Invertebrates
Decorator Crab
Camposcia retusa · also called decorator crab, spider decorator crab
The decorator crab is a small spider crab that builds its own disguise. It trims pieces of sponge, seaweed, hydroids, and debris from the reef around it and fastens them to its own shell and legs using hooked bristles that work like Velcro. Some species end up so completely overgrown that there is no visible crab at all — just a walking clump of reef.
Overview
Decorator crabs are found on reefs and rocky bottoms in every ocean; this species lives on tropical Indo-Pacific reefs. The body is only a few centimetres across, on long spindly legs, and is nearly always hidden under attached material. The crab is a slow scavenger and grazer, relying on its cover rather than speed.
Hooked bristles
The crab’s carapace and legs carry stiff, hook-tipped hairs. It nips off a fragment of sponge or seaweed with its claws and presses it against these hooks until it catches and holds — a natural hook-and-loop fastener.
A living costume
Much of the decoration is alive and keeps growing on the crab, so an individual becomes steadily more encrusted. Decorator crabs tend to choose sponges and hydroids that sting or taste bad, adding a chemical defence, and wearing a particular sponge also masks the crab’s own smell.
Redecorating
When it moves habitat or outgrows its shell at a moult, the crab plucks off the old decorations and re-attaches them, or fresh ones, to the new surface — a deliberate, ongoing grooming behaviour.
Status
Decorator crabs have not been formally assessed. They are common worldwide and not considered threatened, though they depend on healthy communities of the organisms they use for cover.
Decorator Crab: common questions
Why do decorator crabs decorate themselves?
Mainly for defence, in two ways at once. Visually, a crab covered in sponge and seaweed simply does not read as a crab to a hungry fish or octopus; it looks like a lump of reef. Chemically, the crab tends to choose sponges, hydroids, and other organisms that predators find distasteful or that sting, so an attacker that does investigate gets an unpleasant mouthful. Some of the cover also masks the crab's own smell. Because the disguise does the work, decorator crabs can afford to be slow and can live out in the open on the reef where a bare crab would quickly be eaten.
How does the crab attach the pieces?
Its shell and legs are studded with tiny stiff hairs that are curved into hooks at the tip. The crab uses its claws to nip off a suitable fragment of sponge, algae, hydroid, or even a small anemone, sometimes chewing the base to soften it, and then presses and rubs it against these hooked hairs until the fibres of the fragment catch and hold. The effect is like pushing the loop side of Velcro onto the hook side. Living pieces then often anchor themselves further by growing onto the crab.
Do the decorations stay alive on the crab?
Often, yes. Many of the sponges, hydroids, bryozoans, and algae a decorator crab attaches are still alive and continue to grow while riding on it, feeding from the water as they normally would. This is good for the crab, since the cover thickens over time, and it is not obviously bad for the passengers, which get moved around to new feeding spots. When a decorator crab moults it climbs out of its old shell and transfers the still-living garden onto the new one.
Are decorator crabs dangerous or hard to keep in an aquarium?
They are harmless to people, with small claws and no venom, and they are popular in reef aquariums as scavengers and as a curiosity. The main considerations for keepers are that a decorator crab needs a supply of soft materials to decorate with, that it may borrow bits of prized corals or sponges from the display to wear, and that a large or hungry individual can nip at sessile invertebrates. Given enough of its own decorating material and enough food, it generally leaves the rest of the tank alone.