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What is a Sponge Crab?

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  • Written By: J.M. Densing
  • Edited By: Jacob Harkins
  • Last Modified Date: 29 October 2016
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A sponge crab is an intriguing type of saltwater crab that carries around a living piece of sponge as a disguise. It belongs to the family Dromiidae in the classification system, and is sometimes confused with the decorator crab. It is a small- to medium-sized crab, and it carries the sponge resting on its carapace so that it appears to be wearing a sponge hat. Viewed from above it often appears to be merely a moving piece of sponge, with the crab completely hidden underneath.

The sponge crab can vary in size depending on the specific species, from 0.2 to 4 inches (0.5 to 10 cm). It has a smooth rounded body, and is sometimes covered with fine, fuzzy hairs. It has small round eyes that protrude from its body on short stalks. It has a small set of front pincer claws that it uses to defend from attackers and for feeding. The last set of legs to the rear is modified to hold the sponge, bending up and over the carapace, or top shell.

The habitat of the sponge crab is in warm coastal areas in various locations around the world, such as Singapore, Australia, the Caribbean, and many others. It is commonly found in areas with sandy environments with lots of sponges, seagrass areas, kelp patches, rocky areas, coral rubble and jetty piles. It prefers an environment with hiding places and a plentiful food supply.

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The sponge crab fashions its disguise by using its pincers to cut a pieces of living sponge out of a larger sponge mass. It snips the sponge into a shape close to its own, and holds it up over its carapace using the rear legs. An advantage of using living sponge is that the sponge disguise grows right along with the crab, and can be trimmed down for a better fit as needed. If the sponge is lost, the crab will make efforts to retrieve it before replacing it with a new one. If sponge isn't available it will use soft coral or a sea squirt, and there have even been reports of lost flip flops being used.

While carrying a piece of sponge as a camouflage, the sponge crab is almost impossible to spot unless it moves. This provides it with valuable protection from predators. It also hides in rock formations, coral crevices, and other sheltering areas when they are available. In addition to camouflage protection, the sponge offers added protection by having a bad taste that makes predators reluctant to try to eat it.

Often hiding for most of the day, the sponge crab is a scavenger that usually feeds at night. It is an omnivore, and its primary method of finding food is foraging slowly around the sea floor for small creatures and plants to eat. Another unusual feature of these sponge crabs is that some species carry the eggs until they hatch, and the baby crabs stay with the mother for a few weeks being cared for by her. They usually stay hidden underneath her until they are large enough to survive without a mother's protection.

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