sea star Archaster typicus
Family Archasteridae updated
learn only 3 things about them ...
They are no longer common on all our shores.
Their method of mating is unique among sea stars
use water to pump up their bodies. Don't remove them from
water for too long.
its name, this elegant sea star is no longer common on all our shores.
In the North, Chek Jawa used to be the only place where it was seen
in large numbers. But following massive flooding in Johor in early
2007, there were mass deaths among these animals (more about the mass
deaths on the Chek Jawa project
blog). It is not no longer found on Changi or the East Coast.
But it remains the most commonly encountered sea star on Southern
shores. Generally in shallow sandy or silty areas near seagrasses
Features: Adults diameter with arms to about 12-15cm. Smaller juveniles are
rarely seen. Body somewhat rounded (not flat). Arms long and tapered
to a sharp tip, and edged with short flat, blunt spines. Most have
five arms, but those with three, four and six arms are sometimes also
seen.The underside is pale, with large tube feet tipped with suckers.
Colours and patterns on the upperside are highly variable in shades
of greyish blue, to brown and beige.
sometimes four or six arms Chek
Jawa, Dec 03
The white structure
is the madreporite.
due to massive floods. Chek Jawa, Jan 07
specimen Chek Jawa, Nov 06
Wiith seven arms.
Cryene Reef, Jul 10
With six arms. Kusu Island, Feb 07
Sometimes confused with the Eight-armed
sand star. The Eight-armed sand star
has, well, eight arms and large tube feet with pointed tips. While
most Common sea stars have five arms (although some may have six),
and all have large tube feet with sucker-shaped tips.
What does it eat? According to
Schoppe, it feeds on detritus, decaying plants and tiny animals. To
eat, a common sea star sticks out its greenish 'stomach' through its
mouth on the underside. The 'stomach' spreads out to mop up any edible
titbits on the sand surface.
can become flexible to turn
themselves over if accidentally flipped.
Chek Jawa, Mar 05
greenish stomach sticks out through the mouth to 'mop up' edible
bits on the ground.
Chek Jawa, Feb 02
Small male on
top of larger female. Kusu Island, Sep 10
Uncommon mating ritual: Sea stars
of the genus Archaster have a unique mating behaviour. The
male, which is usually smaller, seeks out a female during the breeding
season. He then moves on top of her, his arms alternating with hers.
Their reproductive organs do not actually meet. Sperm is merely released
by the male when the female releases her eggs, for external fertilisation.
This behaviour is believed to increase the chances of external fertilisation.
So much so that the males do not need to be large and are thus usually
smaller than the females, a rare gender difference among echinoderms.
The only other sea star known to practice this kind of mating behaviour
is the closely related Archaster angulatus.
Juveniles are found in prop roots of mangroves and gradually inhabits
sandy shores, seagrass areas and shoals as they age, where they are
burried slightly in the sand.
Living on a star: A tiny parastic
snail (Parvioris fulvescens) is said to be sometimes found
on the underside of the common sea star. Here's some of the parasitic Ulimid
snails (Family Eulimidae) seen on our Common sea stars.
Is our sea star special? According
to the Singapore Red Data Book, the form found in Singapore is larger
than other examples elsewhere in the west Pacific and may be a distinct
variety of subspecies.
Status and threats: Although plentiful
in the past, common sea stars are now listed as 'Vulnerable' on the
Red List of 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.
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.
J.W. and Didier Vandenspiegel. 2003. A
Guide to Sea Stars and Other Echinoderms of Singapore.
Singapore Science Centre. 187pp.
et al. 1998. The
Asteroid fauna (Echinodermata) of Singapore with a distribution
table and illustrated identification to the species. The Raffles
Bulletin of Zoology 1998 46(2): 431-470.