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Department of Planning and Natural Resources
Division of Fish and Wildlife
U.S.V.I. Animal Fact Sheet #13
Long-Spined Sea Urchin
To the casual observer, the long-spined sea urchin,
Diadema antillarum, appears to be little more than a collection
of sharp black spines protruding from the seafloor. And the
unwary swimmer will not soon forget a close encounter the
spines can easily penetrate the skin and break off, leaving a
painful reminder. Despite this sea urchin's treacherous spines,
it is a remarkable marine creature that plays a vital ecological
role on the reefs of the U.S. Virgin Islands.
All those spines are attached to a hard shell known as a
"test." The urchin's internal organs are located inside. The
test of an adult is round, slightly flattened from top to bottom,
and about 4 inches (10 cm) in diameter. True to its name, the
spines can be very long over 12 inches (30 cm) and help
defend the urchin against predators. The spines are movable,
and urchins will wave its spines in the direction of anything
that disturbs it. Among all those spines, and harder to see, are
thin tube feet tipped with suckers that grasp the bottom.
Urchins use their delicate tube feet and spines to move about,
to gather food, and to maintain a firm grip. This enables the
urchins to avoid being swept away by currents and surge in
shallow wave-exposed waters.
On the underside of the test lies the urchins' mouth. The
jaws of the mouth are made from 5 teeth held in a muscular
sling. Together these jaws form a five-pointed beak called
Aristotle's lantern that is very effective at scraping algae from
rocks and other hard surfaces.
There are 5 different sea urchin species that commonly
occur in coastal waters of the U. S. Virgin Islands. The long-
spined sea urchin is easily recognized by its long slender
spines and by its coloration. Adult long-spined sea urchins are
usually completely black, although some adults may have a
few gray or white spines. The young have white bands on
their black spines.
In the U.S.V.I., locals refer to all urchin species as sea
eggs. The long-spined sea urchin is also known as the black
urchin and erizo negro (Spanish).
Sea urchins are marine animals that belong to the phylum
Echinodermata (meaning "spiny skin") a group that includes
sea stars, sea feathers, brittle stars, sea cucumbers and sand
dollars. All sea urchins are in the class Echinoidea. Long-
spined sea urchins belong to the genus Diadema and the
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Distribution & Habitat
Long-spined sea urchins occur throughout most of the
tropical Atlantic Ocean. In the Western Atlantic, they range
from Florida and Bermuda, southward throughout the
Caribbean, to Brazil. In the eastern Atlantic, they occur in the
Azores, Cape Verde Islands, Madeira Islands, and also in the
Gulf of Guinea (Coastal Africa). They are not found in the
Long-spined urchins occur in almost all nearshore marine
habitats, including rocky areas, coral reefs, mangroves, sea
grass beds, and sand flats. They are most common in shallow
waters (usually less than about 30 feet deep) but may also
occur in deeper habitats.
The long-spined sea urchin is perhaps the single most
important herbivore (plant-eater) on Caribbean reefs. These
urchins move about grazing rather unselectively upon a
variety of different algae. Their feeding has two important
consequences. First, urchin grazing reduces the total amount
of algae on a reef, similar to a lawnmower keeping the grass
short. This enables corals (which compete with algae for
space and sunlight) to grow better. Second, when urchins
scrape algae from rocks, they create vacant spaces that can
then be colonized by the larvae of other marine animals
(corals, sponges, gorgonians). This helps to keep the diversity
of reef animals high. In the absence of urchin grazing, coral
reefs may become overgrown with algae, and the diversity of
reef animals may be reduced.
Scientists learned about the important ecological role of
the long-spined urchins following a catastrophic die-off.
During 1983-84, a disease outbreak struck the wider
Caribbean and killed over 93% of the long-spined sea urchins.
During the following years, corals decreased and reefs were
covered with unprecedented levels of algal growth. Now after
almost 20 years, the long-spined urchin appears to be making
a recovery on reefs in the U. S. Virgin Islands and elsewhere
in the Caribbean. Their comeback bodes well for our reefs.
By their nature, long-spined urchins are gregarious that
is, they occur in dense clusters. It is not uncommon to find
groups of more than 100 urchins crowded into a space of
about 5 square meters (roughly 16 square feet).
During daylight hours, long-spined urchins tend to remain
in the protection of reef crevices or around coral heads. At
night, they move out to forage over relatively small areas.
Their nocturnal activity is thought to result from the threat of
predation during daytime.
Several fish species eat long-spined sea urchins. For
example the queen triggerfish (Balistes vetula, also known as
the oldwife) can "blast" the urchins off the bottom. By
overturning the urchin, the fish exposes its vulnerable
underside. Porgies, grunts, wrasses, and one type of snail (the
king helmet snail, Cassis tuberosa) also eat long-spined
Reproduction & Growth
Scientists can determine size and age of sea urchins by
measuring the diameter of the test. Long-spined sea urchins
reach a maximum size of about 4 inches (10 cm) test diameter.
The urchins reach this size after about 4 or 5 years, and this is
thought to be their maximum life expectancy.
Urchins begin reproducing during their 2nd year of life,
when they reach a size of 1-2 inches (3-6 cm) test diameter.
Urchin mating involves something known as mass spawning.
During the late winter through early summer, all the urchins in
an area will respond to some unknown trigger, simultaneously
releasing their gametes (eggs or sperm) into the surrounding
seawater. On occasion, the fortunate diver may observe
clouds of whitish sperm (released by males) and yellow eggs
(released by females) oozing from the tops of the urchins. The
eggs are fertilized by sperm as they drift above the adults.
Fertilized eggs develop rapidly into small swimming larvae
that drift freely in the ocean for over a month.
Eventually, currents bring some of the sea urchin larvae
back to the reefs. At this point, the young urchins are still
very small (only about 1/8 inch long). They settle to the hard
bottom, abandoning their ocean-going lifestyle, to take refuge
in small crevices. There, they adopt the lifestyle of adult
urchins, grazing on plants such as algae and seagrasses.
Urchins don't attack people. But people can be injured if
they accidentally come into contact with the spines of an
urchin. Avoidance is the best precaution, especially in shallow
waters with rocky bottoms. Look where you walk. Avoid
walking or wading in shallows where sea urchins are plentiful.
Wearing sneakers, slippers, or aqua socks may offer some
protection, but beware urchin spines are capable of
penetrating the soles of shoes! When swimming, wear a mask
or goggles you will be able to see the urchins and avoid
brushing against them.
What to do about an urchin injury
If you are "spined", the tips may break off under the skin.
Remove the exposed spines with tweezers, but do not dig
excessively into the skin to get at spine fragments. Instead,
treat for infection, which is a bigger concern. Your tissues will
naturally dissolve the spines over several weeks. Applying a
weak acid, like vinegar, lemon juice, or ammonia may help. A
purple color (pigment from the urchin) may linger under the
skin but it is harmless. The spines carry a weak toxin that
does not affect most people, but allergic reactions are possible.
Seek medical attention if the injury is extensive or if signs of
For more information on this and other animals in the Virgin
Islands please visit our web site at:
WRITTEN BY WES TOLLER IN 2003.
THIS PUBLICATION WAS PRODUCED WITH
FUNDS FROM THE WILDLIFE
CONSERVATION AND RESTORATION
FOR MORE INFORMATION ON
OUR NATIVE ANIMALS CONTACT
DIVISION OF FISH AND WILDLIFE
6291 ESTATE NAZARETH, 101,
ST. THOMAS, VI 00802
PHONE 340-775-6762 FAX 340-775-3972
45 MARS HILL, ST. CROIX, VI 00840
PHONE 340-772-1955 FAX 340-772-3227