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echinoderms text index | photo index
Phylum Echinodermata
Echinoderms
Phylum Echinodermata
updated Dec 08

if you learn only 3 things about them ...
They include sea stars, sea urchins, sea cucumbers and sand dollars.
Most have spines and a five-part symmetry.
They use water to pump up their bodies. Don't remove them from water for too long.

Where seen? Almost everyone is familiar with sea stars. Together with their relatives, these decided 'stars of the shores' are echinoderms. Some kind of echinoderm can usually be found on all our shores. The richest variety of echinoderms appear to be found on our Northern shores such as Changi and Chek jawa.

What are echinoderms? They belong to the Phylum Echinodermata with about 6,500 known species. Most are bottom dwelling.

The Phylum Echinodermata is made up of these groups...

Subphylum Crinozoa
  Class Crinoidea
  In various Orders (sea lilies)
Order Comatulida (feather stars)

Subphylum Asterozoa
  Class Stelleroida
  Subclass Ophiuroidea (brittle stars and basket stars)
Subclass Asteroidea (sea stars)

Subphylum Echinozoa
  Class Echinoidea
In various orders: sand dollars, sea urchins, heart urchins

Class Holothuroidea (sea cucumbers)

Splendid spines:
'Echinodermata' means 'spiny-skin' in Greek. Most echinoderms have a spiny skin. The spines are most obvious in sea urchins. Brittle stars and some sea stars also have prominent spines along their arms. Sand dollars and most sea stars have tiny spines. Sea cucumbers generally lack hard spines.

Give me five! The most well known echinoderm, the sea star, demonstrates the five-point (pentamerous) symmetry typical in the group. In contrast, a human has a 2-point (bilateral) symmetry; you can draw a line from head to toe and have a mirror image of most of our body parts on the left and right. An echinoderm, however, can be divided into five similar parts (or multiples of five) along a circle; much like cutting up a round birthday cake into five equal slices!

Although echinoderms can take on different shapes, they are all generally symmetrical along five axes. Here are some diagrams explaining this in greater detail.


Marvellous Morphing: Echinoderms have an internal skeleton made up of an arrangement of ossicles (plates made mostly of calcium carbonate), connected by a special kind of connective tissue called 'catch connective tissue' or 'mutable collagen' or 'mutable connective tissue'. This tissue is made of collagen and echinoderms can rapidly control the consistency of this tissue from rock hard to almost liquid within seconds. By changing the consistency of this tissue, echinoderms can move, feed, defend themselves and reproduce asexually. This property allows the sea urchin to move and lock its spines; a brittle star and sea star to bend or even purposely break off an arm by softening the tissues; a sea cucumber to flow into narrow places then harden so predators cannot pry it out.

Awesome ossicles: All echinoderms have ossicles, but these have different shapes and are arranged differently. Sea cucumbers have microscopic ossicles and most of their bodies are made up of connective tissue. This is why they appear softer than their cousins. The ossicles in sea stars and and especially brittle stars are larger. In brittle stars these articulate with to form flexible arms. In sea urchins and sand dollars, the ossicles are large and fused to one another, forming a rigid skeleton which is the spherical test in sea urchins, and flattened into a thin disk in sand dollars. Echinoderms have internal skeletons and not externals ones like arthropods. Although some, like sea urchins, appear to have an external skeleton, the skeleton of a living echinoderm is actually covered by a thin tissue.

Water of life: Another unique feature of echinoderms is their water vascular system, a network of internal canals supported mainly with seawater. By expanding or contracting chambers in the system, the water pressure in the canals can be directed and changed. Sea stars pump up and move their tube feet to move, brittle stars pump up and move their entire flexible arms. This water vascular system is connected to the outside through the madreporite, a porous plate usually on the upper side of the body.

Sea urchins and sand dollar move using their spines, rather than by using their hydraulic system. Sea cucumbers move their entire body in a more worm-like manner, using muscles.

Handy Feet: Most echinoderms have tube feet. These are extensions of, and are connected to, the water vascular system which pumps them up. Tube feet have muscles to retract them, but no muscles to extend them.

These tube feet have many uses. In sea stars and sea urchins, tube feet are used to move, gather food and breathe. Sea stars appear to have special glands in their tube feet that secrete a glue so the feet stick to things, and another substance to release the tube feet.

Some sea urchins 'carry' bits of debris using their tube feet. In brittle stars, tube feet are tiny and used mainly to gather food and breathe.

Brittle stars use their highly flexible arms to move, and not their tube feet. In sea cucumbers, tube feet around the mouth are modified into feeding tentacles to gather food.

In sand dollars, the minute tube feet on the underside are used to gather food, while tube feet emerging from the upperside are used to breathe with. Tube feet in general may also be used to excrete wastes, and to sense chemicals with. An echinoderm can have as many as 2,000 tube feet!

Heartless and brainless! An echinoderm doesn't have a heart. The internal fluids do not have a one-directional flow and simply ebb and flow within the animal. It has a simple nervous system but no brain. It does, however, have a complete digestive system with a mouth and an anus.

What do they eat? Most echinoderms gather tiny edible bits from the water or the ground. Some sea stars, however, hunt and eat other animals.

What eats them? Some fishes may eat sea urchins and many animals snack on brittle stars. But most other adult echinoderms don't seem to be considered tasty by many animals.

Living on a star: Many different kinds of animals may live with echinoderms. A kind of worm-like animal is often seen nestled around the mouth of the Black sea urchin. Tiny snails may live on sea stars. Some shrimps and small fishes have adaptations to live with feather stars, sea urchins and sea cucumbers. Echinoderms in turn may also live with other animals. Tiny brittlestars are often found on and even inside sponges.

Echinoderm babies:
Most echinoderms practice external fertilisation, releasing their eggs and sperm simultaneously into the water. Most 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. They eventually settle down and develop into miniatures of their parents. Some echinoderms can reproduce asexually by purposely dividing themselves or budding off a part of their body.

Human uses: Echinoderms are eaten and some are major trade items (e.g., sea cucumbers, sea urchins). Other echinoderms are harvested for the live aquarium trade (some sea cucumbers). Yet others are killed simply to make cheap ornaments (sea stars, sand dollars).

Status and threats: Many of our echinoderms are listed among the threatened animals of Singapore. They are threatened mainly by habitat loss due to reclamation or human activities along the coast that affect the water quality. Trampling by careless visitors and over-collection can also have an impact on local populations.

Sea star
Chek Jawa, Jun 05


Brittle star
Pulau Sekudu, Aug 04


Feather star
Beting Bronok, May 03


Sea urchin
Chek Jawa, Oct 01

Test of a dead sea urchin
Pulau Sekudu, Jun 06


Sand dollar
Chek Jawa, Sep 03


Heart urchin
Kusu Island, Jul 04


Long pointed tube feet of the Sand sea star.
Chek Jawa, Apr 05



Long tube feet of the White sea urchin
Changi, May 05



Sea cucumber
Changi, Dec 03

This brittlestar is often found
living in a feather star!
Raffles Lighthouse, Jul 06

Synaptid sea cucumbers are
often mistaken for worms!
Cyrene Reefs, May 08

Ball sea cucumbers usually lie buried with
only their feeding tentacles above ground.
Changi, Jul 04

Echinoderms on Singapore shores
text index and photo index of echinoderms on this site

Threatened echinoderms 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.
*from Lane, David J.W. and Didier Vandenspiegel. 2003. A Guide to Sea Stars and Other Echinderms of Singapore.

Asteroidea sea stars

  *Family Archasteridae
  *Archaster typicus (Common sea star) (VU: Vulnerable)

  Family Asterinidae
  Cryptasterina sp. (Cryptic sea star) (VU: Vulnerable)

  Family Echinasteridae
  Echinaster callosus (EN: Endangered)

  Family Goniasteridae
  Iconaster longimanus (Icon star) (VU: Vulnerable)

  Family Luidiidae
  Luidia maculata (Eight-armed sand star) (EN: Endangered)
Luidia penangensis
(VU: Vulnerable)

  Family Ophidiasteridae
  Fromia monilis (Peppermint sea star) (VU: Vulnerable)

  Family Oreasteridae
  Anthenea aspera (Cake sea star) (VU: Vulnerable)
Protoreaster nodosus (Knobbly sea star) (EN: Endangered)

  Family Pterasteridae
  Euretaster insignis (EN: Endangered)

Echinoidea

sand dollars

  Family Laganidae
  Laganum depressum (Laganum sand dollar) (Vulnerable)

  Family Astriclypeidae
  Echinodiscus truncatus (VU: Vulnerable)

sea urchins

  Family Cidaridae
  Prionocidaris baculosa (VU: Vulnerable)

  Family Diadematidae
  Chaetodiadema granulatum (EN: Endangered)

  Family Echinothuridae
  Asthenosoma varium (VU: Vulnerable)

Holothuroidea sea cucumbers

  Family Cucumariidae
  Pseudocolochirus violaceaus/axiologus (Sea apple sea cucumber) (VU: Vulnerable)

  Family Holothuriidae
  Holothuria leucospilota (Black long sea cucumber) (VU: Vulnerable)
Holothuria scabra
(Sandfish sea cucumber)

  Family Phyllophoridae (Ball sea cucumber)
  Phyllophorus parvipedes (Tennis-ball sea cucumber) (VU: Vulnerable)

  Family Stichopodidae
  Stichopus ocellatus (Eye-spotted sea cucumber) (VU: Vulnerable)

Crinoidea feather stars

  Family Himerometridae
  Himerometra robustipinna (DD: Data deficient)

  Family Mariametridae
  Stephanometra oxyacantha (VU: Vulnerable)

Ophiuroidea brittle stars

  Family Euryalidae (Basket star)
(formerly Family Gorgonocephalidae)
  Euryale aspersa (DD: EN? Data deficient, possibly Endangered)

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