Tanaids: Tanaidacea

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.