AMPHIPODS: Amphipoda

PHYSICAL CHARACTERISTICS

Amphipods come in a wide variety of shapes and sizes, but most have long, c-shaped bodies that are flat from side to side. Amphipods usually measure 0.2 to 0.6 inches (5 to 15 millimeters) in length, but some deep-sea species reach up to 9.8 inches (250 millimeters).

All amphipods have bodies that are made up of three regions: head, thorax, and abdomen. Both pairs of antennae are well developed and unbranched, or uniramous (YU-neh-RAY-mus). The compound eyes, if present at all, are not set on stalks. Each compound eye has multiple lenses. The eight-segmented thorax has eight pairs of uniramous appendages. The first thoracic (thuh-RAE-sik) segment is tightly joined, or fused, to the head. Its appendages are called maxillipeds (mack-SIH-leh-pehds). Maxillipeds are thoracic limbs that work together with the mouthparts. The remaining seven pairs of thoracic limbs are called pereopods (PAIR-ee-oh-pawds). The first two pairs of pereopods end in pincherlike claws that are used for grasping. The last five pairs of pereopods are used for burrowing, crawling, and jumping. Respiratory organs, or gills, are also found on the thorax.

The six-segmented abdomen has three pairs of appendages called pleopods (PLEE-oh-pawds). The pleopods are used for swimming and for moving oxygen-carrying water through the burrow. Another three pairs of abdominal appendages are called uropods (YUR-oh-pawds). The uropods are used for burrowing, jumping, and swimming. At the end of the abdomen is a small, taillike segment called the telson. Depending on the species, the telson is sometimes fused to the last abdominal segment.

GEOGRAPHIC RANGE

Amphipods are found worldwide.

HABITAT

Most amphipods live on the ocean bottom and burrow in mud or debris. Many live in the open sea and are usually found on floating, jellylike animals, such as jellies and ctenophores. Others burrow into sandy beaches. About 1,200 species are found in fresh water, where they live among decaying leaves. About one hundred species live in moist habitats on land.

DIET

Amphipods eat plants, small animals, or scavenge dead bodies. One group lives on the bodies of marine mammals and eats their skin. Others attach themselves to the bodies of jellies or feed inside the bodies of tunicates.

BEHAVIOR AND REPRODUCTION

Some marine amphipods spend their entire lives attached to floating mats of algae or to jellies and their relatives. One group lives as external parasites on the bodies of dolphins, porpoises, and whales. Beach hoppers and their relatives live under decaying vegetation or dig burrows in mud or sand.

Both males and females are required for reproduction. Males attach themselves to the females and transfer their sperm directly to the opening of her reproductive organs. Females brood their eggs in a pouch under the thorax made up of special thoracic plates. Newly hatched amphipods look very similar to the adults.

ENDANGERED AMPHIPODS LIVING IN THE SHADOWS OF THE NATION'S CAPITAL

The Hay's Spring amphipod, Stygobromus hayi, lives only in five underground springs along Rock Creek in Washington, DC. First discovered in a spring at the National Zoo in 1938, they were listed as Endangered, or facing very high risk of extinction in the wild, in 1982. Very little is known about these small (0.4 inches; 10 millimeters), blind, and colorless animals. They are threatened by limited habitat and pollution in the form of fertilizers and pesticides.

AMPHIPODS AND PEOPLE

In many freshwater and marine habitats, amphipods help break down and recycle decaying plant and animal matter. They are also eaten by other animals that are harvested as food for people.

CONSERVATION STATUS

Seventy-one species of amphipods are listed by the World Conservation Union (IUCN). Two species are listed as Extinct, or no longer in existence; seven as Critically Endangered, facing extremely high risk of extinction in the wild; six as Endangered or facing a very high risk of extinction in the wild; and fifty-six as Vulnerable or facing high risk of extinction in the wild. Nearly all of these are freshwater species found in caves or underground springs. The parasitic species of dolphins, porpoises, and whales are not listed, even though many of their hosts may be considered threatened or endangered.

SKELETON SHRIMP (Caprella californica): SPECIES ACCOUNTS

Physical characteristics: Skeleton shrimp measure up to 1.38 inches (35 millimeters) in length. They have long, slender bodies with very small abdomens. The pincherlike claws of the pereopods are used for grasping and climbing.

Geographic range: This species is found only along the coast of central and southern California.

Habitat: Skeleton shrimp live just below the tide line, where they cling to algae and the slender, plantlike structures of some groups of marine animals.

Diet: They eat both plant and animal tissues found on the bottom or floating in the water.

Behavior and reproduction: Skeleton shrimp scrape the sea bottom to gather food.

Fertilized eggs are brooded in a chamber made by broad, leaflike thoracic appendages. The newly hatched juveniles look very much like the adults.

Skeleton shrimp and people: Skeleton shrimp do not impact people or their activities.

Conservation status: Skeleton shrimp are not considered to be threatened or endangered. ∎

SPERM WHALE LICE (Neocyamus physeteris): SPECIES ACCOUNTS

Physical characteristics: Unlike many amphipods, the yellow to orange bodies of this species are flattened top-to-bottom, not side-to-side. They measure up to 0.4 inches (10 millimeters). They use their hooklike claws to latch on to the skin of sperm whales. Their thoracic gills are clearly visible and resemble clumps of tiny fingers.

Geographic range: Sperm whale lice are found on sperm whales, which occur in all of the world's oceans.

Habitat: They live only on the bodies of sperm whales.

Diet: They scavenge organisms attached to the whale's skin and eat the skin itself.

Behavior and reproduction: Sperm whale lice spread to other whales by jumping onto them when infested whales rub up against other whales. Whale calves are infested by amphipods that come off their mothers when they touch.

Eggs are brooded in a pouch made by broad, leaflike appendages underneath the thorax. Newly hatched sperm whale lice strongly resemble the adults.

Sperm whale lice and people: This species does not impact people or their activities.

Conservation status: Sperm whale lice are not considered by the World Conservation Union (IUCN) to be threatened or endangered, although their only habitat, the sperm whale, is listed as Endangered, or facing very high risk of extinction in the wild. ∎

BEACH HOPPER (Orchestoidea californiana): SPECIES ACCOUNTS

Physical characteristics: Beach hoppers measure up to 1.1 inches (28 millimeters) in length. They have curved bodies that are flat from side-to-side. Their compound eyes are very small. The long second pair of antennae is bright orange to rosy red.

Geographic range: Beach hoppers are found along the west coast of North America, from Vancouver Island, British Columbia, south to Laguna Beach, California.

Habitat: They live on beaches with fine sand and backed by dunes.

Diet: They eat seaweed washed up on the beach.

Behavior and reproduction: Beach hoppers feed at night to avoid both the daytime heat and being eaten by shorebirds. Adults burrow down to 12 inches (300 millimeters) beneath the surface and stay there during the day. They jump by using the rear of their abdomens and their uropods as a spring.

Adults mate in their burrows from June until November. The male deposits a jellylike mass of sperm on the underside of the female and soon leaves the burrow. The dark blue eggs are brooded inside a pouch made by broad, leaflike appendages on the thorax. Newly hatched juveniles closely resemble the adults.

Beach hoppers and people: Beach hoppers do not impact people or their activities.

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

FOR MORE INFORMATION

Books:

Brusca, R. C., and G. L. Brusca. Invertebrates. Second edition. Sunderland, MA: Sinauer Associates, Inc., 2003.

Morris, R. H., D. P. Abbott, and E. C. Haderlie. Intertidal Invertebrates of California. Stanford, CA: Stanford University Press, 1980.

Periodicals:

Holsinger, J. R. "The Freshwater Amphipod Crustaceans (Gammaridae) of North America." Biota of Freshwater Ecosystems, Identification Manual No. 5, Environmental Protection Agency (1972): 17-24.

Web sites:

Amphipoda (Pericarida, Malacostraca). http://www.crustacea.net/crustace/www/amphipod.htm (accessed on March 22, 2005).

The Biology of Amphipods.http://www.mov.vic.gov.au/crust/amphbiol.html (accessed on March 22, 2005).

Endangered Species Bulletin. "Endemic Amphipods in Our Nation's Capital—Brief Article." http://endangered.fws.gov/esb/2002/01-02/toc.html (accessed on March 22, 2005).

Subterranean Amphipod Database.http://web.odu.edu/sci/biology/amphipod/ (accessed on March 22, 2005).