learn only 3 things about them ...
They may have more or fewer than 5 arms.
They take a long time to regenerate lost arms.
are hurt if they are removed from water for a long time.
Sea stars are encountered on most of our shores. Even the most "beat
up" shore will have some kind of sea star. But our Northern shores
appears to have the richest variety of sea stars. Some sea stars are
small and well hidden. Others are large and colourful.
What are sea stars? Although often
called starfish, these creatures are not fish at all! So it is more
correct to call them sea stars. Sea stars belong to the Phylum Echinodermata
and Subclass Asteroidea. There are about 1,800 known species of sea
stars, of which about 300 are found in shallow waters. Sea stars form
the second largest group of Echinoderms after the brittle stars (Subclass
Features: Almost everyone knows
what a sea star looks like! 'Asteroidea' means 'star-like'. Like other
echinoderms, sea stars are symmetrical along five axes, have a spiny
skin and tube feet.
An Armful: Sea stars have arms
that blend into one another before joining the central disk. Some
sea stars seem to be all arms with long narrow arms and a small central
disk. Others have arms that are so short they look like pentagons.
Most species of sea stars have five arms, although some may have more.
However, sometimes, you might come across a sea star with fewer than
five arms. Some kinds of sea stars are really flat, others may have
more cylindrical arms, and some are so round that they look like cushions.
Each arm is usually tipped with one or more sensory tube feet, and
an eye spot that detects light and dark but does not form an image.
Sometimes confused with brittle
stars. Unlike sea stars, brittle stars have very flexible and
long arms attached to a small central disk. Most brittle stars are
much smaller than sea stars, although some have very long arms.
Handicapped Stars: Sea stars are
famous for their ability to regenerate lost arms. But this takes time
and resources. Some species take up to a year to replace a lost limb.
In the meantime, the sea star is probably disadvantaged. If the central
disk is damaged, the sea star may die. Only a few species of sea stars
are known to regenerate from a piece of an arm. So you won't necessarily
get two sea stars when an arm of a sea star is separated. So please
don't purposely mutilate sea stars.
Mouth to the ground: The mouth
is on the underside facing the ground. Some sea stars have jaws made
up of five or more teeth arranged in a star around the mouth. Some
sea stars can extend their stomachs out of their mouths! Part of the
digestive system of a sea star extends into its arms.
Not all sea stars have an anus. Those that don't, spit out indigestible
bits through their mouth. In those that do, the anus is on the upper
surface of the central disk.
In the groove: Radiating from
the mouth and extending under each arm are grooves (ambulacral grooves).
These grooves usually contain 2-4 rows of tube feet. The margins of
the groove are guarded by moveable spines that can close over the
groove. Out of the water, a sea star will usually retract its tube
feet into the grooves so it looks rather lifeless.
Fancy footwork: Sea stars use
their tube feet to move around. Unlike brittle stars, sea stars move
mainly by undulating waves of their tube feet and not by bending their
arms. 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. In some sea stars, the tube feet ends in
suckered disks. These act like suction cups when pressure is applied
by the sea star. Burrowing sea stars may have suckerless tube feet
that end in points to better dig into the ground. Sea stars also use
their tube feet to manipulate food. Some sea stars also breathe through
their tube feet!
Skin and bones: Sea stars have
an internal (not extenal) skeleton. A sea star's body is made up of
tiny ossicles (plates made mostly of calcium carbonate), connected
by a special kind of connective tissue called 'catch connective tissue'.
This connective tissue can rapidly change from almost liquid to rock
hard and allows them to slowly bend and move their arms to climb,
right themselves and clasp prey. Sea stars can also purposely drop
off an arm when stressed or attacked, by rapidly changing the consistency
of this tissue. The entire sea star has a skin that covers all of
the body, including the spines.
Jaws all over the body! Some sea
stars also have tiny structures called pedicellariae that look like
a pair of jaws, or tiny clams. The main function of these is to keep
the body of the sea star free of parasites, encrusting organisms and
debris. These little jaws can snap and those on big sea stars can
even pinch inquisitive human fingers! Pedicellariae may also be used
to collect food.
Water of Life: Like other echinoderms,
sea stars have a water vascular system, a network of internal canals
supported and pumped mainly with seawater. They suck seawater into
their bodies through the madreporite: a sieve-like structure that
usually appears as a spot on the upperside near the centre. By expanding
or contracting chambers in the internal system, the water pressure
in canals within the body can be directed and changed. This is how
they move their tube feet. A study also found that the water within
a sea star may help
it keep cool when exposed at low tide. As they rely on seawater,
it is stressful for sea stars to be left out of water for too long.
Try not to remove sea stars from the water. If you have to do so,
please return them quickly to where you found them.
What do they eat? Some sea stars
placidly gather edible bits from the water or surface. But most sea
stars are scavengers or carnivores, 'sniffing' out their meal by the
chemicals released by the prey or dead animals. Among the more common
prey are snails, bivalves, crustaceans, worms and other echinoderms.
Some sea stars specialise in a certain prey. Some sea stars feed on
sponges, sea anemones and corals. Some carnivorous sea stars eat detritus
when there's nothing better to eat.
Some prey of sea stars have developed various ways to escape from
sea stars. Bivalves such as scallops
(Family Pectinidae) may leap, while others burrow away quickly, some
snails may somersault.
Stomach Turning Table Manners:
Some sea stars, especially those with long arms, can evert their stomachs.
This ability is particularly useful for carnivorous sea stars that
feed on bivalves. How does it do it? A carnivorous sea star uses its
tube feet to hold the bivalve againsts its central mouth. It then
pushes out its stomach through its mouth and inserts its stomach into
the bivalve's shell through imperfections in the fit of the two shells.
If there are no such imperfections, the sea star simply pulls the
shells apart to create a tiny gap! Once inside the shell, digestive
juices are poured on the hapless victim. Digested material is moved
by cilia (minute hairs) on tracks into the sea star. Thus the prey
is partially digested in its own shell! The Shape of Life page on
the PBS website has a cool
video clip of a sea star digesting a mussel in its shell!
The Crown-of-Thorns sea star (Acanthaster planci) pushes its
stomach out of its mouth to digest coral polyps in their skeletons.
Sea stars that eat detritus may push out their stomachs to mop up
whatever is on the surface.
However, sea stars with short arms usually don't push out their stomachs
and simply swallow their prey whole and digest them in their stomachs.
What eats them? While some fishes
may nibble on adult sea stars, it appears they are not considered
tasty by most other animals.
Dead or Alive? All the sea stars
that you see are probably alive. You are unlikely to come across a
skeleton of a sea star. Dead sea stars disintegrate quickly and do
not leave behind whole skeletons. A live sea star also has moving
tube feet. When removed from the water, however, sea stars will retract
their tube feet and may appear dead.
Don't pick up sea stars! Many
sea stars can purposely drop off an arm if it feels threatened. This
is how they might escape the jaws of a predator, or if a stone should
accidentally trap an arm. If you pick up a sea star by the arm, you
may trigger off the same reaction. Also, it is stressful for a sea
star to be out of water for a long time. So please admire the sea
stars where they are.
Should I put a sea star that is high and
dry on the sand back into the water? Intertidal sea stars
are used to being out of water during low tide. It is best to leave
sea stars were they are.
Don't make a sea star flip over
Not all sea stars can do this easily. Even for those than can, it
consumes energy and if the same sea star is made to do this several
times, it can exhaust and thus injure the animal.
Aren't sea stars bad for reefs? Don't they
eat up all the hard corals? The Crown-of-Thorns sea star
(Acanthaster planci) is notorious for decimating reefs. This
sea star eats the polyps of hard corals leaving behind dead white
skeleton. These sea stars are only a danger to reefs when there is
a population explosion of them. Such a situation is generally is believed
to be due to an imbalance in the natural system. For example, when
their predators are overharvested. When there are low numbers of this
sea star, they do not cause massive damage. This sea star has not
been encountered on our shores.
Living with a star: Tiny snails
may live on the upper surface of a sea star, or under their arms.
Sea star babies: Sea stars have
separate genders and are usually either male or female. Eggs and sperm
are stored in their arms. Most species practice external fertilisation,
releasing eggs and sperm simultaneously into the water. Some can produce
lots of eggs; a single female may produce millions! Sea stars undergo
metamorphosis and their larvae look nothing like the 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 tiny sea stars. Here is a fascinating photo
of a sea star larva on Image
Quest 3-D Marine Library.
Human uses: Sea stars are generally
not eaten, and in fact it is advised not to eat them as many are toxic.
There are stories of pets which have eaten sea stars and died. More
about this on The
Echinoblog. They are also not that popular for the live aquarium
trade as they tend to eat their tank-mates. However, in some places,
sea stars are harvested alive and dried to be sold as cheap ornaments.
This is cruel indeed! In some coastal areas, sea stars are harvested
and chopped up as fish meal or fertiliser. Some sea stars are considered
pests on mussel, oyster and scallop farms.
Status and threats: Many of our
sea stars are listed among the threatened animals of Singapore. They
have become uncommon in Singapore mainly because of habitat loss due
to reclamation or human activities along the coast that affect the
water quality. Trampling by careless visitors and overharvesting can
also have an impact on local populations.
The large Knobbly sea star
is an icon of Chek Jawa.
Chek Jawa, Jun 05
Juvenile Knobbly sea stars are common on
Cyrene Reefs but not elsewhere.
Cyrene Reefs, Apr 08
The Cushion star is more pentangonal
than star shaped.
Terumbu Ular, Apr 06
The Eight-armed sand star
has more than five arms.
Chek Jawa, Jul 07
The shorter arm of this Sand sea star
Changi, Jul 03
Long pointed tube feet of the Sand sea star
helps it move quickly over the sand.
Chek Jawa, Apr 05
Underside of a Common sea star.
Greenish stomach outside the central mouth, tube feet emerging from
groove beneath the arms.
Chek Jawa, Jan 03
Huge bivalved pedicellaria
(pincer-like structures) on the
underside of the Cake sea star.
Chek Jawa, Jun 04
Tiny white snails are sometimes seen on the upperside of a Sand
Changi, Jun 05
A sea star disintegrating, possibly
due to flooding and a drop in salinity.
Chek Jawa, Jan 07
The Crown sea star can come in dull
or bright colours (below).
Chek Jawa, Jan 03
Chek Jawa, May 05
Asteroidea recorded for Singapore
from Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity
*from Lane, David J.W. and Didier Vandenspiegel. 2003. A Guide
to Sea Stars and Other Echinderms of Singapore.
in red are those listed among the threatened
animals of Singapore from Ng, P. K. L. & Y. C. Wee, 1994. The
Singapore Red Data Book: Threatened Plants and Animals of Singapore
callosus (EN: Endangered)
(Icon sea star) (VU:
*Stellaster equestris (Galloping
sp. (Luidia sand star)
(Eight-armed sand star) (VU: Vulnerable)
Luidia penangensis (VU: Vulnerable)
Fromia monilis (Peppermint sea
star) (VU: Vulnerable)
insignis (EN: Endangered)
- Lane, David
J.W. and Didier Vandenspiegel. 2003. A
Guide to Sea Stars and Other Echinoderms of Singapore.
Singapore Science Centre. 187pp.
- Ng, P. K.
L. & Y. C. Wee, 1994. The
Singapore Red Data Book: Threatened Plants and Animals of Singapore.
The Nature Society (Singapore), Singapore. 343 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 Hawai’I
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.