CLASS: Crustacea

Lobsters, crabs, and prawns  

Animals with five pairs of segmented legs, an open circulatory system, and 
a hard outer shell. 

Decorator crab, 2 cm/0.8 inch wide. Moss Beach, California.
Photo by Dr. Robert Sprackland.

Definition: Crustaceans are animals with a bilaterally symmetrical body that is not necessarily divided into obvious segments. The circulatory system is open, meaning that blood does not move through arteries and capillaries. The nerve cord is solid and located ventrally. The body is covered by a hard, chitinous outer skeleton, which is shed periodically during growth. There are several pairs of complex jointed appendages that are differentiated into mouthparts, legs (generally 5 pair), and swimmerets. 

While insects are conspicuously absent from oceanic environments (though they may be quite abundant on beaches!), crustaceans thrive in them, and most species are found in the coastal waters of the world's oceans, including mangroves and bays. Several genera inhabit fresh water, and some, such as the "pillbugs" or "woodlice," are terrestrial. 

A deep-water shrimp bearing large pincers. From the Monterey 
Bay Canyon, California. Photo by Dr. Robert Sprackland.

Crustaceans vary in size and shape to a considerable degree, and range from microscopic parasitic barnacles in the genus Sacculina to tiny millimeter-long brine shrimps to the Alaska king crab with a 2 meter/6.2 foot leg span. In shape they vary from familiar lobsters and crawfish to bizarre barnacles. They all reproduce by laying eggs, and the young go through a stage in which they look quite unlike the adult. Offspring may number in the millions per female, and because they represent such a widely accepted food source, are a highly predated group of animals. In general, crustaceans taken as marine prey are referred to as "krill," though that term is also correctly applied to certain shrimp-like species as well. Whales and large sharks are not the only predators on crustaceans; they form a huge and economically important part of human food sources, typically at high prices.  

Included among the crustaceans are lobsters and crawfishes (with distinct cephalothorax and tail body segments), crabs (but not horseshoe crabs, which are more closely related to spiders), shrimps and prawns, barnacles, and pillbugs.

A striking yellow crab from the canyon walls in Monterey Bay, posing among 
deep-water anemones (Cnidaria: Anthozoa). Photo by Dr. Robert Sprackland.

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