Horned ghost crabs: Using camouflage based on day-night cycle and not simply environmental colours?
A study has revealed that horned ghost crabs (Ocypode ceratophthalmus) change their appearance from day to night for camouflage.
The ghost crab lives in the Indo-Pacific region from East Africa to the Philippines and the Great Barrier Reef. It has extended eyestalks which point skywards resembling horns, hence the name horned ghost crab. These crabs build burrows along beaches to shelter themselves from predators. The crabs are mostly nocturnal and their young are slightly translucent (Gibbs, P.E. 1978).
Biologists investigating the young crabs’ defences found that they mimic the different hues of sand on different parts of the beach. This aids in providing camouflage against their common predators like birds.
However, what makes this little creature so interesting is that it not only matches the colour of the sand around them, but also changes its colour according to the time of the day! Scientists had their suspicions raised about this day to night colour change after noticing differences in images of the same crab taken at different times.
They tested their theory using crabs gathered from the local beaches here in Singapore.
Over a twenty-four period, a distinctive transition from a dark appearance through the night to a lighter appearance at midday was recorded.
However, when the environmental brightness was manipulated by placing some crabs in a dark tank, the crabs did not change colour.
This means the crabs followed a natural daily cycle of colour change instead of directly reacting to the ambient brightness.
The researchers posit that the crabs use their body clock to decide the colour they should take on for a simple reason. If they merely became darker when in shadow, such as when they enter their burrow, they would be very eye-catching when they came out into daylight. Thus, a camouflage based on the time of the day would greatly increase their chances of survival.
Strangely, only the younger crabs exhibit this sort of behaviour. Adult horned ghost crabs are unable to change colours as freely as compared to their young. Biologists say this is due to the thickening of their shells, which become more pigmented and less likely to change in colour.
Gibbs, P.E. 1978. Macrofauna of the intertidal sand flats on low wooded islands, Northern Great Barrier Reef. Philosophical Transactions of the Royal Society of London, Series B 284(999):81-97.
Martin Stevens, Cheo Pei Rong & Peter A. Todd, 2013. Colour change and camouflage in the horned ghost crab Ocypode ceratophthalmus. Biological Journal of the Linnean Society, 1095-8312.
“Horned ghost crabs change camouflage from day to night” By Ella Davies. BBC Nature, 5 April 2013.
URL: http://www.bbc.co.uk/nature/22025789 (accessed on 7th April 2013)