Sea
urchins
Class Echinoidea
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Chek Jawa, sea urchins are not commonly seen. But among the
sea urchins that you might come across on Chek Jawa's seagrass
lagoon is the Pencil sea urchin (Prionocidaris bispinosa).
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'Echinoidea'
means 'porcupine-like'. This class includes sand dollars
and heart sea urchins. There are about 900-1,000 species
of Echinoids. Most have a body
6-12cm in diameter.
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Sea urchins are echinoderms. They are thus symmetrical along five
axes, have tube feet and spines.
Perilously Poky: And boy do they
have spines! Sea urchins are usually covered with lots of long, sharp
spines that deter most predators. The spines also tend to break off
inside their victim's flesh and some sea urchins have venomous spines.
Some sea urchins also have tiny structures called pedicellariae which
look like jaws on stalks. The main function of these is to keep the
body of the sea urchin free of debris and parasites. They may also
be used to collect food and protect from predators. Some sea urchins
have larger venomous pedicellariae which painfully sting predators
and large creatures, and can paralyse small ones.
 Test
of Strength: Sea urchins have an internal skeleton (called
the test) formed out of large ossicles fused together in plates arranged
like a sliced orange in multiples of five. The test is a rigid, hollow
sphere. To grow larger, each ossicle is enlarged, and new ossicles
added near the anus.
 Splendid
spines: There are little knobs all over the outside of
the skeleton. The spines move on these little knobs, articulating
somewhat like the ball-and-socket joint of our knees. Sea urchins
usually have two kinds of spines; one larger and/or longer, and the
other smaller. Like other echinoderms, sea urchins have mutable connective
tissue as well as muscles which help move the spines. These moveable
spines not only protect the sea urchin, but are also used for walking.
Sea urchins can move in any direction because they are spherical.
The spines can also be locked in place to wedge themselves in a safe
hiding place.
Where do the spines of a dead sea urchin
go? Like us, sea urchins have a skin covering the spines
and the test. When a sea urchin dies, the skin decays rapidly and
all the spines fall off, leaving only the spherical test. The inside
of a sea urchin is mostly empty except during mating season when it
is full of sperm or eggs.
Tube Feet Too! Like other echinoderms,
sea urchins also have tube feet. If we go back to the image of the
test as a sliced orange, the tube feet emerge from holes along five
'slices'. Like other echinoderms, the tube feet are operated hydraulically
with the water vascular system that all echinoderms have. These tube
feet end in suckered discs that can stick to things and thus allow
the animal to move, climb up vertical surfaces, dig or collect food.
The tube feet are also used to sense chemicals, breathe, as well as
excrete wastes!
Urchin Food: Most sea urchins
graze on algae, detritus from hard surfaces or on immobile creatures
such as sponges or encrusting animals. They have a complex jaw structure
on their underside, facing down. The jaw is made of a circle of five
plates that meet in the middle to form a beak-like structure. The
entire structure can be extended outwards to chomp on their food.
It is called the Aristotle’s Lantern after the Greek philosopher Aristotle
who first described it. New 'teeth' grow to replace those that are
worn down. A sea urchin's anus is on the opposite side of its mouth,
on the upperside of its body.
Baby sea urchins: Sea urchins
have separate genders and are usually either male or female. They
practice external fertilisation, releasing eggs and sperm simultaneously
into the water. Each female sea urchin can release millions of eggs
at a time! undergo metamorphosis and their larvae look nothing like
their adults. The form that first hatches from the eggs are bilaterally
symmetrical and free-swimming, drifting with the plankton. At this
stage, they have several long 'arms' which are believed to funnel
food particles into the central mouth. They eventually settle down
and develop into a more sea urchin-like shape. Here is a fascinating
photo
of an echinoid larva on Image
Quest 3-D Marine Library
Role in the habitat: Grazing sea
urchins keep algae growth in check. An excessive algae 'bloom' can
deplete oxygen, smother life forms and upset the ecological balance.
Despite their spines, sea urchins are eaten by many creatures including
crabs, fish and birds.
Humans uses: The roe (egg mass)
of some sea urchins are relished as a Japanese delicacy and sea urchins
are commercially harvested for this reason in various parts of the
world. Sea urchins have been extensively studied to better understand
egg fertilisation and embryo development for other applications. This
is because their eggs are large and easy to study.
Status and threats: In Singapore,
the main threat is habitat loss due to reclamation or human activities
along the coast that pollute the water.
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Pencil sea urchin
Prionocidaris bispinosa
Close-up showing spines and five joints where the plates of the
test meet |
