semaphore crabs (ilyoplax spp.)

CONTENTS

Fauna index

Crustacea

Peter K L Ng and N Sivasothi (editors)

Semaphore crabs
Ilyoplax spp.
Family Ocypodidae

Size: about 0.5-0.8 mm

There are five species of semaphore crabs in Singapore. All are very small crabs and can be easily missed. Crabs communicate in a wide variety of ways. Some are able to stridulate (rubbing granules against a rough surface), making a distinct scraping sound.

Many mangrove species, however, communicate with hand and body signals, much like the days of old before radio when people used a series of coloured flags to get important messages across, like a visual morse code!

Crabs, especially those of the genus Ilyoplax also use semaphore signals in the same manner. Their messages are equally important—signalling territorial rights and a readiness to mate.

above and below:
Ilyoplax delsmani signalling

Some males, like those of the White semaphore crab (I. delsmani) even change their body colour to a stunning white which does stand out on a drab mangrove background and then jumping frantically waving their claws. When resting, they turn a dull grey to brown!

Another common species is Tweedies semaphore crab (I. obliqua).

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From "A Guide to Mangroves of Singapore", Peter K. L. Ng and N. Sivasothi (editors)
Volume 1: The Ecosystem and Plant Diversity and Volume 2: Animal Diversity
Authors: Kelvin K. P. Lim, Dennis H. Murphy, T. Morgany, N. Sivasothi, Peter K. L. Ng,
B. C. Soong, Hugh T. W. Tan, K. S. Tan & T. K. Tan
BP Guide to Nature Series published by the Singapore Science Centre, sponsored by British Petroleum
© 2001 Raffles Museum of Biodiversity Research, The National University of Singapore & The Singapore Science Centre