Crabs

By Kristen Washer, Program Director

October 7, 2010

Crabs are an important part of the ecosystem. Throughout their lives they are a source of food for fish, birds and other animals. As adults they eat other crabs, crustaceans, fish and dead plants and animals. Crabs come in a variety of sizes and live in many different habitats, but all crabs begin as eggs and must go through larval stages before becoming adults. When crabs are in the larval stages they are zooplankton, i.e. animal plankton that are the base of our food chain, but as they grow into adults they fall into the class of crustaceans.  This type of critter is also called meroplankton.  Meroplankton are organisms that are planktonic for only a part of their life cycles.

Female crabs carry can produce between 800,000 and 8 million eggs. These eggs begin to develop under the female crab's apron, which is the hinged section in the middle of the crab's abdomen. When the eggs are ready, she moves them outside of her body. When the eggs hatch, these tiny larva, known as zoea, are left on their own to drift about the water column becoming part of the zooplankton community. Zoea provides an important food source for fish, and feed on phytoplankton by trapping its food in its bristles.  Zoea, barnacle larva and copepods are among the most abundant and favorite zooplankton that Salish students observe in their research aboard the Carlyn.

When crabs grow, they shed their exoskeletons. This process is called molting. When the zoea molts, it moves into the next stage of its development and becomes a megalopa. A megalopa can take anywhere from a few weeks to several months before they molt again and become tiny crabs. Unlike the zoea stage that eats phytoplankton, the megalopa eat zooplankton, even other megalopa. When the megalopa gets too heavy, they sink to the bottom of the sea and start to live as a crab.

An interesting fact that I have recently learned about crabs is that barnacle larva sometimes gets inside a young male crab and takes nourishment from the crab’s blood.  Having this barnacle take up residence inside the crab and feeding on its blood causes the crab to change its sex hormones.  The next time a crab with a barnacle living inside it molts, it becomes female.