Crabs and lobsters

Crab and Lobster Facts

Classification:

• Kingdom: Animalia
• Phylum: Arthropoda
• Subphylum: Crustacea
• Class: Malacostraca
• Order: Decapoda
• Families: Homaridae or Nephropsidae (true lobsters),
• Palinuridae (spiny lobsters or sea crayfish),
• Scyllaridae (slipper, Spanish, or shovel
• lobsters), Polychelidae (deep-sea lobsters);
• Brachyryncha (true crabs), Anomura (irregular-tailed crabs)
• Geographical location: All the earth’s oceans, except at the poles
• Habitat: Mostly marine, although some crab species live much of their lives on land near salt water
• Gestational period: Lobsters, eggs hatch in one year; crabs, depending on the species, can range from a few weeks to almost a year.
• Life span: Lobsters, up to fifty years; crabs, depending on the species, can live anywhere from one to seventy years
• Special anatomy: Segmented exoskeleton; large claws on front pair of legs (one often larger than the other); eyes on the ends of stalks

Crabs and lobsters are joint-legged animals that belong to the phylum Arthropoda. This diverse phylum of animals also includes the insects, spiders, ticks, mites, millipedes and centipedes. Phylum characteristics include a chitinous exoskeleton, jointed appendages, a ventral nervous system, and a dorsal brain. Both crabs and lobsters are crustaceans, which are placed in the class Malacostraca along with the shrimp. Crabs differ from lobsters in having a broad upper shell or carapace and an abdomen that is shorter and either tapered or tucked forward beneath the carapace.

Because they have five pairs of legs, crabs and lobsters belong to the order Decapoda. The first one or two pairs of legs are typically enlarged and modified as chelicera, which are variously used for feeding or defense. In some decapods they may also have a sexual function. The remaining three or four pairs of legs are used for walking, or, in some species, swimming or climbing.

Crabs

There are two kinds of crabs. The irregular-tailed crabs, or Anomurans, include the hermit, porcelain, and mole crabs. Some have a small, tapered abdomen, and others, especially the hermit crabs, have a somewhat coiled abdomen that is tucked into a discarded snail shell. The true crabs, or Brachyurans (short-tailed crabs), have a greatly reduced abdomen that is tucked forward beneath the carapace.

Contrary to popular belief, crabs do not always scuttle sideways; they can walk forward or backward with equal facility. A few of the fast moving crabs seem to use two pairs of legs when running. The blue crab (Callinectes sapidus) family, which also includes the familiar lady crab (Ovalipes ocellatus) have a paddlelike last pair of appendages and are fast and agile swimmers.

Crabs vary greatly in size and shape. The smallest are pea crabs, which measure only a few millimeters across the carapace, while the legs of a Japanese spider crab can span up to four meters. The differences in size and shape have allowed crabs to colonize a wide range of habitats. Most of the 4,500 species of true crabs are marine, inhabiting the rocky intertidal zone or burrowing in sand and mud. Some species are benthic and others invade freshwater, especially in the tropics. Still others are terrestrial or semiterrestrial, often ranging far away from water, although they generally prefer moist places. A tree-dwelling tropical species feeds on coconuts.

Ecologically, almost all crabs function as scavengers, feeding on dead or dying animals or organic debris of any kind. Many species are also active predators, capable of killing small fish and breaking open the shells of different mollusks. Porcelain crabs, for example, have one of their mouthparts equipped with long, feathery setae that are swept through the water. They catch small particles and transfer them to other parts of the mouth, where they are eaten. Organic matter is dismembered by the pincers and conveyed to the mouth by a series of appendages called maxillipeds and maxillae. A pair of comblike mandibles guard the mouth and chop the food into tiny pieces, which are then swallowed. A short esophagus leads to a gastric mill analogous to the gizzard of other animals, where food is ground up.

Crab Reproduction

Reproductive behaviors vary, but during courtship males may use various chemical, visual, or acoustic signals including pheromones, waving chelipeds back and forth, or drumming at the entrance to the female’s burrow. Other males build pyramid-shaped mounds to attract females. Following courtship the male transfers sperm to her in a packet called a spermatophore. The eggs are attached to the female’s abdominal appendages and form an egg mass between the abdomen and the thorax. The abdomen is therefore out of place and hangs downward when the female is carrying eggs. The eggs are usually brightly colored with carotenoid pigments, and can be red, yellow, blue, brown, or green. They range in size and number between species, but a large marine crab can lay up to a million eggs at a time. The tiny larvae are called zoea and are very common summer components of marine plankton. It takes one or two more molts before the crablike form is evident. Many of the semiterrestrial and freshwater species develop directly into miniature crabs.

Lobsters

Millions of American lobsters (Homarus americanus) are harvested for food each year, making them possibly the most commercially important of all crustaceans. Lobsters differ from crabs in having an extended thorax. Most are large: up to sixty centimeters in length and twenty-two kilograms in weight. The first pair of walking legs is modified into large claws or chelipeds. The European lobster (Homarus gammarus) is similar in shape but somewhat smaller. Lobsters walk forward using their walking legs, but when threatened swim rapidly backward using their broad tail segment or telson.

Ecologically, lobsters are creatures of rocky subtidal zones that rarely extend activities shoreward. Eggs are laid in late summer, generally July and August of every other year, and are glued to the female’s abdominal appendages. They hatch in about a year and the young remain attached for some time before drifting free and becoming part of the plankton.

The spiny and slippery lobsters of the tropics and pantropics are not closely related to the American lobster. Some reach the size of American lobsters, but lack pincers. Spiny lobsters are noted for their mass migrations during fall, in which up to fifty individuals march together, heads-to-tails, in long queues. Females are capable of producing four million eggs during the spring reproductive season. The transparent juveniles attach to jellyfish and undergo a series of molts for about a year, gradually transforming into a five-centimeter lobster. Lobsters actively forage for live prey, including worms, mollusks, crabs, and small fishes, as well as organic debris.

Crustaceans as Food

Crabs are food for a variety of other animals, including fish and larger invertebrates. To protect themselves, some species carefully place sea anemones on their carapace, while others, such as the decorator crab, glue pieces of shell all over their carapace. Still other species, such as the pea crab, take refuge inside oysters and other shellfish, while boring crabs dig burrows into mud substrates or soft limestone rocks.

Both crabs and lobsters are important food for humans as well. Crabs are caught in traps or by crab dredges. The most highly prized are soft-shelled crabs, which are really the ordinary species of crabs that are caught just after molting and before they have had time to secrete a new shell. Both American and European lobsters are the centers of a fisheries industry measured in the billions of dollars annually. They are caught in baited lobster pots. Coastal pollution and over-fishing coupled with an apparent die-off of the American lobster population in the Northeast has spawned millions of federal dollars in research to determine causes and provide restoration protocols. With the demise of the American lobster population, most of the frozen lobster tails shipped to market are the spiny or slippery lobsters, which are caught in tropical and subtropical waters of the world.

Principal Terms

abdomen: posterior portion of a crustacean

cephalothorax: combined head and thorax

chelicerae: also called pincers, claws, or chelipeds, these are the enlarged first and sometimes second pairs of walking legs, which are used for feeding, defense, or sexual recognition

decapods: animals with ten appendages (from Greek deca, “ten,” plus poda, “foot or leg”)

thoracic: related to the middle section of the body of arthropods, which are generally subdivided into a head, thorax, and abdomen; both thorax and abdomen may have several segments

Bibliography

Burgren, W. W., and B. R. McMahon, eds. Biology of Land Crabs. New York: Cambridge University Press, 1988. Print.

Cobb, J. S., and B. F. Phillips, eds. The Biology and Management of Lobsters. 2 vols. New York: Academic Press, 1980. Print.

Gosner, K. Field Guide to the Atlantic Seashore. Boston: Houghton-Mifflin, 1978. Print.

Ingle, R. W. Crayfishes, Lobsters, and Crabs of Europe: An Illustrated Guide to Common and Traded Species. London: Chapman, 1997. Print.

Moe, Martin, Jr. Lobsters: Florida, Bahamas, and the Caribbean. Plantation: Green Turtle, 1991. Print.

Warner, G. F. The Biology of Crabs. New York: Van Nostrand Reinhold, 1977. Print.

Weis, Judith S. Walking Sideways; The Remarkable World of Crabs. Ithaca: Cornell UP. 2012. Print.

Williams, A. B. Shrimps, Lobsters, and Crabs of the Atlantic Coast of the Eastern United States, Maine to Florida. Washington, D.C.: Smithsonian Institution Press, 1984. Print.