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 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. There are between 50,000 and 67,000 crustacean species, including 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 grow as backward extensions of their heads. Carapace texture varies from tonguelike 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. Slow-swimming 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 over 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, inhabit saltwater and freshwater (mostly on shallow ocean floors), and eat smaller animals and plants. Some smaller 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's 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 crayfish. Their narrow, dark green bodies are up to two feet long and weigh two to fifteen pounds. In each lobster, two large, body-length claws stretch forward to grasp prey, and a fan-shaped tail is used for propulsion. The group of true lobster species are very important foods in North America and Europe. American and European lobsters (Homarus americanus and Homarus gammarus) have enlarged claws. Cape lobsters (Homarus capensis) were formerly included in this group. Another popular species, Norway lobsters (Nephrops norvegicus) have longer, thinner claws. In 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, left-clawed, or ambidextrous.

The lobster head has two pairs of antennae, and its eyes are compound on the ends of mobile stalks. A female lobster lays 8,000 to two million eggs every two years and carries them under her tail for nine to eleven months until the young hatch. The young, initially three-tenths inches long, drift and swim for a month before settling on the bottom, at one 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. Out of 50,000 eggs, around two lobsters survive to legal fishing age. 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 some species can live for up to one hundred years. The European lobster lives an average of thirty-one years for males, and fifty-four for females, and the average American lobster lives for twenty 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 off Great 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 who 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 a common food crab in Eastern America. Two important groups, 1,500 species of Anomura (hermit crabs, squat lobsters, porcelain crabs, and hairy stone crabs) and 7,000 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. A crab 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. The life span of crab species varies greatly. Coconut crabs may live up to sixty years, while the blue crab usually lives one year.

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 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, often referred to as intersex in the twenty-first century. Some change sexes in the middle of their lives, while others, like the branchiopods, ostracods, and isopods, produce eggs that can successfully hatch without fertilization—a process called parthenogenesis. 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.

Principal Terms

Carapace: a hard, chitinous outer covering, such as a crustacean shell or insect exoskeleton

Cephalothorax: the anterior (front) section of a crustacean, consisting of a fused head and thorax

Chitin: a semitransparent, hard, horny substance forming much of crustacean shells and insect exoskeletons

Decapod: any crustacean having five pairs of locomotor appendages (legs)

Dorsal: at the hind (posterior) end of a living organism

Hermaphrodite: an organism having both male and female reproductive systems; intersex is more often used in the twenty-first century

Molting: shedding part of or all of a crustacean carapace (shell)

Bibliography

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