Barnacles and Relatives: Thecostraca

Adult barnacles and their relatives have very unusual bodies compared to other crustaceans. In fact, it is only during the free-swimming larval stages that most species are recognizable as crustaceans at all. The first pair of antennae, or antennules (an-TEN-yuls), of the larvae (LAR-vee) is used for grasping. They help the larvae, or young animals, to attach themselves to a host animal or object, their final home as adults. The bodies of the adults are different because of their sessile (SHE-sihl) or parasitic (PAIR-uh-SIH-tik) lifestyles. Sessile barnacles are unable to move because they are permanently attached to other objects, and the parasitic species spend most of their lives feeding inside the bodies of other animals. Most adult barnacles and their relatives have very small heads, lack eyes, and have abdomens with no appendages underneath. The second pair of antennae is usually absent. Thecostraca (thee-koh-STRAH-kay), is divided into three groups: barnacles and their parasitic relatives, ascothoracids, and the y-larvae.

Barnacles are the only group of sessile crustaceans. However, some species are able to move because they attach themselves to animals or floating objects. Adults have white, pink, red, purple, yellow, or brown bodies and live on their heads, upside down, in cone-shaped shells. In most crustaceans, the carapace (CARE-eh-pes) is tough and shieldlike. The carapace of barnacles is soft, sacklike, and called the mantle (MAN-tuhl). The mantle surrounds the body and gives off a substance that forms the cone-shaped shell. The shell is made up of thick, protective plates that join together in the shape of a round or flat cone. The plates are either distinct or tightly joined (fused) together. Inside the mantle is a multi-purpose space, or chamber, where the mouth and reproductive organs are located. It is also where solid waste is released from the barnacle's body. The eggs are kept in this chamber, and the larvae develop there until they are released into the open sea. Adults have a six-segmented thorax with six pairs of cirri (SIH-ree). Cirri are long, bristly, tentaclelike limbs and are either branched or not. They are used together for breathing and like a net to catch bits of food floating in the water. In acorn barnacles, the shell is attached directly to a solid object, but those of gooseneck barnacles are located on top of a soft, flexible stalk. Barnacles without stalks measure up to 9 inches (230 millimeters) in length and 3.1 inches (80 millimeters) across. Stalked species measure up to 27.5 inches (700 millimeters) in length.

There are two small groups of parasitic crustaceans closely related to barnacles. They burrow into corals or into the shells of mollusks (mussels, clams, snails), or they invade the bodies of decapods and isopods. They do not live inside a shell and measure only a few millimeters in length. In one group, the bodies of the adults are completely enclosed inside a folded carapace. Adult females are much larger than the males. The tiny male lives like a parasite and is permanently attached to the female. In the other group, the adults look almost plantlike. They have rootlike bodies that spread out through the tissues of their hosts to soak up nutrients.

The bodies of ascothoracids are completely surrounded by a folded carapace. Their mouthparts are cone-shaped and used to suck the body fluids from their hosts. The thorax has up to six pairs of cirri, and the tip of the abdomen has a pair of appendages.

As their nickname suggests, the y-larvae are y-shaped. They are very small, measuring less than 0.039 inches (1 millimeter) and are free-swimming. The fact that they have grasping antennules and hooked mouthparts suggests that the adults are probably parasites. However, the y-larvae have never been seen clearly associated with their adults.