you learn only 3 things about them ...
| They include sea stars, sea urchins, sea cucumbers and
Most have spines and a five-part symmetry.
use water to pump up their bodies. Don't remove them from
water for too long.
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
The Phylum Echinodermata is made up of these groups...
various Orders (sea lilies)
Order Comatulida (feather
|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.
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.
|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
|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
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 surface. 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. Large snails may eat sand dollars. But most
other adult echinoderms don't seem to be considered tasty by many
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, like sea cucumbers,
can reproduce asexually by purposely dividing themselves or budding
off a part of their body.
|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.
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
echinoderms of Singapore
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.
callosus (EN: Endangered)
longimanus (Icon star) (VU: Vulnerable)
Luidia maculata (Eight-armed
sand star) (EN: Endangered)
Luidia penangensis (VU: Vulnerable)
Fromia monilis (Peppermint sea star) (VU: Vulnerable)
insignis (EN: Endangered)
baculosa (VU: Vulnerable)
granulatum (EN: Endangered)
varium (VU: Vulnerable)
Holothuroidea sea cucumbers
Phyllophorus parvipedes (Tennis-ball
sea cucumber) (VU: Vulnerable)
Crinoidea feather stars
Himerometra robustipinna (DD: Data deficient)
Stephanometra oxyacantha (VU: Vulnerable)
Euryale aspersa (DD: EN? Data deficient, possibly Endangered)
- 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
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 Coleman's
World of Water, Australia. 64pp.