No need to stretch the imagination here … these really are miniature, aquatic orangutans! Members of the spider or decorator crab subfamily Inachinae, the orangutan crab (Achaeus japonicas) is a macro-photographer’s delight, measuring just two centimetres across.
Covered from head to claw-tip in long, reddish-brown ‘fur’, these well-loved critters are masters of disguise, attaching strands of filamentous algae or cyanobacteria to their skeleton. Decorator crabs are famous for their ingenious tactic of actively sticking all manner of camouflaging debris to their bodies, perfecting their ability to go unnoticed.
Their long ‘arms’ help the orangutan crabs move around the reef with surprising agility in pursuit of safety and better feeding grounds. In fact, if time allows, observing an individual for long enough may just unveil the same arm-swinging behaviour so characteristic of their somewhat larger, arboreal namesakes. On one occasion, an individual was observed curling up into a ball and falling through the water column until it landed on a coral further down the reef wall. Even the orangutan itself would be impressed by this trick!
As is often the case with crustaceans, the orangutan crab is more active at night, crawling around on sponges and corals in search of any organic material worth nibbling. They may also use their long ‘fur’ to help trap particulate matter passing by in the current.
These cute creatures can be found in shallow reef habitats of the tropical Indo-Pacific at depths down to 30m. Not a great deal is known about the orangutan crab, although they do seem to like hanging around the inflated grape-like protuberances of the beautiful bubble coral (Plerogyra sinuosa). The dead parts of this coral’s skeleton are frequently covered by the same clumps of red-brown cyanobacteria used by the crabs, which supports the idea that these crafty crustaceans are indeed trying to blend in with their background. It is possible, however, that they are more often observed in association with this particular coral due to the ease of spotting them against the pearly white, uniform background. The protection offered by the coral’s stinging flesh could also offer more than enough reason to stay put, as many other bubble-loving commensals would agree. These distinctive bubbles are generally only inflated during the day when the coral maximises the photosynthetic yield of its algal symbionts (zooxanthellae) which provide the host colony with the majority of its nutritional requirements.
Two unlikely animals found living together: a tiny crab that sports a humorously similar appearance to one of the world’s great apes, and a coral that resembles a bunch of succulent white grapes. One would never imagine there to be a link between such things, but that’s the wonder of the natural world for you.