For those who like to explore marshes, bays, and estuaries with a hand net or small seine, the catch often includes representatives of the crustacean group. Several species of marine crabs are commonly found on Cape Cod in its shallow waters. The “shell” of the crab is called a carapace and this exoskeleton is composed of a material called chitin. There are three major parts to a crab: the head, thorax, and abdomen. These animals also have jointed appendages and use gills for breathing. They grow through a process called molting in which the carapace splits open and the animal emerges.
The crabs are not difficult to identify. One of the most commercially important species is the blue crab (Callinectes sapidus). An aggressive species, this animal is bright blue and has nine teeth along both sides of its shell. The hind legs are shaped like a paddle and it is a very fast swimmer.
Green crabs (Carcinus maenus) are quite abundant under small rocks in the intertidal zone. This animal was introduced from Europe but has become one of the most commonly seen crabs on the Cape’s shoreline. One of the reasons it has been so successful is its tolerance for changes in salinity. The green crab feeds mainly on worms and clams.
Another common crab on Cape Cod is found in estuaries and bays, but look carefully because this animal is an expert at camouflage. The spider crab (Libinia emarginata) has a brown carapace with long, spindly legs. It has the ability to stick small shells and algae on its carapace. Unlike the blue crab, this animal is harmless and can be picked up without any difficulty.
To examine one of the most interesting coastal crabs go to a salt marsh in the summer at low tide. Scurrying in the mud will be hundreds of fiddler crabs (Uca pugnax), and they can be easily caught. Their burrows are scattered throughout the banks and extend about three feet deep. The males are distinct from the females in that they have a large claw that is used for courtship and defense. These animals are ecologically important to the salt marsh. They are a food source for many animals, and they aerate and fertilize the marsh sediments.
Sometimes you might collect a crab that is in the process of molting. If it’s just about to emerge from its shell, it’s called a peeler. If it has recently grown from its old shell it will still be soft. It takes a little bit of time for the carapace to harden. Some animals may also appear to be at different stages of regeneration as they slowly replace lost limbs or claws.
Copyright Gil Newton 2009 Thanks to Chris Dumas for logo image.