ANASPIDACEANS: Anaspidacea

PHYSICAL CHARACTERISTICS

Anaspidaceans (an-ah-spih-DAY-see-ans) are usually dull brown. They measure up to 1.9 inches (50 millimeters), but some species are less than 0.39 inches (10 millimeters). They have a head, thorax, and abdomen. They may or may not have a beaklike rostrum. The eyes are found either on the tips of stalks or on the head. Some species do not have eyes at all. The first pair of antennae, or antennules (an-TEN-yuls), is either branched or not. The second pair of antennae is biramous (BY-ray-mus) or branched. The mandibles are uniramous (YU-neh-RAY-mus) or unbranched. They do not have a shieldlike carapace covering the head and thorax. The first thoracic (thuh-RAE-sik) segment is tightly joined with the head. It has a pair of maxillipeds, thoracic limbs associated with the mouth.

The pairs of thoracic legs are either unbranched or branched. In biramous legs, the inner branch is called the endopod (IHN-doh-pawd), while the outer branch is called the exopod (EHK-soh-pawd). The flaplike gills, organs used for breathing underwater, are located on the bases of the legs. The movements of the exopods keep oxygen-carrying water flowing over the gills. The abdomen also has segments. There are one, two, or five pairs of pleopods (PLEE-oh-pawds), or limblike structures attached to the underside of the abdomen. The tip of the abdomen has a pair of long appendages called uropods (YUR-oh-pawds). The uropods are found on either side of a central tail segment, or telson . The uropods and telson sometimes form a fanlike tail.

GEOGRAPHIC RANGE

Anaspidaceans are found only in southeastern Australia (including Tasmania), New Zealand, Chile, and Argentina.

HABITAT

The larger species of anaspidaceans usually prefer to live in cool mountain streams, lakes, and swamps. Stream-dwelling species forage on boulders and smaller rocks on the stream bottom. Species living in swamps are found in the burrows of crayfish. Lake species live in mats of algae growing on the bottom. Smaller species live in the sands surrounding underground springs.

DIET

Anaspidaceans eat plant and animal materials. Larger species scavenge these materials by scraping them off rocks submerged in water.

BEHAVIOR AND REPRODUCTION

Anaspidaceans are poor swimmers and usually crawl about their habitat. The exopods of their thoracic legs move constantly to circulate oxygen-carrying water over the flaplike gills. When they walk, their thoracic and abdominal limbs move together in a smooth, rhythmic motion.

Both males and females are known, but mating has not been observed. Unlike most crustaceans that carry their eggs or young, female anaspidaceans lay their eggs individually on plants or stones. They do not guard or provide any care for them. The eggs hatch in 30 to 60 days as larvae (LAR-vee) that have working antennae and mouthparts only. Adulthood is reached through a series of molts, or sheddings of the exoskeleton. Additional appendages are added with each molt. Sometimes young anaspidaceans hatch with fewer than the adult number of appendages, adding additional appendages as they molt.

ANASPIDACEANS AND PEOPLE

Anaspidaceans and bathynellaceans are considered to be very primitive relatives of krill, crabs, shrimps, lobsters, and crayfish. Studying them may reveal how these animals have evolved, or gradually changed to survive in their environment, over millions of years.

SOMETHING FISHY DOWN UNDER

Water pollution is not the only threat to aquatic animals in Australia. Like all native freshwater crustaceans, anaspidaceans evolved in the absence of trout. Australian colonists introduced these fish from Europe. Without natural defenses against trout, anaspidaceans can only survive in isolated branches of streams and rivers that are out of reach from these hungry predators. Four species of anaspidaceans are listed as Vulnerable, but more species may be considered as scientists learn more about them.

CONSERVATION STATUS

Four species of Australian anaspidaceans are listed by the World Conservation Union (IUCN) as Vulnerable, or facing a high rate of extinction in the wild. These include the Tasmanian anaspid crustacean (Allanaspides helonomus), Hickman's Pygmy Mountain shrimp (Allanaspides hickmani), Great Lake Shrimp (Paranaspides lacustris), and Eucrenonaspides oinotheke.

NO COMMON NAME (Anaspides tasmaniae): SPECIES ACCOUNT

Physical characteristics: The body of this species is long, and the eyes are on stalks. The thoracic and abdominal segments are similar in length. The pleopods are long and leglike. The telson and uropods form a fanlike tail.

Geographic range: This species is found only in Tasmania, Australia.

Habitat: Anaspides tasmaniae live in freshwater streams and shallow pools.

Diet: They feed on both plant and animal tissues.

Behavior and reproduction: This species searches constantly for food, chewing on large pieces of plants or scraping the surfaces of pebbles with its mouth. They will also scavenge the bodies of small, dead animals. They walk by using both thoracic and abdominal appendages. To avoid danger, they will snap their body trunk and jump straight up.

Mating has not been observed. Eggs are laid on plants or bark under water. The young hatch as juveniles with fewer appendages than adults.

Anaspides tasmaniae and people: Anaspides tasmaniae are of interest to scientists studying crustacean evolution.

Conservation status: This species is not considered endangered or threatened. ∎

FOR MORE INFORMATION

Books:

Schram, F. Crustacea. Oxford, U.K.: Oxford University Press, 1986.

Periodicals:

Swain, R., and C. I. Reid. "Observations on the Life History and Ecology of Anaspides tasmaniae." Journal of Crustacean Biology 3 (1983): 163-172.

Web sites:

Lowry, J. K. Crustacea, the Higher Taxa: Description, Identification, and Information Retrieval. Version: 2 October 1999. http://www.crustacea.net/crustace/www/anaspida.htm (accessed on February 15, 2005).