Ubin "crop circles"?

During my visit to Ubin on Sunday, 5 Sept 2004, I explored the entire stretch of Noordin Beach which I have never done before.

The tide was relatively low and much of the natural sandy beach was exposed. While walking along the beach, I started noticing meticulously-created equal-sized balls of sand littered along the beach, forming very interesting patterns.



The more I walked, the more I discovered. I was about to suspect that aliens have landed on Noordin till I noticed that the balls of sand usually span outwards from a hole in the middle. Then I figured it was probably the handicraft of a crab. I was unable to take a picture of the elusive sand artist but I managed to caught sight of a few sandy-coloured legs scurrying into the hole. Not only are they found along the beach, they are also found on the sandy banks of the river behind the beach, seperated by mangroves.



After consulting a graduate student friend studying crabs at NUS, based on my description, she concluded that this was the work of the Sand Bubbler Crab (Scopimera).

According to the Guide to Seashore Life in Singapore, "the common name of this crab derives from the balls of sand it leaves behind after sifting it for organic detrital matter. They are related to the soldier crabs (Dotilla) and prefer sandier areas. They have numerous stiff hairs on their legs, and lack the transverse row of abdominal hairs. Their hairs are instead located at the base of their legs, and are used for the same purpose as Dotilla's. (Body diameter up to 1.5cm)

BBC's Blue Planet TV series also mentioned this fastidious worker to be "the best 'back-heel' in the world. These beach-dwelling [crabs] convert an entire beach into minute footballs as they work at breakneck speed to filter food out of sand grains. They religiously practise their skills every day as the tide goes out."

I must have been very lucky to have caught them in action during the low tide then!

Links
Pulau Ubin Stories Gallery Photos from Ubin taken on 5 September 2004

Sand Bubbler Crab in A Guide to Seashore Life in Singapore by Dr Leo W H Tan and Peter K L Ng. Published by the Singapore Science Centre and Sponsored by BP.

BBC - Science & Nature - Sea Life - Blue Planet TV series

Sand Bubbler Crab in Online Guide to Chek Jawa

Photo of Sand Bubbler Crabs taken from A Guide to Seashore Life in Singapore. Taken by Kang Nee. (source)