Creature Feature - Harmonica Pebble Crab

Andrew Hosie's blog | Created 6 years ago

Harmonica Pebble Crab, Oreophorus reticulatus

Sometimes at the museum we come across a species that can be baffling. This crab with its pock-marked, lumpy and misshapen carapace is a bizarre sight. The row of circular pits on the pincers reminded us of a harmonica, which is why we are dubbing this species the Harmonica Pebble Crab. This species was first reported in Australian waters in 2004 from the Dampier Archipelago at a depth of 40 m.

But why the bizarre appearance? Mimicry: this is a common phenomenon where an animal or plant resembles, either through appearance or behaviour, another organism. Mimicry can serve several purposes: to avoid being eaten, to help in capturing food, or to help reproduce. For example orchids are known to mimic the shape and colour of insects to help attract pollinators, while certain spiders mimic ants in order to go undetected by their prey.

Mimics that are attempting to avoid being eaten often resemble another organism that is unpalatable or is otherwise dangerous to potential predators. In the case of this pebble crab it is blurring the boundary between mimicry and camouflage. The unusual sculpture of pits and ridges on the carapace give the Harmonica Pebble Crab the appearance of a small piece of dead coral and who would want to eat a dead piece of coral?

The areas around coral reefs where this species lives are littered with pieces of dead coral of varying sizes. This species blends in almost perfectly. This crab is not likely to be a fast mover which helps to build the illusion that it is just another broken piece of coral on a rubble strewn seascape.

The Harmonica Pebble Crab compared to a piece of coral

The Harmonica Pebble Crab from above, front and a close up of the pincer showing the unusual sculpturing of the carapace. Compare and contrast with a dead piece of the coral Porites sp. in the lower right.
Image copyright WA Museum