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Phylum Echinodermata > Class Echinoidea | sand dollar | sea urchin | heart urchin
Echinoids
Class Echinoidea
updated Jan 14
if you learn only 3 things about them ...
Echinoids are related to sea stars and have five-part symmetry.
They include sand dollars, sea urchins and heart urchins.
Some burrow beneath the sand and may be damaged by trampling.

Where 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!

Baby 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 shape.


Large group of living sea urchins.
Changi, Jul 04


Sand dollars hidden in the sand.
Chek Jawa, Dec 01



Keyhole sand dollar
Echinodiscus truncatus

Cake sand dollar
Arachnoides placenta

Thorny sea urchin
Prionocidaris sp.

Black sea urchin
Temnopleuris sp.

Passion urchin
Salmacis virgulata

White urchin
Salmacis sphaeroides
Status 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

Class Echinoidea recorded for Singapore
For lists of species see individual pages for sand dollars | sea urchins | heart urchins

Links
  • Class Echinoidea Tan, Leo W. H. & Ng, Peter K. L., 1988. A Guide to Seashore Life. The Singapore Science Centre, Singapore. 160 pp.
  • The 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.
References
  • Lane, David J.W. and Didier Vandenspiegel. 2003. A Guide to Sea Stars and Other Echinoderms of Singapore. Singapore Science Centre. 187pp.
  • 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. 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.
  • Coleman, Neville. 2007. Sea stars: Echinoderms of Asia/Indo-Pacific. Neville Coleman’s Underwater Geographic Pty Ltd, Australia.136pp.
  • Miskelly, Ashely. 2002. Sea Urchins of Australia and the Indo-Pacific. Capricornia Publications. 180pp.
  • Gosliner, 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
  • Pechenik, Jan A., 2005. Biology of the Invertebrates. 5th edition. McGraw-Hill Book Co., Singapore. 578 pp.
  • Hendler, Gordon, John E. Miller, David L. Pawson and Porter M. Kier, 1995. Sea Stars, Sea Urchins, and Allies: Echinoderms of Florida and the Caribbean. Smithsonian Institution Press. 390 pp.
  • Schoppe, 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.
  • Coleman, Neville. undated. Sea Stars of Australasia and their relatives. Neville Coleman's World of Water, Australia. 64pp.

 

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