learn only 3 things about them ...
Sand dollars are alive! They are animals. Don't step on
They are related to sea stars and have five-part symmetry.
sand dollars have tiny moving spines that tickle when
you hold them.
dollars are seen on many of our sandy shores and can be particularly
common on Chek Jawa, Tanah Merah and some Southern shores. Usually
half buried in the sand.
What are sand dollars? Sand dollars
are animals! Seeing one for the first time, it may be hard to believe
that these are living creatures. They are so flat and appear lifeless.
Sand dollars belong to Phylum Echinodermata
and Class Echinoidea which includes
sea urchins and heart urchins.
Features: 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.
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
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.
Why do some sand dollars have holes in them? The Keyhole
sand dollar got its common name from the intriguing slot-shaped
holes in the body (called lunules). Suggestions for the function of
these slots range from helping the animal to burrow, right itself,
find food or to prevent the waves from lifting it out of the sand.
The last is the most widely accepted explanation.
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
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
Status and threats: Some of our
sand dollars are listed among the threatened animals of Singapore.
The main threat is habitat loss due to reclamation or human activities
along the coast that pollute the water. Like other creatures of the
intertidal zone, they are affected by human activities such as reclamation
and pollution. Trampling by careless visitors and over-collection
can also have an impact on local populations.
Sometimes, sand dollars are tiny!
Tanah Merah, Apr 05
A sand dollar can move quite far!
Lazarus Island, Jun 09
A living sand dollar is covered with spines!
Berlayar Creek, Aug 10
A dead sand dollar has no spines.
Chek Jawa, Nov 02
The petalloid is made up of holes where
tube feet emerge. This is more obvious
in the skeleton of a dead sand dollar.
Changi, Jun 06
The central mouth is on the underside.
Changi, Jun 06
dollars recorded for Singapore
Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in
*additions from Lane, David J.W. and Didier Vandenspiegel. 2003. A
Guide to Sea Stars and Other Echinderms of Singapore.
red is that listed among the threatened animals of Singapore
from Davison, G.W. H. and P. K. L. Ng and Ho Hua Chew, 2008. The
Singapore Red Data Book: Threatened plants and animals of Singapore.
Family Clypeastreridae (Arachnoididae)
- Lane, David
J.W. and Didier Vandenspiegel. 2003. A
Guide to Sea Stars and Other Echinoderms of Singapore.
Singapore Science Centre. 187pp.
G.W. H. and P. K. L. Ng and Ho Hua Chew, 2008. The Singapore
Red Data Book: Threatened plants and animals of Singapore.
Nature Society (Singapore). 285 pp.
- Wee Y.C.
and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore.
National Council on the Environment. 163pp.
Ashely. 2002. Sea Urchins of Australia and the Indo-Pacific.
Capricornia Publications. 180pp.
- Edward E.
Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate
Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
Jan A., 2005. Biology
of the Invertebrates.
5th edition. McGraw-Hill Book Co., Singapore. 578 pp.
Gordon, John E. Miller, David L. Pawson and Porter M. Kier, 1995.
Stars, Sea Urchins, and Allies: Echinoderms of Florida and the
Smithsonian Institution Press. 390 pp.
Sabine, 2000. Echinoderms
of the Philippines: A guide to common shallow water sea stars,
brittle stars, sea urchins, sea cucumbers and feather stars.
Times Edition, Singapore. 144 pp.
Neville. undated. Sea
Stars of Australasia and their relatives. Neville Colemanís
World of Water, Australia. 64pp.