Tanaids: Tanaidacea

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TANAIDS: Tanaidacea

NO COMMON NAME (Apseudes intermedius): SPECIES ACCOUNT

PHYSICAL CHARACTERISTICS

Most tanaids are small, shrimplike crustaceans measuring 0.039 to 0.078 inches (1 to 20 millimeters), but the largest species may reach 3.15 inches (80 millimeters). Some species have color patterns of mostly yellowish and blue or gray. Both pairs of antennae are branched, or biramous (BY-ray-mus). The compound eyes, each having multiple lenses, are on the tips of stalks. The head and first two segments of the thorax are closely joined together into one region, the cephalothorax (SEH-feh-lo-THOR-acks). A shieldlike carapace covers the cephalothorax. All of the thoracic and abdominal limbs are biramous. The first two pairs are called maxillipeds, but they are not part of the mouth. The second pair of maxillipeds has pincherlike claws. The remaining five segments of the thorax each have a pair of limbs called pereopods (PAIR-ee-oh-pawds). The pereopods are usually similar in shape and used for swimming. In some species the first pair is flat and probably used for digging.

In females, the bases of some of the legs form flattened plates and are used to form a brood pouch, or marsupium (mar-SOUP-ee-uhm). The appendages underneath the abdomen, called pleopods (PLEE-oh-pawds), are either present or absent. If present, they are usually used for swimming. But in species that live in tubes, the pleopods are used to create a flow of oxygen-carrying water through the tube. The last two abdominal segments are tightly joined with the tail segment, or telson. On either side of the telson is an appendage called the uropod (YUR-oh-pawd). The telson and uropods together form a fanlike tail.

GEOGRAPHIC RANGE

Most tanaids live in the ocean, but a few species prefer the brackish waters of estuaries where rivers meet the ocean.

HABITAT

Most tanaids live at a wide variety of depths on the bottom of oceans and estuaries. Many species live at depths of more than 656 feet (200 meters); some are found below 29, 527 feet (9,000 meters). A few species live in the open ocean as free-floating plankton. Still others live in the cracks of sea turtle shells, while others live inside snail shells occupied by hermit crabs. Other species glue bits of sand and other particles into open-ended tubes and live inside.

DIET

Most tanaids eat bits of plants, animals, and other microorganisms. Larger pieces of food are handled by the maxillipeds and passed on to the mouth. In some species, the smaller branches of the pereopods and the first pair of pleopods have special structures that are used to stir up mud and sand and strain out bits of food. A few predatory species use their max-illipeds and mouthparts to attack small animal prey.

BEHAVIOR AND REPRODUCTION

Nothing is known of the behavior of tanaids.

SWITCHING SIDES

The tanaid Leptochelia dubia lives in population densities usually greater than 50,000 individuals per 1.2 square yards (1 square meter). If there are few or no males around, females have the ability to change their sex and become male. Males fight with other males over females, and the largest male usually wins. Since males do not feed and cannot grow, the largest males are females that have switched to males.

Both males and females are usually required for reproduction. Some species are hermaphrodites (her-MAE-fro-daits), with individuals having the reproductive organs of both males and females. The eggs are held in the marsupium until they hatch. The hatchlings, or larvae (LAR-vee), go through several distinct larval stages inside the marsupium. They do not leave the pouch until they have developed most of their appendages. Unlike many crustacean larvae, tanaids do not live in the open water with other plankton.

TANAIDS AND PEOPLE

Tanaids do not affect people or their activities.

CONSERVATION STATUS

No species of tanaids is considered endangered or threatened.

NO COMMON NAME (Apseudes intermedius): SPECIES ACCOUNT

Physical characteristics: The last five thoracic segments have rounded bristly expansions on their sides. The marsupium is not well developed in this species. The telson is twice as long as it is wide. The outer branch of the biramous uropods has seven segments.

Geographic range: This species is found around the Cape Verde Islands, the Mediterranean Sea, and off the coast of Brazil (Rio de Janeiro).

Habitat: They live in shallow waters, in sandy or muddy bottoms.

Diet: This species eats bits of plant and animal materials, as well as microorganisms. They probably stir up bits of food from the bottom with their pereopods and pleopods.

Behavior and reproduction: Nothing is known about the behavior of this species. In some habitats tanaids are the most abundant of all animals. Males and females are known; hermaphrodites are not.

Apseudes intermedius and people: This species does not directly impact people or their activities.

Conservation status: Apseudes intermedius is not considered endangered or threatened. ∎

FOR MORE INFORMATION

Books:

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 2003.

Martin, Joel W., and George E. Davis. An Updated Classification of the Recent Crustacea. Los Angeles: Natural History Museum of Los Angeles County, Science Series 39, 2001.