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Phylum Echinodermata > Class Echinodea > Order Clypeasteroida
Cake sand dollar
Arachnoides placenta
Family Clypeasteridae
updated Mar 2020
Where seen? This rather featureless disk-shaped animal is commonly seen on some of our Northern shores, on sand bars and sandy areas near seagrasses. Often found in groups of large numbers of individuals, half buried in the sand. Sometimes also seen on the Southern shores.

Features: Body diameter 6-8cm. Seen in various sizes, some may be tiny (about the size of a 10cent coin). It has no slots in the body. While the star-shaped petalloid is not very obvious in living specimens, they are clearly seen in the test of a dead sand dollar. The colours of a living sand dollar may range from deep reddish-purple, to brownish-purple or beige.

Changi, May 08


They are often found in huge numbers.
Chek Jawa, Jan 09


Mouth on the underside.
Dead or alive? Sand dollars may appear dead, but they are very much alive. A living sand dollar is covered with fine spines and appears velvety. The skeleton (test) of a dead one is smooth, without any spines, and the details of skeleton can be seen more clearly. The skeleton is fragile and will shatter at the slightest pressure.

Upperside of test

Underside of test

Living sand dollar moving under wet sand.

Changi, Jun 06

Living sand dollars, not moving under sand.
Lazarus Island, Jun 09
What eats sand dollars? Some snails such as the Grey bonnet are believed to feed on sand dollars. A Knobbly sea star was seen with its stomach stuck to a sand dollar. A Haddon's carpet anemone was also seen in the process of engulfing one. Kok Sheng also shared a video clip (below) of hermit crabs arguing over a sand dollar. Tiny parasitic snails are sometimes seen on them too.

Is this Grey bonnet snail eating a sand dollar?
Cyrene Reef, May 11
Photo shared by Marcus Ng on flickr.

Being eaten by a Knobbly sea star?
Cyrene Reef, May 11
Photo shared by Marcus Ng on flickr.

Tiny parasitic snail on a Cake sand dollar.
Cyrene, Aug 11

Cake sand dollar being swallowed
by a Haddon's carpet anemone.
Chek Jawa Feb 04

Pecked to death? On Chek Jawa, many sand dollars are observed flipped over with their undersides broken. From the prints around the sand dollars, it seems they were flipped and pecked by birds. Strangely, some sand dollars are flipped but unpecked. Perhaps only egg-bearing females are pecked?

Bird footprints surround the sand dollar.
Chek Jawa, Mar 10

It seems birds have flipped over some
and pecked out the underside.

Some are flipped but not harmed.
Only egg-bearing females are pecked?
Chek Jawa, Mar 10
Status and threats: The Cake sand dollar is not listed among the threatened animals of Singapore. The main threat is habitat loss due to reclamation or human activities along the coast that pollute the water. Like other creatures of the intertidal zone, they are affected by human activities such as reclamation and pollution. Trampling by careless visitors and over-collection can also have an impact on local populations.

Cake sand dollars on Singapore shores
On wildsingapore flickr

Other sightings on Singapore shores

Lazarus, Feb 09
Shared by Loh Kok Sheng on his blog.

Pulau Tekukor, Jun 16
Photo shared by Jianlin Liu on facebook.

Small Sisters Island, Aug 20
Photo shared by Marcus Ng on facebook.

Terumbu Pempang Tengah, May 11
Photo shared by Loh Kok Sheng on his blog.

Pulau Sudong, Dec 09

Filmed at East Coast Park
Give me back my dollar! from Loh Kok Sheng on Vimeo.

Links References
  • Calvin Jiah Jay Leow. 31 August 2020. Parasitic snail, Eulima adamsii, on sand dollar Arachnoides placenta. Singapore Biodiversity Records 2020: 127 ISSN 2345-7597
  • Lane, David J.W. and Didier Vandenspiegel. 2003. A Guide to Sea Stars and Other Echinoderms of Singapore. Singapore Science Centre. 187pp.
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