CEPHALOCARIDS: CephalocaridaNO COMMON NAME (Hutchinsoniella macracantha): SPECIES ACCOUNT
Cephalocarids (sef-fal-oh-KAR-ids) are small crustaceans, measuring 0.078 to 0.146 inches (2 to 3.7 millimeters) in length. The eyeless head is short, broad, and covered by a horseshoe-shaped shield. There are two pairs of antennae and two pairs of jaws. The first pair of antennae is not branched; it is uniramous (YU-neh-RAY-mus). The second pair is branched or biramous (BY-ray-mus). The second pair of jaws is also biramous. It is the maxillae (mack-SIH-lee), which follows the uniramous first pair of jaws and resembles the appendages on the rest of the body. Maxillipeds (mack-SIH-leh-pehds), fingerlike limbs associated with the mouth, are absent. The thorax, or midbody, has eight segments, each with a pair of paddlelike, biramous limbs. The abdomen or tail section has 11 segments with no limbs at all. The tip of the tail, or telson, has a pair of long, threadlike, uniramous appendages.
Cephalocarids are usually found on or just below the surface of the muck that settles on the sea bottom, from shallow waters to depths of 5,250 feet (1,600 meters). This muck is rich in plant and animal materials. A few species are found in sand or the rubble that accumulates around coral reefs.
Using the rhythmic beat of their paddlelike limbs, cephalocarids draw water with bits of food into their mouth. As they spread their limbs, food-carrying water is pulled into a groove on the underside of their bodies that leads to the mouth.
BEHAVIOR AND REPRODUCTION
Cephalocarids swim and burrow through the muck by using their limbs. They sometimes fold their bodies in half to clean themselves.
Cephalocarids are hermaphrodites (her-MAE-fro-daits), with individuals having both male and female reproductive organs. Eggs are carried by small appendages on the midbody. The hatching cephalocarid is eyeless and has only three pairs of functional limbs, all located on the head. This is one of its three stages as a larva (LAR-vuh), or young animal that must go through changes in form before becoming an adult. As the larvae (LAR-vee; plural of larva) grow and molt, or shed their exoskeletons, additional body segments and limbs are added.
CEPHALOCARIDS AND PEOPLE
Cephalocarids are of scientific interest because they are thought to have features similar to the most ancient of crustaceans.
A CLASS IS BORN
In 1954, eight individuals of a new crustacean species were dredged up from the bottom of Long Island Sound. Their eyeless, horseshoe-shaped heads, long, slender bodies, similar midbody limbs, no rear segment appendages, and other features distinguished them from other groups of crustaceans. Dr. Howard Sanders of Yale University named the new class Cephalocarida from the Greek words kephale, meaning head, and karis, or shrimp. Cephalocarids are thought to be among the most primitive of all living crustaceans.
No cephalocarids are considered threatened or endangered.
Physical characteristics: The head and midbody segments are wider than the abdomen or tail section. Some pairs of limbs are reduced in size and resemble small rounded projections.
Habitat: Hutchinsoniella macracantha live in the muck that settles on the sea bottom.
Diet: They eat bits of plant and animal materials.
Behavior and reproduction: Hutchinsoniella macracantha are usually found on or just below the surface of the sea bottom. The larva molts 19 times before reaching adulthood.
Hutchinsoniella macracantha and people: This species is not known to impact humans or their activities.
Conservation status: This species is not considered threatened or endangered. ∎
FOR MORE INFORMATION
Hessler, R. R. Cephalocarida: Living Fossil Without a Fossil Record. In Living Fossils. N. Eldredge and S. M. Stanley, eds. New York: Springer Verlag, 1984.
Schram, F. Crustacea. Oxford, U.K.: Oxford University Press, 1986.
Sanders, H. L. "The Cephalocarida, a New Subclass of Crustacea from Long Island Sound." Proceedings of the National Academy of Sciences 41 (1955): 61-66.
Brachypods (Cephalocarida). http://www.crustacea.net/crustace/www/brachyp.htm (accessed on January 21, 2005).
Introduction to the Cephalocarida.http://www.ucmp.berkeley.edu/arthropoda/crustacea/cephalocarida.html (accessed on January 21, 2005).