THE SPONGE DECORATOR CRAB

The sponge decorator crab Composcia sp.

Can you imagine an animal, a small one wearing a disguise? How would it look like to have a crab decorating itself? Are you shrugging away in disbelief? Strange but true, this is what the ‘Decorator Crab’ is all about. Let’s take a closer look at this interesting little fellow.

 Decorator crabs are distributed worldwide grouped under the family Majidae and Inachidae. The family Majidae includes crabs that have tear shaped eyes and typical decoration. These crabs stick living plants or animals or both to their bodies to hide/ wade off or warn predators. This phenomenon is called “Camouflage”. The carapace or the body is covered with spines and knobs with hooked hairs on them. The projecting hooked hairs aid as attachment sites for algae, sponges and hydroids.

Some species of decorator crabs attach sponge fragments on their carapace and so these varieties are called “sponge decorator crabs”. The crab in the picture is Composcia retusa, a common species in Palk Bay and Gulf of Mannar. They are found in depths ranging from 2 meters to 15 meters in the sea. This species can grow to about 3 cm (carapace length). The eyes and antennae are short for this crab. A typical sponge decorator crab can attach fragments of sponges to their body and some reports show even attachment of sea anemones along with sponges. The claws are short and the appendages/ legs, long. Once sponge is attached to its body, the crab resembles a ‘weight lifter’, fitted with muscles and all set for a performance! While decorator crabs move on top of a sponge colony, they camouflage perfectly and are totally concealed from predators. Scientists often quote this as “highly specialized behavior”.

Front view where eyes and antennae can be seen

Another important species of sponge decorator crab is Hyastenus elatus. This variety shows worldwide distribution but its diversity is more in the Indo-Pacific region. For Composcia sp, the pincers or the claw is the only place devoid of any sponge attached!

The presence of decorator crabs is often unnoticed during the day time because they prefer to hide in crevices or holes in coral reefs. However, the species is very active during the night and towards dawn and dusk when illumination is less. Decorator crabs are carnivorous, feeding on a wide diversity of organic matter and marine fauna. They hunt down their invertebrate prey during the night time.

The attachments found on decorator crabs play a dual role. It aids in easy concealment and at the same time, due to the distasteful nature of sponges (this is because of the presence of some metabolites), predators dislike coming near the sponge decorator crab. Presence of hydroids in the body of some decorator crabs gives a sting to the predator.

Decorator crabs are also called as “velcro crab” because of the presence of the projecting hooked hairs possessing the capacity to stick the sponge fragments collected to the body which is exactly how velcro sheets work!

Some examples of decorator crabs include little seaweed crab, grace decorator crab, masking or moss decorator crab and spider decorator crab.

 © V. Deepak Samuel