Crabs, Shrimps, and Lobsters: Decapoda

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RED KING CRAB (Paralithodes camtschaticus): SPECIES ACCOUNTS


Decapods, including crabs, shrimps, lobsters, and crayfishes, are among the most familiar of all crustaceans. They come in a wide variety of sizes, ranging from tiny pea crabs to the giant Japanese spider crab Macrocheira kaempferi with spidery legs spanning up to 12 feet (3.7 meters) across. Many species have distinctive color patterns, and some are able to change their colors. Despite their incredible variety, all decapods have the same basic body plan with three body regions: head, thorax, and abdomen. The head and thorax are closely joined together, or fused, to form the cephalothorax (SEH-feh-lo-THOR-acks). A shieldlike carapace (CARE-eh-pes) covers the cephalothorax. The carapace also covers the sides of the body and protects the breathing organs, or gills.

The head sometimes has a beaklike projection called a rostrum and two distinct pairs of long antennae. The first pair of antennae, or antennules (an-TEN-yuls), is branched, or biramous (BY-ray-mus). They are used mostly to detect odors. The second pair of antennae is uniramous (YU-neh-RAY-mus), or not branched. This pair is used mainly as organs of touch. The compound eyes are set on the tips of stalks. Each compound eye has multiple lenses. Depending on the species, the uniramous jaws, or mandibles, are used for slicing flesh, grinding plant materials, or crushing shells.

The first three segments of the thorax are closely joined, or fused, with the head. The appendages of these first three segments are called maxillipeds (mack-SIH-leh-pehds). Maxillipeds are thoracic (thuh-RAE-sik) limbs that work together with the mouthparts. The leglike limbs, or pereopods (PAIR-ee-oh-pawds), of the remaining five thoracic segments are either uniramous or weakly biramous and are used mainly for walking. The first few pairs of legs, especially the first pair, often have claws that are used for feeding, mating, and defense. Fast, slender claws are used to grab alert prey. Large strong claws with toothlike surfaces are used to crush the shells of clams, snails, and other hard-shelled prey.

The six-segmented abdomen has pairs of appendages underneath called pleopods (PLEE-oh-pawds). In lobsters, crayfishes, and shrimps, the abdomen is long, thick, and powerful and is used for swimming. At the end of the abdomen is a pair of slender biramous appendages, the uropods (YUR-oh-pawds). In between the uropods is a taillike segment called the telson. The telson is not tightly joined with, or fused, to the last abdominal segment. The telson and uropods work together to form a fanlike tail. Hard-bodied decapods, such as lobsters, snap their abdomens with fanlike tails forward underneath their bodies to propel themselves backward through the water. Shrimps and other softer-bodied species use their abdomens and tails to swim forward in the water. However, the bodies of crabs are relatively short and compact. They have lost most of their abdominal appendages. The crab's abdomen is short, flat, and folded forward under the body. It plays no role in swimming and has just a few pairs of pleopods that are used only for carrying eggs (females) or mating (males).


Decapods are found worldwide.


Approximately ninety percent of all species live in the ocean. They are found in all kinds of habitats, including mud flats, mangrove forests, rocky shores, muddy or sandy beaches, sea grass beds, coral reefs, open water, and sea bottoms, including deep sea geysers known as hydrothermal vents. Many species dig burrows in these habitats and come out only under the protection of dark to look for food. Freshwater species live in a variety of habitats, including mountain streams, rivers, ponds, and along lakeshores. Only one percent of all decapods are terrestrial. These species are sometimes found at elevations of up to 3,280 feet (1,000 meters) or as far as 9 miles (15 kilometers) away from the ocean. They still depend on the sea and must return there to reproduce.


As a group, decapods are omnivorous, which means they eat both plant and animal materials. Lobsters, shrimps, and many crabs mostly prey on other animals, including fish, worms, mollusks, and other crustaceans. Occasionally, when the opportunity arises, they will scavenge the dead bodies of these and other marine animals. Some crabs and shrimps are filter feeders. They use their bristly antennae, maxillipeds, or legs to strain out bits of food floating in the water. Other filter feeders stir up sand and mud to strain out food that has settled on the bottom. Many marine and freshwater decapods eat plants, but will sometimes feed on animal flesh when it is available.


Decapods show many complicated and even amazing behaviors. For example, Caribbean spiny lobsters form a long row of up to 65 individuals that march single-file toward deeper water. It is not well understood why they do this, although it may have something to do with avoiding storms during the winter. Juvenile red king crabs often gather together into mounds that may contain thousands of individuals. This behavior is believed to prevent them from being eaten by predators.

Some species form special relationships with other organisms. Certain shrimps establish cleaning stations where fishes line up to have their parasites removed. Fish parasites are worms, crustaceans, and other animals that live on their bodies and eat their blood and other body fluids. Other shrimps share a burrow with a fish. The fish watches for danger as the shrimp builds and maintains the burrow.

Some species make signals that are seen or heard by other members of the same species. Decapods that spend some or most of their lives on land produce sounds by rubbing one part of their body against another. Marine and freshwater species release special chemicals, or pheromones (FEH-re-moans), into the water along with their waste through special glands on the antennae. The pheromones are used to attract mates.

Except for one kind of crayfish, all decapods require males and females to reproduce. In some shrimps, the adults mature first as males and later develop into females. A few species keep both male and female reproductive structures and are called hermaphrodites (her-MAE-fro-daits).

Decapods have many kinds of courtship. In fiddler crabs the male has claws that are much larger than those of the females. Useless for feeding, the oversized male claws are used only for fighting with other males and attracting females. Depending on the species, mating is very brief or may occur only after males and females have spent long periods of time together. In these cases the female can only mate just after she molts, or sheds her external skeleton (exoskeleton). As they prepare to molt, adult females release pheromones to attract males. A male will sometimes grasp the female with its legs for several days or weeks until she finally molts. Sperm is transferred to the female as a fluid or inside packets. The sperm is deposited directly into the reproductive organs of the female or into a special storage sac in her body. In some species the male stands guard over the female to prevent other males from mating with her.

Most female decapods hold their egg masses with their pleopods. They keep the eggs clean and make sure that plenty of oxygen-carrying water is circulated around them. Just before hatching, the eggs release a chemical that tells the female to shake the mass to help release the larvae (LAR-vee), or young animals, into the water as they hatch. After hatching, parental care is rare. Young crayfishes will stay with their female parents for protection. In some tropical crabs that breed in freshwater trapped at the bases of plants growing on tree limbs, the females provide food for their larvae and protect them from predators.


Lobsters and crabs are capable of learning. They can be trained to find their way through a maze. When some crabs are offered a prey item that they have never seen before, they will quickly learn the most efficient way to eat it. Some species can be taught to act a certain way in response to a specific situation. Others can even learn to distinguish between different colors.


Most of the crustaceans harvested as food for humans are in the order Decapoda. Shrimps and crayfishes are raised on aquatic farms and sold as food throughout the world. Depending on where and what decapods eat, some species can become poisonous to the people that eat them. Others, if they are not cooked properly, may carry parasites that infest the bodies of people and make them sick.

In some areas, land crabs are considered pests in rice fields. They eat the rice plants and dig burrows that drain water away from the plants. The accidental introduction of the European green crab to the eastern coast of the United States has caused serious harm to clam beds harvested for food. Foreign species of crayfishes have damaged crops and threatened to reduce populations of native crayfish species.


There are 197 species of decapods listed by the World Conservation Union (IUCN), most of which are species of freshwater crayfish that live in very small or limited habitats. They are especially vulnerable to habitat destruction and loss and may become threatened in the future.


Physical characteristics: The dark red, nearly black body of the red swamp crayfish measures up to 4.7 inches (120 millimeters). Its pincherlike claws are long and slender, with bright bumps and red tips.

Geographic range: Originally from the southern United States and northern Mexico only, this species is now widely established in Europe, Africa, Central and South America, and Southeast Asia.

Habitat: This species is tolerant of a wide range of habitats, even slightly salty water. They prefer to live in slow moving streams, swamps, and ponds, where there are plenty of plants growing along the shores and leaves on the bottom. Their populations are the largest in habitats that flood each year.

Diet: Red swamp crayfishes are omnivorous and eat many different kinds of plants and small animals, such as snails, insects, fishes, and tadpoles.

Behavior and reproduction: During the dry season they will sometimes dig burrows down 2 feet (0.6 meters) or more to reach water.

Adult males have two body forms. The first form has large claws, hooks at the bases of some legs, and spends its time looking for a mate. After mating, males molt into the second form, which has smaller claws. Females can store sperm for long periods of time. They brood their eggs for two to three weeks and produce two generations each year. The hatchlings resemble small crayfishes and stay with the female for several weeks. They live for a total of twelve to eighteen months in the wild.

Red swamp crayfishes and people: This species is sold as a pet and as fish bait. It has been introduced around the world because it grows fast and can live in a wide variety of habitats. Red swamp crayfishes were introduced to eastern Africa to eat snails that carry parasites harmful to humans.

Conservation status: The World Conservation Union (IUCN) does not consider this species to be threatened or endangered. ∎


Physical characteristics: Adult Harlequin shrimp measure about 2 inches (50 millimeters) in length. The claws on the second pair of legs are very large, distinctly flat, and platelike in shape. Both the body and claws have bright purple markings or red blotches on a white or cream-colored background.

Geographic range: This species is found along the shores of East Africa, the Red Sea to Indonesia, and across northern Australia to Hawaii, Panama, and the Galápagos Islands.

Habitat: This species lives in and hides among coral reefs.

Diet: Harlequin shrimp pry sea stars off coral reefs with their large, flat claws and eat them.

Behavior and reproduction: Pairs are territorial. Single individuals are much more active than those in pairs.

Adult males and females live and defend their territory together. Females molt every eighteen to twenty days and mate soon after. They produce about one thousand eggs at a time. Eggs hatch within eighteen days. The larvae are well developed and spend a short time floating in the water with other plankton. Plankton is made up of plants and animals that live in open water and are at the mercy of ocean currents.

Harlequin shrimps and people: Harlequin shrimp are popular pets because they are easy to breed and raise in captivity. They might also play a role in conserving coral reefs because they eat small, coral-eating crown-of-thorns sea stars.

Conservation status: The World Conservation Union (IUCN) does not consider this species to be threatened or endangered. ∎

RED KING CRAB (Paralithodes camtschaticus): SPECIES ACCOUNTS

Physical characteristics: Red king crabs are a very large, reddish brown species with a carapace measuring up to 11 inches (280 millimeters) wide. Their legs stretch more than 6 feet (1.8 meters) across. Both the carapace and legs are covered with lots of sharp spines. The right claw is larger than the left. Only three pairs of walking legs are visible.

Geographic range: This species lives in the Sea of Japan to northern British Columbia. It was introduced into the Barents Sea and has spread westward to Norway.

Habitat: Red king crabs live at depths of 10 to 1,190 feet (3 to 366 meters) and prefer open habitats with sandy or muddy bottoms.

Diet: They prey on a variety of bottom-dwelling invertebrates, including brittle stars, sea stars, sand dollars, and sea urchins, barnacles, worms, mollusks, and sponges.

Behavior and reproduction: Two-year-old juveniles often gather by the hundreds or thousands to form spectacular mounds known as pods in shallow water. The pods are thought to discourage predators; they disperse shortly after dusk, as the crabs forage for prey, and form again before dawn.

Mating occurs just after the female molts, and her exoskeleton is still soft. The male uses his small fifth pair of walking legs to spread sperm over the female's pleopods. Anywhere from 150,000 to 400,000 eggs come out of the female's body right away, but they take almost a year to hatch. The larvae do not resemble the adults and molt four times before settling on the ocean bottom.

Red king crabs and people: Red king crabs were once the target of one of the most valuable fisheries off Alaska in United States waters. Commercial harvesting is carried out with large baited pots.

Conservation status: The World Conservation Union (IUCN) does not consider this species to be threatened or endangered. However, populations off the coast of Alaska and in the western Bering Sea have suffered as a result of over-fishing. A population introduced to the Barents Sea appears to be growing. ∎


Physical characteristics: The carapace of adult sand fiddler crabs are about 1 inch (26 millimeters) wide. The eyestalks are very long and slender. The claws of males are distinctly unequal in size, while those of females are smaller and equal in size.

Geographic range: This species is found along the eastern coast of the United States, from Cape Cod, Massachusetts, south to Pensacola, Florida.

Habitat: Sand fiddler crabs live on sandy beaches that open to the sea or on sand-mud beaches in protected areas along salt marsh edges and tidal creek banks.

Diet: They eat bits of plants, bacteria, and other organisms.

Behavior and reproduction: Sand fiddler crabs feed at low tide, scraping up mud with their claws and pulling out bits of food with their mouth parts. Males only use their small claw to feed, while females use both claws. They dig and live in simple burrows in the sand and come out only at low tide to feed. When the tide starts to come in, they will plug the burrow entrance overhead with sand.

Males sit near the entrances of their burrows, where mating occurs, waving their large claw and beating it against the sand to attract females. The female remains in the burrow for about two weeks, until the eggs are about ready to hatch. The larvae do not resemble the adults, and are released into the water at night during high tide and carried away by the current. After six to eight weeks and five molts, the larvae return and settle on the bottom.

Sand fiddler crabs and people: They are occasionally sold as pets.

Conservation status: The World Conservation Union (IUCN) does not consider this species to be threatened or endangered. ∎


Physical characteristics: The dark bodies of adult giant tiger prawn have several distinct black and white bands and reach 13.2 inches (336 millimeters) in length.

Geographic range: They are found off the eastern coast of Africa and the Red Sea, east to India, Australia, and Japan.

Habitat: Juveniles live near the shore and in mangrove estuaries (EHS-chew-AIR-eez). The adults prefer waters with silty or sandy bottoms and live down to depths of 525 feet (162 meters).

Diet: Giant tiger prawn eat mostly smaller shrimps, crabs, mollusks, and other animals, as well as algae (AL-jee). They will also swallow sand and silt mixed with bacteria and bits of plants and animals. Bacteria are single-celled organisms that break down the tissues of dead organisms.

Behavior and reproduction: During the day, giant tiger prawn remain buried on the sea bottom. Groups of two hundred to three hundred individuals have been observed swimming in shallow water at dawn and dusk.

Giant tiger prawn mate at night, just above the sea bottom, right after the female molts. The male deposits sperm in a special structure underneath the female's thorax. Females release 250,000 to 800,000 eggs into the water, where they hatch in less than eighteen hours. They do not resemble the adults and have only mandibles and two pairs of antennae for appendages. The non-feeding larvae use these appendages to swim in the water. The larvae reach adulthood in about twelve days and live less than a total of two years.

Giant tiger prawns and people: Giant tiger prawn are harvested by boats dragging nets in the water. They are an important species for aquatic farms because they grow large very quickly and bring a high price at the market.

Conservation status: The World Conservation Union (IUCN) does not consider this species to be threatened or endangered. ∎



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Debelius, H. Crustacea Guide of the World. Frankfurt, Germany: IKAN, 1999.

Factor, J. R. Biology of the Lobster. New York: Academic Press, 1995.


Herrnkind, W. F. "Strange March of the Spiny Lobster." National Geographic (June 1975) 147, no. 6: 819-831.

Web sites:

Crayfish Homepage. (accessed on March 15, 2005).

Crustacean Gallery. (accessed on March 15, 2005).

Crustacean Printouts. (accessed on March 15, 2005).

Fiddler Crabs (Genus Uca). (accessed on March 15, 2005).

Lobster FAQs. (accessed on March 15, 2005).