Horned ghost crabs change camouflage from day to night: A Singapore study

Ella Davies Reporter BBC Nature 5 Apr 13;

Horned ghost crabs change their appearance from day to night for camouflage, a study has revealed.

The species Ocypode ceratophthalmus builds burrows on beaches from Japan to East Africa to shelter from predators.

Researchers investigating young crabs' defences found they fine-tune their brightness to mimic their background.

The crabs reflected changes in their environment throughout the day, becoming lighter in the daytime and darker at night.

The same crab can look very different in the day (top) and at night (bottom)

The findings are published in the Biological Journal of the Linnean Society.

Horned ghost crabs are named for their eyestalks which extend upwards resembling horns. The crabs are mostly active at night and juveniles are slightly translucent.

Dr Martin Stevens, working for the University of Cambridge, undertook the study with colleagues from the National University of Singapore.

"They have a remarkable match to the sand on which they live to provide camouflage against predators like birds and primates," he told BBC Nature.

"They can even match the colours of particular grains of sand found on the beach where they occur."

Scientists first suspected the crabs' unusual day to night colour change after noticing differences in images of them taken at different times.

They tested their theory in the laboratory using crabs collected from beaches in Singapore.

Observing the colour of the crabs over a twenty-four period, they recorded a distinctive change from a dark appearance through the night to the lightest appearance at midday.

When placed in a dark tank, the crabs did not change colour but they did become brighter when placed a on a white versus a black surface.

Dr Stevens explained that, rather than directly reacting to the ambient light, the crabs combined a natural daily rhythm of colour change with reactions to the colour of the surface they occupied.

"This changes their camouflage so that they are very well camouflaged against the yellow sand during the day, and dark at night - we think to blend in with shadows on the beach," he said.

Dr Stevens added that if the crabs simply became darker when in shadow, such as when they enter their burrow, they would then appear very noticeable against the sand when they next ventured out in daylight.

This camouflage trick directly contrasts with how other species of crabs alter their appearance to suit their surroundings.

Fiddler crabs from the Uca genus are known to appear darker during the day and lighter at night. Scientists have suggested this may help the crabs to regulate their temperature or protect them from UV radiation in the day.

A further oddity of the ghost crabs, according to Dr Stevens, is the fact that only the juveniles can change their appearance in this way.

"The adults probably don't change colour as much... because the carapace becomes thick and dense with pigment when the crabs get big. But why that should happen I'm not sure," he said.

The biologist intends to investigate other crab species in order to understand more about their ability to change colour for concealment.