Crustaceans

Crustaceans include crabs, lobsters, shrimp, krill, barnacles, and related species. There are approximately 40,000 crustacean species, the great majority of which are aquatic. Crustaceans are the only primarily aquatic group in the phylum Arthropoda, which also includes the insects and spiders. A few crustaceans, such as the familiar pillbug, have invaded terrestrial (land-based) habitats.

Like other arthropods, crustaceans are characterized by a segmented body, jointed appendages, and an external skeleton that offers protection from predators. The external skeleton, called a cuticle, is made of chitin . The cuticle is periodically molted in order to allow for growth. The period after molting can be a dangerous one, because the new shell is still soft and affords little protection.

Crabs

There are nearly 5,000 crab species. Most are marine, although terrestrial species exist as well. Crabs have ten jointed appendages, including two large claws for food capture called chelipeds, and eight walking legs that are used for walking sideways. An elliptical carapace protects the rest of the body.

Crabs have two stalked eyes and a pair of sensory antennae. Respiration occurs via gills, and special jointed mouthparts process food. Crabs have diverse diets: some are scavengers ; others are predators on clams and snails; and still others are herbivores , feeding on vegetation. The largest crab species reach sizes up to 3.7 meters (12 feet) across, including the legs. Crabs are an important seafood species.

Hermits and Fiddlers.

Two familiar crab groups are hermit crabs and fiddler crabs. Hermit crabs have the unique life history strategy of using discarded snail shells for protection. These crabs find new shells to inhabit as they grow.

Fiddler crabs are known for their asymmetry in claw size—one claw is much larger than the other. The large claw is used to communicate with other individuals—males raise and wave their claws in order to defend their territories, intimidate other males, or attract females. These displays allow competition to be resolved without dangerous physical combat.

Lobsters

Like crabs, lobsters have ten appendages, two claws and eight walking legs. A lobster can also snap its tail to propel itself quickly backward—this is most often used as an escape response when confronted with potential predators.

Lobsters have compound, stalked eyes, chemosensory antennae, and sensory hairs on various parts of the body to detect touch and motion. The antennae are particularly sensitive, responding to environmental chemical cues regarding food, potential mates, and predators.

Lobsters are predatory, and use their large claws to attack prey such as clams. The two claws of lobsters are adapted to different tasks—the crusher claw is used to break shells, whereas the ripper claw, which has finer teeth, is used to tear flesh. Legs and jointed mouthparts are used to manipulate prey items.

There are both marine and fresh-water lobster species. The largest lobsters grow to lengths of 1.2 to 1.5 meters (4 to 5 feet) and may live over 100 years. Like crabs, lobsters are considered a delicacy and represent an important seafood species.

Shrimp

Shrimp are small crustaceans that, like crabs and lobsters, have ten jointed legs. However, shrimp also have special swimmerets, small appendages along the abdomen, which enable them to swim. In addition, shrimp use sweeping motions of the tail to propel themselves backwards.

Shrimp species are marine or fresh-water. The shrimp's chitinous external skeleton is thinner than that of crabs and lobsters and is shed as the animal grows. The largest species reach nearly 23 centimeters (9 inches). By weight, more shrimp are eaten by humans worldwide than any other crustacean.

Barnacles

Barnacles are commonly seen attached to solid substrates such as piers, boats, or rocks, and are sometimes mistaken for mollusks because of their shells and sedentary lifestyle. Barnacles live in the high intertidal zone and filter small food particles from the water.

Before they attach permanently to a substrate, barnacles go through a mobile larval stage. Larvae then attach head first and begin to secrete shells of chitin, which are expanded as they grow. These shells are kept closed when the tide is out in order to prevent drying out. When submerged, barnacles open their shells and filter-feed by waving their legs to capture plankton .

Respiration occurs through gills on the legs. Many barnacles are hermaphrodites, producing both eggs and sperm.

Krill

Krill are small crustaceans that form large, dense swarms in Antarctic waters. They are intimately associated with sea ice. Krill are important in the diet of numerous Antarctic species, including fish, whales, seals, and penguins.^*

Krill are filter feeders that feed at night on the water surface. Their diet consists primarily of phytoplankton and algae.

Like all crustaceans and arthropods, krill shed their exoskeleton over time. Unlike other crustaceans however, shedding is not always the result of growth—krill sometimes shrink in size, using their own body resources for metabolism during the long, dark Antarctic winter when food availability is low.

The largest krill species attain lengths of up to 14 centimeters (5.5 inches). Krill are consumed by humans in some parts of the world.

Other Aquatic Arthropods

The horseshoe crab is not technically a crustacean—rather, it is an arthropod distantly related to spiders, which are arachnids. Horseshoe crabs have a large carapace and long tail. They eat clams and other invertebrates, crushing shells with their legs. Mating and egg-laying occur on beaches. Horseshoe crabs sometimes are described as "living fossils" because they do not appear to have evolved much in the last 400 million years.

There are numerous species of aquatic insects and arachnids. Many insects play important roles in aquatic ecosystems, including as crucial elements of the food chain. For example, mayflies spend a lengthy larval period in the water, living either in the water column or in the muddy bottoms. Mayflies are detritivores (animals that feed on dead organic matter), and play an important role in breaking down nutrients.

Dragonflies and damselflies are also found in aquatic habitats—their larvae are aquatic and highly predatory, capturing worms, small fish, and small amphibians. Insects such as mosquitoes are also characterized by an aquatic larval stage. Aquatic spiders feed on aquatic insects or small fish. Some aquatic spiders maintain an underwater air pocket to which they repeatedly transport fresh air.

see also Food from the Sea; Mariculture; Oceanography, Biological.

Jennifer Yeh

Bibliography

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 1990.

Gould, James L., and William T. Keeton, with Carol Grant Gould. Biological Science, 6th ed. New York: W. W. Norton & Co., 1996.

Hickman, Cleveland P., Larry S. Roberts, and Allan Larson. Animal Diversity. Dubuque, IA: Wm. C. Brown, 1994.

Internet Resource

The Lobster Conservancy. <http://www.lobsters.org>.

^* See "Ecology, Marine" for a photograph of a krill swarm.

Crustaceans

The crustaceans are a group of animals that belong to the class Crustacea in the phylum Arthropoda (organisms with segmented bodies, jointed legs or wings, and an external skeleton). The class includes a wide variety of familiar animals, such as barnacles, crabs, crayfish, copepods, shrimp, prawns, lobsters, water fleas, and wood lice. More than 30,000 species of crustacea have been identified, the majority of which live in water. Species that live in moist habitats on land, such as wood lice and pill bugs, are believed to have evolved from marine species.

Most crustaceans are free-living but some species are parasitic. Some even live on other crustaceans. Some species are free-swimming, while others crawl or burrow in soft sediments.

General structural characteristics

Despite such an extraordinary diversity of species, many crustaceans have a similar structure and way of life. The distinctive head usually bears five pairs of appendages (limblike attachments). Two pairs of these appendages are antennae that are used to detect food as well as to sense changes in humidity and temperature. Another pair of appendages are mandibles (jaws) that are used for grasping and tearing food. The final two pairs of appendages are maxillae, armlike projections used for feeding purposes.

The main part of the body is taken up with the thorax and abdomen. Both of these segments are covered with a tough outer skeleton, or exoskeleton. The exoskeleton is generally harder than it is in other arthropods because it contains limestone in addition to chitin (pronounced KITE-in), the usual skeletal material.

Attached to the trunk region are a number of other appendages which vary both in number and purpose in different species. In crabs, for example, one pair of appendages may be modified for swimming, another for feeding, another for carrying eggs, and yet another for catching prey.

Life patterns

Crustaceans exhibit a wide range of feeding techniques. The simplest of these techniques are used by species such as the tiny shrimps and copepods that practice filter feeding. In filter feeding, an animal rhythmically waves many fine oarlike structures known as setae back and forth. This motion creates a mini water current towards the mouth. Plankton and other suspended materials are carried into special filters and then transferred to the mouth.

Words to Know

Appendage: A limblike attachment extending from the main part of an animal's body.

Arthropoda: The largest single animal phylum, consisting of organisms with segmented bodies, jointed legs or wings, and exoskeletons.

Dioecious: A type of animal that exists as either male or female.

Exoskeleton: An external skeleton.

Free-living: An organism that is able to move about in its search for food and is not attached to some other organism as, for example, a parasite.

Hermaphrodite: An organism with both male and female sex cells.

Larva: An immature form of an organism capable of surviving on its own.

Molt: The process by which an organism sheds its outermost layer of feathers, fur, skin, or exoskeleton.

Parasite: An organism that lives in or on a host organism and that gets its nourishment from that host.

Seta: A thin, whiskerlike projection extending from the body of an organism.

Larger species such as crabs and lobsters are active hunters of small fish and other organisms. Other species adopt a scavenging role, feeding on dead animals or plants and other waste materials.

Smaller forms of crustaceans obtain the oxygen they need by gas exchange through their entire body surface. Most crustaceans, however, have special gills that serve as a means of obtaining oxygen. Simple excretory organs provide for the removal of body wastes such as ammonia and urea. Most crustaceans have a series of well-developed sensory organs that include not only eyes, but also a range of chemical and tactile (touch) receptors. All crustaceans are probably capable of detecting a light source, but in some of the more developed species, definite shapes and movements may also be detected.

Breeding strategies vary considerably among crustaceans. Most species are dioecious (pronounced die-EE-shus), that is, are either male or female. But some, such as the barnacles, are hermaphroditic. An hermaphroditic animal is one that possesses both male and female sex organs. Fertilization usually occurs sexually between two individuals. The fertilized eggs then mature either in a specialized brood chamber in some part of the female's body or attached directly to some external appendage

such as a claw. Most aquatic species hatch into a free-swimming larvae that progress through a series of body molts (where they shed their skin) until finally arriving at the adult size.

Crustaceans

Scientific Name: Amphipoda
(2 types of legs)

Scuds

There are over 50 different species of the scud. This fascinating crustacean looks and acts like saltwater shrimp so they are often called freshwater shrimp. Scuds have been successfully introduced into man-made tail waters, lakes and reservoirs. There is a wide variety of colors even among the same species, same generation and area. Males are smaller than the females and their diet and growth rate will vary. Scuds have two long antennae on top of the head that curve forward and down. On the six abdominal segments there are 6 pairs of appendages with 7 thoracic segments. The first thoracic segment and the head are fused together. There are 7 pairs of legs with the first 2 pairs being club-like claws. Scuds resemble an armadillo when they die because of the C-shaped fetal position they form. Their body colors are different grays, browns, olives, tans, creams, some bright green, opalescent blue, red and some transparent colors.

Sow Bugs: Isopoda

Common names for sow bugs are cress bugs and pill bugs. They are closely related to scuds and have the same life cycle as the scud. Other relatives are terrestrial potato bugs. Sow bugs have 8 very distinct flat segments, 2 pairs of antennae–one small and one large. On the rear segment there are two paddle-like flat tails. The underside of the body is white. These bugs are very slow crawlers and will helplessly drift in the current and may roll up in a pill shape. They are nocturnal and can be active in very cold water. Cold water conditions in the off seasons (fall, winter and spring) are excellent for fishing.

crustaceans See CRUSTACEA.