Crustaceans

The majority of animals in the oceans are crustaceans. The name indicates their class, Crustacea, which consists of aquatic arthropods having jaws and two pairs of antennae, such as crabs, lobsters, and shrimps. Crustaceans are among the most successful of animals, dominating the oceans as the insects dominate the land. They also live in freshwater, and some even live in moist land habitats. Most crustaceans are small, but they can differ widely in body form, size, and habits. The twenty-five thousand crustacean species include lobsters up to two feet long and giant spider crabs with leg spans of over ten feet.

Physical Characteristics of Crustaceans
All crustaceans are covered by hard chitin coats (carapaces) acting as external skeletons (or exoskeletons), which growas backward extensions of their heads. Carapace texture varies from bonelike to tough and leathery. The hardness of the covering depends on the amount of calcium salts in the chitin. The exoskeleton acts like armor, to protect the animal. Crustacean bodies are made up of several segments and divided into sections. Each section usually has one pair of jointed legs. Crustacean heads are fused with several segments just behind them, forming the cephalothorax. This is followed by the abdomen. The head holds two pairs of sensory organs, jaws (mandibles), and antennae. Behind the mandibles are two other pairs of mouth parts, the maxillae. These limbs, which evolved under and in front of the mouth, are used to hold, tear, and taste food. Antennae provide the sense of touch. Each crustacean head also holds a pair of compound eyes, an unpaired eye, or both. Each eye is on a movable stalk and can be turned in any direction. The cephalothorax also holds limbs used in locomotion and others used as gills in respiration. Special legs under the body are used for walking. Large legs called pincers or claws are used to catch fish, crack mollusks, dig burrows, and fight. Slowswimming legs under the tail can be used to hold eggs. The legs at the end of the tail are flattened into a fan-shaped fin, also used in swimming. The main body cavity is a blood circulatory system, pumped by a dorsally located heart. The crustacean intestine is a straight tube containing glands that secrete digestive fluid and absorb food. All crustaceans also have at least rudimentary brains, composed of ganglia near sense organs and below the intestine.

Shrimp, Lobsters, and Crabs
Shrimp make up two thousand crustacean species. Structurally similar to lobsters and crayfish but flattened laterally, they are green or brown. They range from insect size to ten inches long, inhabiting salt water and freshwater (mostly on shallow ocean floors) and eat smaller animals and plants. Some species, krill, live deep in the oceans and are eaten by whales. Shrimps have eight pairs of appendages; the first three are used for eating and the rear five are for walking. The shrimp abdomen also contains five pairs of swimming legs and a fanlike tail. Lobsters are marine decapods, with five pairs of thoracic locomotor appendages. They belong to the crustacean suborder Reptantia and are related to freshwater crayfishes. Their narrow, dark green bodies are up to two feet long and weigh two to fifteen pounds. In each lobster, two large, bodylength claws stretch forward to grasp prey, and a fan-shaped tail is used for propulsion. The three true lobster species are very important foods in North America and Europe. American and European lobsters (Homarus americanus and Homarus vulgaris) have enlarged claws. Norway lobsters (Nephrops norvegicus) have longer, thinner claws. In these true lobsters, one claw is heavy and has blunt teeth to crush prey. The other is smaller and has sharp teeth to tear prey up. Not all lobsters have the heavy claw on the same side. They are "right clawed" or "left clawed." The lobster head has two pairs of antennae and its eyes are compound, on the ends of mobile stalks. Afemale lobster lays 5,000 to 160,000 eggs every two years, and carries them under her tail for up to eleven months, until the young hatch. The young, initially 0.3 inches long, drift and swim for a month before settling on the bottom, at 1 inch long. Survivors of the next few months on the dangerous ocean bottom dig shallow burrows beneath rocks, or inhabit crevices in the ocean bed. During the day, a lobster stays inside its burrow waiting for prey.At night it comes out to search for dead or live food. Lobsters grow by molting and may live up to fifteen years. All are primarily scavengers. American lobsters occur only off eastern North America, from Labrador to North Carolina. Most live on the ocean bottom, at depths of ten to one hundred feet. Caught in baited lobster pots, they average lengths of ten inches and weigh up to five pounds. Norway lobsters are most abundant near France and Spain. European lobsters are caught offGreat Britain, France, Italy,Norway, and Portugal. Rock lobsters, lacking the enlarged claws of the true lobsters, are found off South Africa, Australia, New Zealand, Japan, Brazil, the United States, Mexico, and the Bahamas. Crabs, related to lobsters and shrimp, can move sideways, burrow, and swim. They are decapods, whose smallest members, tiny pea crabs, consist of females which live in the shells of live oysters and males who live in the outside world and visit them to mate. The giant spider crabs of Japan are the largest crabs, often ten feet in circumference (including legs). Many kinds of crabs are used as human food. For example, the blue crab is the common food crab in eastern America. Two important groups, 1,500 species of Anomura (hermit crabs) and 4,500 species of Brachyura (true crabs), have similar body shapes, with small abdomens and large, broad anterior bodies, though hermit crabs have fewer walking legs. Seen most as ocean-bottom dwellers, crabs also inhabit freshwater, and some live on land. The crab body is covered by a chitin carapace. The small abdomen under the body is most often a brood pouch.Acrab has five pairs of walking legs, two sense antennae, and front legs which are pincers (claws) for feeding and defense. Crabs also have complex nervous systems, with keen compound eyes and the ability to smell and taste food. They enjoy complex mating rituals and communicate by pincer waving. Crabs often mate just after a female molts, when its shell is soft. Eggs are kept in the brood pouch and may pass through two larval stages: the initial form, zoea, does not resemble adults; the later form, megalops, does. Each time a crab molts after birth, it increases in size. Crabs may live for three to twelve years. There are many different crab species. Some interesting crabs are the lobsterlike anomurans, called squat lobsters; sand crabs, which burrow into sand and filter suspended matter from the water; large spider crabs with long legs and slender bodies; swimming crabs (blue crabs) with paddlelike legs; and fiddler crabs, whose males each have a huge claw that they use in mating and combat. There are even land crabs, omnivores found in the tropics, that release larvae into oceans.

Life Cycle of Crustaceans
Crustaceans reproduce via eggs, which usually hatch underwater. Some crustaceans, such as lobsters, carry their eggs and young on the hairs of swimming legs. The eggs of different crustaceans hatch at different stages of development. Young lobsters and crayfish look like their parents; young crabs do not. After hatching, young crustaceans grow until their shells become too tight. Then the crustacean sheds its old shell for a larger new one. The process of changing shells (molting) takes place several times during growth. The new shell is formed inside the old one, and is soft and wrinkled until exposed to the environment. When a lobster molts, its shell splits along the back and the lobster leaves through the opening. Sometimes molting accidents will occur. For example, a leg or a feeler often breaks off in the process of leaving the old shell. When the animal molts again, it grows a replacement limb. The new limb is small at first, but becomes full-sized after several molts. Some crustaceans, such as barnacles, are hermaphrodites. All barnacles live in oceans. Their larvae are free-swimming, but adults attach to foreign objects, such as ship bottoms, wharf piles, rocks, and whales. There are five orders of barnacles. Four are parasites of shellfish. The fifth order includes stalked barnacles, originally found in warm waters. However, because barnacles attach to ships, they are found worldwide.