Orangutan crabs

Orangutan feature

Achaeus japonicus

COPYWRITE OF SUBMERGE PUBLISHERS An orangutan crab showing off its hairy claws.

10 2009 10 SUBMERGE SUBMERGE •• October/November August/September 2009

Text and images by Fiona Ayerst

Phy

Chromodor

crabs

yllidia ocellata.

ris annae.

feature

These are particulary hard to spot,

furry little critters that you will generally only see if they move along the reef as they look exactly like a tiny blob of algaecovered seaweed. They are, in fact, small decorator crabs (carapace length 1.2cm) with an oval body, red eyes, long legs and short claws. Once you get a good look at this little guy, it is obvious where its rather descriptive name comes from. Like its namesake, the crab’s entire body and legs are covered with a thick, rusty, maroon-coloured hair. “Orangutan” is actually derived from an Indonesian word (oran hutan) that means “forest person”. The orangutan crab comes from the decorator family who purposely stick debris, particles or detritus on to their hair for further camouflage (as if they need it!). Completely “naked” specimens can occasionally be seen, with a typical banding pattern on the legs – if I have ever seen

A small decorator crab camouflaged against the corals.

COPYWRITE OF SUBMERGE PUBLISHERS October/November August/September 2009 • SUBMERGE 11

feature

COPYWRITE OF SUBMERGE PUBLISHERS

This picture leaves no doubt as to why it is called the orangutan crab.

The whole body is covered with thick reddish hair, which helps with camouflage.

An orangutan crab hides itself in a variety of corals.

12 2009 12 SUBMERGE SUBMERGE •• October/November August/September 2009

one without hair, I did not recognise it as an orangutan crab.

Living along the external reef, this species feeds on plankton and suspended particles that collect in their hair (on the first pair of walking legs or from the fur of other crabs). These crabs are often associated with hard corals, particularly of the family Caryophyllidae (mushroom coral, Euphyllia glabrescens or bubble coral, Plerogyra sinuosa). These crabs are much more mobile at night, when they clamber over sponges or soft corals. I have most often seen them peeking out from the extended polyps of bubble and mushroom coral, possibly due to the fact that they are easier to see with their purple fur against the fat, white bulbous coral.

The orangutan crab has several pairs of appendages attached to its segmented body. The first two are sensory antennae and the next five pairs serve as walking legs. The chelae or pincers are located on the front legs and are used for feeding, courtship displays or fighting. All crustaceans shed their outer shell in a process called moulting, which allows them to grow and regenerate lost appendages. Crabs are scavengers of any type of edible matter they find, be it animal or plant material. Some will require a deep substrate to burrow in during the day, while others are always on the move.

Strangely enough, this rusty-red-coloured crab enjoys publicity as it sits on the edge of the bubble coral, highly visible against the coral’s white pearly colour. I always enjoy finding and watching this crab as it bobs up and down in the bubble coral with eyes like Christmas tree lights glowing brightly.

Besides delighting divers and taste buds all over the world, the crab has more noble duties. A lot of them are of substantial economic significance in fisheries, medicine and ecology.

There is a need to identify potential food crabs, bio-indicators and endangered species. Many species are also habitat specific and thus excellent bio-indicators of habitat health and environmental degradation, especially coral reefs and mangroves. S

feature

COPYWRITE OF SUBMERGE PUBLISHERS

A fuzzy-looking crab climbing through an anemone.

October/November August/September 2009 • SUBMERGE 13