Introduction to Crustacean
Crustacean, a member of a large class of animals with segmented bodies. Crustaceans belong to the same major division, or phylum, of the animal kingdom as insects and spiders. Included among them are lobsters, shrimp, crabs, wood lice, barnacles, water fleas, and many other animals. All told, there are about 25,000 species of crustaceans. Most of them live in the water; a few, such as wood lice, live on land.
Wood lice are small crustaceans.The crustacean's body is made up of as many as 50 segments, but most of the higher crustaceans have only 19. The body usually has three regions—head, thorax, and abdomen. The head and thorax are often fused into a single unit called the cephalothorax. A hard substance, chitin (kī'tīn), covers the body, forming an external skeleton like a coat of armor. This exoskeleton does not grow with the body of the animal. Periodically it is shed and a new one is formed.
Crustaceans have compound eyes (consisting of many lenses) that are sometimes on stalks. Extending from the head are two pairs of jointed antennae (feelers) that act as sense organs and sometimes as swimming organs. Usually the two outer antennae are much longer than the inner pair. Armlike appendages on the thorax, usually including a pair of large, pincerlike claws, are used for walking, grasping, tearing, and eating. Appendages on the tail are swimming structures.
Crustaceans reproduce by means of eggs. The eggs of saltwater crustaceans hatch into tiny larvae that are unlike the adults. Young freshwater crustaceans resemble adults except that they are smaller.
The smaller crustaceans are important mainly as food for fish. Lobsters, crabs, and shrimp are excellent human food, and cray-fish are also eaten by many people. Crustaceans also serve as scavengers, helping to keep beaches and streams clean. On the other hand, some crustaceans are serious pests, destroying crops or boring into and destroying wharves and sea walls.
The class Crustacea of the phylum Arthropoda contains two subclasses. The Entomostraca are mostly small, without distinct body regions. They include the fairy shrimp, water fleas, cyclops, fish lice, barnacles, and copepods. The Malacostraca include forms with the typical 19 segments. They include crayfish, lobsters, shrimp, sow bugs, pill bugs, sandhoppers, and wood lice.
Which Crustaceans Are Seafood?
Many types of crustacean are edible, or fit to be eaten. People in kitchens all over the world take advantage of this.
Throughout the world, the crustaceans most popular as seafood are shrimp, crabs, and lobsters. In addition to these animals, other crustaceans are prized as food in specific regions. For instance, barnacles are a delicacy in parts of Europe and South America. Crayfish are a centerpiece of spicy Creole and Cajun cuisines of Louisiana.
Ecologically, crustaceans are an important source of food for marine animals of all sizes and shapes. Tiny crustaceans called copepods are food for small fish. Krill are the main source of nutrients for many whales and penguins. Because they are so rich in nutrients, krill also have been thought of as a potential source of food for people.
Do All Crustaceans Live In Water?
Most crustaceans live in water. However, there are a few crustaceans that are completely adapted to terrestrial, or land, life. The wood louse, also called the sow bug or the pill bug, is one of these unusual crustaceans. It lives its whole life on land. Although it may look very much like a bug, it is actually a crustacean.
You may be able to find wood lice in your backyard, patio, schoolyard, or local park. Wood lice may live in rotting logs, under the bark of trees, under flowerpots, or beneath thick piles of leaves. Wood lice are almost always found in damp places, though there are a few species of desert wood lice.
When a wood louse is touched, it curls into a small ball like a miniature armadillo.
Which Is the Smallest Crustacean?
Water fleas make up a group of tiny animals. They mostly live in freshwater ponds and lakes, but a few species live in the ocean. Water fleas can be as tiny as 1⁄125 inch (0.2 millimeter) long. Larger species can grow to be as long as 3⁄4 inch (18 millimeters), which is still very small.
Water fleas have a see-through shell. If a water flea is big enough, you can look through its shell to see its heart pumping and its other organs working.
Water fleas are not really fleas, they are crustaceans. But they look as if they are hopping through the water, as fleas hop on land, and that is probably how they got their name.
Which Is the Most Common Type of Crustacean?
Crustaceans called copepods are the most abundant of all the crustaceans. Some scientists believe that copepods are the most numerous multicellular animals (animals with more than one cell) found in water. Most copepods are about 1⁄17 inch (1.5 millimeters) long.
Copepods are part of the plankton (PLANGK tuhn), a large variety of tiny organisms, that float or swim near the water’s surface. They live in both fresh and saltwater. Copepods feed on other, smaller members of the plankton. In turn, other animals, especially small fish, eat copepods. The protein that copepods supply to the other animals that feed upon them is an important part of the aquatic food chain.
Not all copepods are free swimmers. Many are parasites (PAR uh sytz), which are organisms that live and feed on other animals. Parasitic copepods infect the skin, gills, mouth, and eyes of fish.
Are Crustaceans In Danger?
Most crustaceans are not in danger. Most crustaceans are so small, numerous, and widely distributed that scientists are not concerned for their survival. There are plenty of most types of these small animals.
Crustaceans harvested for human consumption are a different story. However, commercial fishers, marine biologists, and environmentalists often disagree about the status of species that are harvested commercially for human consumption. They disagree about the reasons for the decline of these species in the wild.
One thing is certain, however—things have changed in the sea. The giant American lobsters that once were found in the waters of the Atlantic coast of the United States are no longer seen. The average size of a lobster caught today is about 1 pound (0.45 kilogram). In the past, 20-, 30-, and 40-pound (9-, 14-, and 18-kilogram) lobsters were common.
Crustacean Fun Facts
The largest kind of crustacean, the giant spider crab of Japan, measures up to 12 feet (3.7 meters) across between its outstretched claws. The smallest crustaceans, such as water fleas, can be smaller than 1⁄125 inch (0.2 millimeter) long.
Lobsters have teethlike structures in their stomach, which grind partially digested food. The structure is called a gastric mill.
Why are they called fiddler crabs? Male fiddler crabs have one large claw, which they wave in the air to impress females and intimidate other males. If you use some imagination, it looks like the male is playing a fiddle, or violin.
Some species of shrimp are luminescent (LOO muh NEHS uhnt), meaning they produce a faint light.
Soft-shelled crabs are blue crabs that have just molted their shells