| On
Chek Jawa, sand dollars are common, especially on the northern
arm of the sand bar. |
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'Echinoidea'
means 'porcupine-like' in Greek. Besides sand dollars,
this class includes sea urchins and heart sea urchins.
There are about 900-1,000 species of Echinoids.
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They got their
name because they resemble a one-dollar coin. Like other echinoderms,
sand dollars are symmetrical along five axes, and have tube feet and
spines. They can be considered flattened sea urchins. Their flat disk-like
shape is an adaptation for life on the sea bottom where they gather
detritus.
Coat of spines: Instead of the
ferocious, long spines of their spherical sea urchin cousins, sand
dollars have tiny, soft spines. These spines are moveable and used
like tiny spades to dig into the sand or to move around. The dense
layer of spines also keeps sediments off so there is a flow of oxygenated
water across the body. Like the sea urchins, sand dollars also have
tiny structures called pedicellariae which look like jaws on stalks.
The main function of these is to keep the body of the sand dollar
free of debris and parasites. They may also be used to collect tiny
food particles.
Flat skeleton: Sand dollars have
an internal skeleton (called the test) formed out of large ossicles
(pieces made of calcium carbonate) fused together into plates in multiples
of five. The test is a rigid, hollow, flattened disk. To grow larger,
each ossicle is enlarged, and new ossicles added near the anus. In
some sand dollars, there is internal buttressing to support the test.
Nevertheless, sand dollars are still quite fragile so please handle
live ones with care.
Dead or alive? Sand dollars may
appear dead, but they are very much alive. A living sand dollar is
covered with fine spines and appears velvety. A dead one is smooth,
without any spines, and the details of skeleton can be seen more clearly.
The skeleton is fragile and will shatter at the slightest pressure.
Breathing petals: The petal design
on the upperside of a sand dollar is called a petaloid. The petaloid
is a series of tiny holes in the skeleton. Tube feet emerge through
these holes and the sand dollar breathes through these feet! These
breathing tube feet are short and flattened.
A sand dollar's mouth is on its underside, facing the ground. Its
anus is on the its underside as well, usually, this is located off-center.
Some, but not all, sand dollars have jaws made of a circle of five
plates that meet in the middle. Unlike those of the sea urchin, however,
the sand dollar's jaws cannot be extended outwards.
Picky eaters: Most sand dollars
are deposit feeders and process sand to feed on detritus in the sediments.
They don't just process any sand. The dense layer of tiny spines keep
out larger particles and only let in fine ones. Tiny tube feet and
cilia (minute beating hairs) move these fine particles to the food
grooves and along these grooves to the mouth in the center.
Damaged dollars: Like other echinoderms,
sand dollars can repair minor damage. If you come across an 'uneven'
sand dollar with a part of its body obviously chewed off, look closely
and you might see spines growing on the chewed edge. But if a large
part of the sand dollar is broken, it will probably die.
How do upside down sand dollars right themselves?
They dig one side into ground and stick the other end out. Eventually,
the waves and currents flip them over. This is laborious and they
usually need to be in water to achieve this. So please put sand dollars
back the right way around.
Dollar babies: Sand dollars have
separate genders and are usually either male or female. They practice
external fertilisation, releasing eggs and sperm simultaneously into
the water. Sand dollars 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 sand dollar-like shape. One species of
sand dollars (Dendraster excentricus of the US) is known to
settle in response to a substance released by adults. This might explain
why so many sand dollars of the same species may be found in one place.
Here is a fascinating photo
of a sand dollar larva on Image
Quest 3-D Marine Library.
Human uses: Sand dollar eggs have
been extensively studied to better understand cell division and thus
some diseases such as cancer, which is associated with uncontrolled
cell growth.
Status and threats: In Singapore,
the main threat is habitat loss due to reclamation or human activities
along the coast that pollute the water. Chek Jawa is one of the few
places left where sand dollars are still commonly seen. |
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More sand dollars
in the sand
Cake sand dollar
They can be small
Sand traveller
Sand dollars can
move quite a distance
across the sand
Keyhole sand dollar
The living sand dollar
above is quite different
from the skeleton
of a dead one below
Sand dollar mouth
Healing dollar
Fine spines
were growing on
the broken edge
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