| |
Common
sea star
Archaster typicus
Family Archasteridae
Chek Jawa is one of the few places in Singapore where these sea stars
are still common. Often, large numbers are buried just beneath the
sand, leaving star-shaped patterns on the surface.
Sea
star parts: The arms of the Common sea star are long and
tapered, and edged with short spines. Most have five arms, but you
might come across some with only three or four arms. Some may even
have six arms. They have variable patterns and colours in shades of
brown and beige. Their large tube feet end in suckers. The Common
sea star has a madreporite (sieve plate) that appears as a white spot
on the upper surface of the central disk. Seawater is sucked into
the body through the madreporite. Its anus also lies on the upper
surface of the central disk.
Flexible arms: Although the arms
of the Common sea star appears stiff, these can become quite flexible.
Like other echinoderms, instead of a hard skeleton, the Common sea
star's body is mostly made up of a tissue that can rapidly change
from almost liquid to rock hard. This tissue is called 'catch connective
tissue".
Common food: This sea star feeds
on detritus. It sticks out its greenish stomach through its central
mouth and spreads this out on the sand surface to 'mop up' an edible
titbits. It also eats decaying plants and tiny animals that gets 'mopped
up'.
Common babies: Common sea stars
have a unique mating behaviour. The male, which is usually smaller,
lies on top of the female, his arms alternating with hers. Their reproductive
organs do not actually meet and they merely release sperm and eggs
simultaneously. This behaviour is believed to increase the chances
of external fertilisation.
Role in the habitat: In some places
a tiny parastic snail (Parvioris fulvescens) is found on the
underside of the Common sea star.
Human uses: In the past in Singapore,
the Common sea star was often collected as a teaching aid in zoology
and biology. This practice can threaten the population if it results
in overcollection.
Status and threats: Although plentiful
in the past, Common sea stars are now listed among the threatened
animals in Singapore due to habitat loss and overcollection. Their
mating behaviour suggests that a certain population density is required
for them to be able to reproduce successfully. |
|
Burrowing into
the sand
A three-armed
sea star
A four-armed
sea star
Poking out the stomach through the mouth
Mating sea stars! |
|
quick
facts
Diameter with arms to about 10cm, common on the sand bar
and sandy parts of the seagrass lagoon.
Classification:
Class Asteroidea
Phylum Echinodermata |
