences in P

between terrestrial and aquatic salamanders may be related to the

thickness of the skin, as this is the only gas exchange surface in

these lungless forms, whereas Siren normally utilizes cutaneous,

































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gill and lung respiration. Ultsch (1971) states that forcibly sub-

merged Siren utilize cutaneous respiration almost exclusively. The

cutaneous surface to weight relationships for Siren and Desmognathus

are virtually identical.

The P values of Clarias are about three times lower than those
c
of Siren of a similar weight, even though the metabolic rate of

Clarias is approximately 3.7 times higher (for 100 g animals). The

differences are probably due to the greatly increased surface area

for gas exchange provided by the gills of Clarias.

Singh and Hughes (1971) found that 58.4% of the oxygen uptake

is through the air-breathing organ when the fish were in air-

saturated water with access to air. However, in the present study,

fish that were breathing bimodally in the metabolic chambers did

not surface very often, and little oxygen (10-20% of total) was

taken in through the air-breathing organs. The event recorder data

indicate that the number of surfacings per hour is related to the

weight of the fish, water P02, time of day, external weather con-

ditions, and the amount of disturbance to the fish. As oxygen

consumption values were taken every hour in the metabolic studies,

it is probable that the low values for aerial breathing may be a

result of disturbances that inhibited surfacing. All experiments

were conducted during daylight hours, and, if they extended into

the evening, lighting remained on, a condition which produced low

air-breathing values. In one case (71 g individual), the lights

were turned off for a few hours in the early evening and the V02

values for aerial breathing doubled. It appears that the gills









provide adequate respiratory exchange when the animal is not active,

but increased oxygen uptake for activity is obtained through the

supplemental exchange surface of the air-breathing organ.

Small fish surface more frequently than larger individuals

(Fig. 10) apparently requiring proportionally more oxygen from air

than larger ones. This is unusual for bimodal breathers. McMahon

(1970) found that juvenile lungfish could survive forcible submergence

longer than adults and Ultsch (1972) showed that Siren lacertina, an

aquatic salamander, can function as a water breather at small sizes

only; animals larger than 800 g are obligate air breathers. Although

I did not have any large (300 g or larger) animals, which may be

obligate air breathers, the size range in this study (30-244 g)

indicates that small fish, when given the opportunity, breathe air

more often than large ones.

Saxena (1966) measured the gill surface area of three species

of Indian catfish; one water breather and two air breathers (in-

cluding Clarias batrachus). The gill surface area of Rita rita,

a water breather, increases curvilinearly with increasing body

weight as is the usual relationship, while C. batrachus (Fig. 14)

showed no such correlation over a size range of 36-67 g. Perhaps

the proportionally higher metabolic rates coupled with gill surface

areas that are not proportional to weight in small fish account

for the fact that small fish breathe air more often than large

fish.

In oxygen deficient water, where the animals are breathing

air only, the rates of metabolism areminimal, but can be raised





































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by increasing the frequency of air breathing. In one case, the

surfacing frequency was measured by event recorder for a fish placed

in nitrogen-saturated water (Fig. 11). The surfacing frequency dur-

ing the day was approximately equal to the values for surfacing in

air-saturated water, but the night values were greatly increased.

When the fish are maintained in water with a low oxygen tension,

activity is curtailed (during daylight hours) and they swim slowly

to the surface for a breath, then fall back to the bottom, often

on their sides. Opercular activity is normally diminished or

completely stopped.

Oxygen uptake during air exposure is comparable to values for

fish in water with access to air. Many air-breathing fishes show

reduced VO2 in air, while others (such as the African and South

American lungfishes and electric eel, which are obligate air

breathers, or forms such as Periophthalmus whidiis active when it

leaves water) show no reduction in O02. Clarias is an example of

the latter case as they are usually active upon emersion. Clarias

is capable of raising the active aerial VO2 to at least three times

the minimal level. The high levels of oxygen uptake in air are

not entirely due to the utilization of the air-breathing organ

because Clarias breathing air in oxygen-deficient water show a 27-46%

decrease in VO2 below values for air-saturated water with access to

air. The skin of these fish may assume up to 50% of the oxygen

uptake in air. Singh and Hughes (1971) found that 16% of the V02

in air-saturated water was cutaneous. The amount of blood circulating

to the skin evidently increases upon emersion, as the skin of albino









individuals becomes pinker and cutaneous O02 increases. Berg and

Steen (1965) found that cutaneous oxygen uptake of Anguilla vulgaris

is about 2.7 times greater in air than in water. The gills of

Clarias also probably play an important role in aerial respiration,

as the widely spaced thickened lamellae would not collapse in air

and probably function similarly to those of species that employ

gill breathing in air, such as Symbranchus, Hypopomus and Periopthalmus.

The high aerial VO2 rates of Clarias batrachus seem somewhat

inconsistent with some of the other aspects of the physiology of

this fish when it is exposed to air. Singh and Hughes (1971)

found that the respiratory quotient of C. batrachus in air was

lower (0.52) than other air-breathing forms they examined (Anabas

and Saccobranchus) even though Anabas is scaled. Howell (1970)

examined the buffering curves of blood of air-breathing fish

(including C. batrachus). These data, plus values for the carp

and bullfrog (from Rahn, 1967), are shown in Fig. 15; all values

are calculated for 250C. Clarias and Neoceratodus, both primarily

water-breathers, have low HCO3 concentrations and are not capable

of buffering blood carbon dioxide much better than the carp. Howell

(1970) states that preliminary laboratory studies indicate that

Clarias goes into severe respiratory acidosis when forced to reside

in air for a few hours. Yet, Clarias is the only one of these air-

breathing fishes that has been reported to leave the water volun-

tarily.

The skin of Clarias is very sensitive to desiccation and unless

it is kept moist, a few hours of air exposure produces necrosis of




























Figure 15.


Buffering capacity of blood of selected
vertebrates at 25"C (from Howell, 1970,
and Rahn, 1967).

A Electrophorus

B Rana

C Protopterus

D Lepidosiren

E Cyprinus

F Clarias

G Neoceratodus














































pH









the skin. Both the sensitivity of the skin and a propensity for

respiratory acidosis in air is probably why Clarias leaves the water

and migrates mainly (if not exclusively) when it is raining. Clarias

also follows thin sheets of runoff that flow over roads and down

into the ditches. These conditions maintain a film of water over

the animal's body surface, which prevents skin damage and presumably

would permit carbon dioxide elimination through the naked skin.

In water, Clarias shows coupling of respiratory and circulatory

adaptations similar to other air-breathing forms such as Electrophorus,

Periophthalmus, and Symbranchus, as maximum cardiac output follows

an air breath. Inflation of the gill chamber in Clarias corresponds

to an increase in heart rate from about 29 to 39 beats/minute.

Electrocardiograms of Clarias show physiological features of a

terrestrially active fish such as Periophthalmus. After the

experimental tank was drained, heart rate fell to approximately

half the initial value until the fish moved and took a breath, then

tachycardia ensued with a 250% increase in heart rate. This is

very similar to the results obtained by Johansen (1966) for

Symbranchus. Gordon et al. (1970) found that the Chilean cling-

fish, Sicyases, a fish which is inactive upon emersion, showed an

immediate drop in heart rate upon removal from water. Heart rate

in this fish remained at low levels for the duration of their

emersion.

Clearly, Clarias batrachus has many characteristics that have

contributed to its success in South Florida. It is able to survive

using aquatic respiration only in aerated water, until the PO2 drops










to very low levels. Air breathing is more common at night, which

tends to decrease susceptibility to predation by diurnally active

predators (herons). When oxygen tensions are low, fish survive by

breathing air and reducing the amount of activity. They can

survive drying (and probably cold in South Florida) by burrowing

in mud or making short excursions to deeper water. During the

rainy season they make extended excursions by water or land to

other waters, which contributes to the dispersal of these species


prior to breeding.














SUMMARY

1. Clarias batrachus is a facultative air breather.

2. The V02 of Clarias varies as a function of weight and access

to air. The metabolic rates of Clarias in air and water with

* access to air are essentially the same, but the 02 of forcibly

submerged fish is lower.

3. The metabolism characteristic of forcibly submerged Clarias

is primarily aerobic, its reduced rate is probably the result

of reduced activity.

4. When the fish are in water with access to air, aquatic breath-

ing accounts for 80-90% of the total oxygen uptake. Small

fish appear to increase aerial respiration at higher oxygen

tensions than do large fish.

5. Clarias are oxygen regulators at oxygen tensions above 40 mmHg.

6. The critical oxygen tension (Pc) of Clarias appears to

increase with increasing body weight, as it does in salamanders.

Because of the increased surface area for gas exchange provided

by the gills of Clarias, the P values are about three times

lower than those of bimodally-breathing aquatic salamanders

(Siren).

7. The number of air breaths per unit time is a function of time

of day (doubles at night), and is inversely proportional to the

weight of fish. This may be due to the lack of a proportional









relationship between gill surface area and weight that is shown

in small Clarias (35-68 gms), and also the proportionally

higher metabolic demands of small fish.

8. Oxygen uptake during air exposure is not reduced, as Clarias

is active upon emersion. Cutaneous oxygen uptake accounts for

up to 50% of the total V02 in air-exposed fish. Cutaneous

vasodilation apparently occurs during emersion.

9. The high oxygen capacity of Clarias blood is probably an

adaptation to facilitate oxygen uptake because of the thickened

and somewhat reduced gill surface area that is necessary for

air breathing and terrestrial activity.

10. Electrocardiographic measurements of heart beat frequencies

demonstrate that Clarias is similar to other air breathers

that are well adapted to emersion.

11. It is suggested that the low HCO3 levels of the blood of

Clarias causing respiratory acidosis after brief periods of

air exposure and the sensitivity of the skin to desiccation

preclude terrestrial activity in Clarias except on rainy

nights when a cutaneous water film (essential for carbon

dioxide exchange) can be maintained.














BIBLIOGRAPHY


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Beckenbach, A. 1969. "Influence of body size on the respiratory
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Belkin, D. 1961. Anaerobic mechanisms in the diving loggerhead musk
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Berg, T. and J. Steen. 1965. Physiological mechanisms for aerial
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BIOGRAPHICAL SKETCH

Jill Ann Jordan was born in Columbia, South Carolina on February 20,

1944. She graduated from Niagara Falls High School in Niagara Falls,

New York and received a Bachelor of Science from Cornell University

in 1966. She received a Master of Science degree from Tulane University

in 1967, and enrolled in the University of Florida. Until the present

time, she has worked toward the Ph.D. in the Department of Zoology.










I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.




B. K. McNab, Chairman
Professor and Chairman of Zoology


I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.




F. G. Nordlie
Associate Professor of Zoology


I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.




Pierce Brodkorb
Professor of Zoology


I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.




H. L. P enoe
Profess r of Soils










This dissertation was submitted to the Department of Zoology in the
College of Arts and Sciences and to the Graduate Council, and was
accepted as partial fulfillment of the requirements for the degree of
Doctor of Philosophy.

June, 1973


Dean, Graduate School








































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