MUSSEL SHRIMP: Ostracoda

PHYSICAL CHARACTERISTICS

The bodies of mussel shrimp are completely surrounded by a folded, shieldlike carapace (CARE-eh-pes) that resembles an upside down taco. Depending on the species, the carapace may be smooth, bumpy, pitted, bristly, or spiked. The front of the carapace may or may not have a beaklike projection. The two halves of the carapace are not the same size and one side fits snugly inside the other. They have fewer body segments and appendages than other crustaceans. Most species are 0.08 inches (2 millimeters) or less, but some "giant" species measure 1.26 inches (32 millimeters). Ostracods resemble clam shrimp, but their shells do not have growth rings.

The compound eyes are not set on stalks, if they have eyes at all. Each compound eye has multiple lenses. There are two pairs of antennae and three pairs of mouthparts. The first pair of antennae, the antennules (an-TEN-yuls), is uniramous (YU-neh-RAY-mus), or not branched. The second pair of antennae is branched, or biramous (BY-ray-mus). Adults use their feathery antennae, and sometimes the antennules, for swimming. The thorax and abdomen usually do not have segments and look very similar to each other. There are one to three pairs of biramous limbs. Depending on the species, these limbs are used for walking or cleaning. The tip of the abdomen has a pair of slender or platelike appendages.

GEOGRAPHIC RANGE

Mussel shrimp are found in all oceans and on all continents.

HABITAT

Most species live on the bottom, or near bottom. Some attach themselves to other organisms living on or near the bottom, including other species of crustaceans. One species lives in the gills of fish. Some live in open water, while others are found in very wet moss and leaf litter on land. Some ocean-dwelling species live at depths of 22,965 feet (7,000 meters).

DIET

Most mussel shrimp eat bits of plant material, although some species prey on small animals or scavenge their dead bodies.

BEHAVIOR AND REPRODUCTION

Mussel shrimp have different ways of getting around. Species living on the bottom open their carapace, extend the antennae and limbs, and walk with a rocking motion. Open water species keep their carapaces closed, with just their antennae and limbs poking out. They swim by rowing their appendages through the water.

Both males and females are required for reproduction. Eggs are released into the water or brooded inside the carapace until they hatch. The young hatch as nauplius (NAH-plee-us) larvae (LAR-vee) with folded carapaces covering their bodies. Nauplius larvae have antennae and mouthparts for appendages and use them for walking or swimming. They molt, or shed their external skeletons (exoskeletons), five to eight times before reaching adulthood, adding more appendages with each molt. Mussel shrimp usually live for one year or less.

A BLAST FROM THE PAST

Mussel shrimp are about 500 million years old. Scientists have described about 65,000 species of fossil ostracods. Distinctive impressions of carapaces left behind by ancient ostracods and other organisms were preserved in layers of sea mud. After millions of years of tremendous pressure, the mud turned to stone. The presence or absence of fossil ostracods in rock samples provides scientists with clues about conditions of ancient habitats and the age of surrounding fossils.

MUSSEL SHRIMP AND PEOPLE

Scientists use fossil ostracods to help them understand conditions in ancient habitats. Fossils are impressions left by species that died and settled on the mud bottoms of ancient seas. Over millions of years the mud hardens into stone.

CONSERVATION STATUS

No mussel shrimp are considered threatened or endangered by the World Conservation Union (IUCN). However, species living on listed species of freshwater crayfish and isopods could become threatened if their hosts disappear.

NO COMMON NAME (Vargula hilgendorfii): SPECIES ACCOUNT

Physical characteristics: This ostracod measures 0.12 inches (3 millimeters) in length. It has a beaklike projection on the front of the smooth, round, clear carapace. The black eyes are visible through the carapace. The carapace has large notches through which the antennae stick out. The appendages at the tip of the abdomen are very large and visible between the folded halves of the carapace.

Geographic range: They are found along the Pacific coast of central Japan.

Habitat: This species is very common in shallow waters with sandy bottoms.

Diet: In captivity they will attack worms, scavenge dead fish, or eat fish food.

Behavior and reproduction: Vargula hilgendorfii remain buried just under the surface of the sand during the day. At night they use their antennae to move across the bottom or swim over the bottom. When threatened they will use the large appendages on the tip of the abdomen to push themselves into the sand. They also use these structures to lift themselves quickly up from the sand and into the water.

Males hold females for about 30 to 60 minutes before mating actually begins. Males transfer a packet of sperm to the female's reproductive organs. Females brood their eggs under the carapace. The larvae molt five times before reaching adulthood. Young larvae are capable of crawling, digging, and swimming.

Vargula hilgendorfii and people: They are eaten by some fish that are caught and used as food for people.

Conservation status: This species is not considered threatened or endangered by the World Conservation Union (IUCN). ∎

FOR MORE INFORMATION

Books:

Benson, R. H., et al. Treatise on Invertebrate Paleontology, Part Q, Arthropoda 3. Lawrence, KS: Geological Society of America and University of Kansas Press, 1961.

Periodicals:

Vannier, J., and K. Abe. "Functional Morphology and Behavior of Vargula hilgendorfii (Ostracoda: Myodocopida) from Japan, and Discussion of Its Crustacean Ectoparasites: Preliminary Results from Video Recordings." Journal of Crustacean Biology 13 (1993): 51-76.

Web sites:

Crustacea, the Higher Taxa. Ostracoda (Maxillipoda.http://www.crustacea.net/crustace/www/ostracod.htm (accessed on March 18, 2005).

IRGO. The International Research Group on Ostracoda. Ostracoda. http://www.uh.edu/rmaddock/IRGO/ostracoda.html (accessed on March 18, 2005).

Recent British Intertidal Ostracoda.http://www.staff.ncl.ac.uk/ian.boomer/gallery/modern-IT-ostracods.htm (accessed on March 18, 2005).