Characteristics of Crustaceans

With estimates ranging from 30,000 to 70,000 known species, crustaceans represent a large contingency of the planet's invertebrates and arthropods. They primarily inhabit saltwater environments, but can also be found in freshwater and, in rare cases, on land. Although there are five distinct classes of crustacean, they almost all share a certain number of distinguishing characteristics. The few exceptions pertain only to a small fraction of this sizable population. You've probably noticed their interesting qualities while dining upon them in the form of lobster, crab or shrimp.

  1. The Exoskeleton

    • All crustaceans are easily identified by the hard, calcified exoskeleton that protects their internal systems, including the open circulatory system--crustaceans do not have hearts--and a primitive, two-part nervous system, which features a single ventral nerve cord and the ganglia system. Crustaceans have an edge on their fellow arthropods when it comes to the strength of the crucial exoskeleton; their protective coverings also contain limestone, an element that further hardens the exoskeleton.

    The Head

    • This crab's mug clearly shows the features of a crustacean's head.

      In addition to two compound eyes and two antennae, which are used to sense fluctuations in temperature and humidity as well to locate food, crustaceans' heads typically boast two more sets of distinct appendages that are primarily used for feeding. These include the maxillae that function much like arms to reach and carry food to the animal's mandibles, which grip and tear into food.

    The Body

    • Like other arthropods, crustaceans have segmented bodies, and although the number of segments may vary from species to species, most posses a thorax and an abdomen. Notably, most crustaceans' abdomens feature "swimmerets"--legs that are used for swimming.

    Respiration

    • The majority of crustaceans actually possess specialized gills to absorb oxygen from the water. However, most of the smaller crustaceans use their full body surface to exchange gas and obtain oxygen. A few familiar crustaceans that use this breathing system include barnacles and copepods.

    Sex and Reproduction

    • Barnacles are a hermaphroditic class of crustacean.

      Most crustaceans--with the rare exception of a few hermaphroditic species, such as barnacles--are either distinctly female or male. Reproduction begins as the eggs--attached to the swimmerets of females--are fertilized by the males, whose first pair of swimmerets transfers their sperm to the female.

    Diet

    • Crabs are outstanding hunters.

      Larger crustaceans like lobsters and crabs hunt their prey, chiefly small fish and other aquatic organisms. Those on the smaller side of the spectrum, such as copepods and particularly small varieties of shrimp, use a filter-feeding method in which the crustacean waves its setae--fine, paddle-like extensions--back and forth to create something of a miniature current that carries plankton through the animal's filters and into its mouth. Still other varieties have adapted a preference for carrion, scavenging for dead plant or animal matter.