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Phylum Echinodermata > Class Echinodea > Order Clypeasteroida
Cake sand dollar
Arachnoides placenta
Family Clypeasteridae
updated Oct 2016
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.

What eats sand dollars?
Some snails such as the Grey bonnet (Phalium glaucum) are believed to feed on sand dollars. One was seen on top of a sand dollar at Cyrene Reef. A Knobbly sea star (Protoreaster nodosus) was seen with its stomach stuck to a sand dollar. A Haddon's carpet anemone (Stichodactyla haddoni) 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 (Family Eulimidae) are sometimes seen on them too.


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?

Changi, May 08

Upperside.

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

Underside.

Mouth on the underside.


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


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.


Upperside of test.

Changi, Jun 06

Underside of test

Living sand dollar moving under wet sand.
Living sand dollars, not moving under sand.
Lazarus Island, Jun 09


Cake sand dollars on Singapore shores

Photos of Cake sand dollars for free download from wildsingapore flickr

Distribution in Singapore on this wildsingapore flickr map


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

Pulau Tekukor, Jun 16
Photo shared by Jianlin Liu 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
  • The Echinoid Directory by Dr. Andrew B. Smith on the London Natural History Museum website: everything you could possibly want to know about sand dollars and sea urchins with lots of large close-up images and explanatory diagrams. With technical descriptions of Family Cylpeasteridae and photos of dead Arachnoides.
  • Arachnoides placenta on SeaLife Base: Technical fact sheet.
References
  • 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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