Sunday, September 30, 2007

Sand Star

Sand Stars (Archaster typicus) is one of the more easily seen sea star on our shores. During low tide, they will emerge from the sandy sediments and they could also be located by the star-shaped impressions left on the sand. This is especially so on the sandy shores of Pulau Semakau. We spotted several mating pairs and the Temasek Poly students wondered how copulation takes place.

A. typicus belongs to the family Archasteridae. This is a small family with only one single genus, Archaster. Sand Star males are slightly smaller than females and a mating pair will always involve the male superposed on top of the larger female. However, fertilization occurs externally where both male and female synchronize the release of sperm and eggs. Thus copulation, which infers internal fertilization, is probably the wrong term to use. Pseudocopulation is the term widely used by scientists.

Sand Star is one of only two sea star species currently known to perform male-on-female superposition sexual behaviour. Most other species conduct aggregate spawning. It is not exactly clear why Sand Stars superpose during breeding. It could either be to increase the chances of fertilization when the eggs and sperm are dispersed into the water or it is a mate guarding behaviour.

Pictures taken at: Pulau Semakau, August 2007.


Komatsu, M, 1983. ‘Development of the Sea-star, Archaster typicus, with a Note on Male-on-female Superposition’, Annotationes Zoologicae Japonenses, vol. 56 (3), pp. 187-195.

Lane, D. J. W. & D. VandenSpiegel, 2003. A Guide to Sea Stars and other Echinoderms of Singapore. Singapore Science Centre, Singapore.

Saturday, September 15, 2007

Gold-spotted Mudskipper

It has been a while since I stepped into a mangrove and I’m beginning to miss the mudskippers, or as Rachel would affectionately call them the ‘doink-doink-doinks’ because of their locomotion on land. Their movement of pectoral fins pulling and dragging the body forward always leaves a distinctive trail on the mudflat.

Gold-spotted Mudskippers (Periophthalmus chrysospilos) have pretty orange spots along their body and some of them can appear yellowish in colour. They are rather common and easy to identify from afar. One of the diagnostic features separating this species from other mudskippers is their fully fused pelvic fins that form a round disc.

P. chrysospilos has a small gill area compared to other mudskippers. They are thus less efficient in aquatic respiration and so more adapted to land than water. Their breeding period is thought to be between May and July when they’ll become less seen as they spend more time in their burrows. Males will attract females to their burrows with vigorous courtship displays. Once paired, the male will defend his burrow and surrounding territory against all comers including mangrove crabs!

Pictures taken at: A southern offshore island, December 2006; A northern offshore island, May 2006 and western part of Singapore, January 2007.


Ip, Y. K., S. F. Chew, L. L. Lim & W. P. Low, 1990. ‘The Mudskipper’, in Essays In Zoology. Papers Commemorating the 40th Anniversary of the Department of Zoology, National University of Singapore, eds, L. M. Chou & P. K. L. Ng, Department of Zoology, National University of Singapore.

Larson, H. K. & K. K. P. Lim, 2005. A Guide to Gobies of Singapore, Singapore Science Centre, Singapore.