learn only 3 things about them ...
Echinoids are related to sea stars and have five-part
They include sand dollars, sea urchins and heart urchins.
burrow beneath the sand and may be damaged by trampling.
seen? Echinoids are sometimes seen on some of our shores.
Sand dollars may be found in sand bars at the low water mark, or at
the bottom of shallow sandy lagoons. Others such as sea urchins may
be found among seagrasses especially during a seaweed bloom, or among
coral rubble and living reefs. Heart urchins are more rarely encountered.
What are echinoids? Echinoids
belong to Phylum Echinodermata.
The Class Echinoidae include sand dollars, sea urchins and heart urchins.
There are 900-1,000 known echinoid species.
Features: 'Echinus' means 'hedgehog'
or 'prickly' and indeed, many echinoids look like prickly hedgehogs.
While sea urchins have rounded, globular bodies, heart urchins have
more oval-shaped bodies, and sand dollar are flat and round like coins.
Like other echinoderms, they are symmetrical along five axes, have
tube feet and spines. The spines of echinoids vary from long, scary
ones on sea urchins, longish soft ones on heart urchins, and tiny
ones on sand dollars. Like other echinoderms, they have mutable connective
tissue as well as muscles which help move the spines. These moveable
spines not only protect the animal, but may also used for walking
and even to collect food from the water. The spines can also be locked
in place to wedge themselves in a safe hiding place.
Test of Strength: Echinoids 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 rigid and hollow. Sand dollars may reinforce
the test with internal struts. To grow larger, each ossicle is enlarged,
and new ossicles added near the anus.
The test has a 5-petalled design on the upperside called a petaloid.
The petaloid is a series of tiny holes in the skeleton. Tube feet
emerge through these holes and the animal breathes through these feet!
Where do the spines of a dead echinoid go?
Like us, echinoids have a skin covering the spines and the test. When
an echinoid dies, the skin decays rapidly and all the spines fall
off, leaving only the test.
Tube Feet Too! Like other echinoderms,
echinoids also have tube feet. Like other echinoderms, the tube feet
are operated hydraulically with the water vascular system that all
echinoderms have. Some have tube feet that 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!
echinoids: Echinoids have separate genders and are usually
either male or female. They practice external fertilisation, releasing
eggs and sperm simultaneously into the water. 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
group of living sea urchins.
Changi, Jul 04
Sand dollars hidden in the sand.
Chek Jawa, Dec 01
and threats: Some of our echinoids 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.
Skeleton of a
dead sea urchin
Changi, May 02
Echinoids have spines as well as tube feet.
Changi, Jul 04
Tan, Leo W. H. & Ng, Peter K. L., 1988. A
Guide to Seashore Life. The Singapore Science Centre,
Singapore. 160 pp.
Echinoid Directory by Dr. Andrew B. Smith on the London Natural
History Museum website: everything you could possibly want to
know about sand dollars and sea urchins with lots of large close-up
images and explanatory diagrams.
- 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.
Neville. 2007. Sea
stars: Echinoderms of Asia/Indo-Pacific. Neville Coleman’s
Underwater Geographic Pty Ltd, Australia.136pp.
Ashely. 2002. Sea Urchins of Australia and the Indo-Pacific.
Capricornia Publications. 180pp.
Terrence M., David W. Behrens and Gary C. Williams. 1996. Coral
Reef Animals of the Indo-Pacific: Animal life from Africa to Hawaii
exclusive of the vertebrates Sea Challengers. 314pp.
- Allen, Gerald
R and Roger Steene. 2002. Indo-Pacific
Coral Reef Field Guide.
Tropical Reef Research. 378pp.
- Edward E.
Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate
Zoology 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. Sea
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 Coleman's
World of Water, Australia. 64pp.