Sea Spiders: Pycnogonida

Approximately one hundred different species, or kinds, of sea spiders live off the coasts of the United States and Canada. Most of these species are small, measuring 0.04 to 0.36 inches (1 to 9 millimeters) in length. Some deep-sea species are quite large, with legs as long as 1 foot (305 millimeters). Sea spiders are usually white or colored to blend in perfectly with their backgrounds, typically tan or brown. Sea spiders have a hard outer skeleton, called an exoskeleton. The exoskeleton is so thin that the spider can breathe through it, eliminating the need for a respiratory system, such as those found in humans and other animals. The exoskeleton is covered with tiny pits and hairlike structures. These structures require more study, but they probably help sea spiders to "taste" and "touch."

The sea spider's body is not clearly divided into distinct regions, like those of other arthropods. The first body region is the largest and contains the four small eyes and mouth. The mouth is at the end of a long, flexible tube called the proboscis (pruh-BAH-suhs). The proboscis in some sea spiders is as long as or even longer than the rest of the body. This organ is used to mix digestive chemicals into the food and suck it up into the digestive system, or those body parts that break down food for the body to absorb. The tip of the proboscis has three lips. In some species of sea spiders, the lips may have teeth, while others have spines. Leglike structures are found on either side of the proboscis and are used to handle food.

Next to the mouthparts is a pair of specialized legs used to carry eggs. In most species these egg-carrying body parts are found only in males, but some females also have them. At times of the year other than the breeding season, females use them to clean and groom themselves. During the breeding season, males use them for courting and carrying the eggs until they hatch.

The remaining body segments form the trunk of the sea spider. Attached to the trunk are four pairs of long, slender walking legs. Some sea spider species have five, or even six, pairs. Each leg has eight segments and a single claw. The trunk is very slender and contains part of the digestive system and the reproductive organs, those body parts that produce young. Both eggs, female reproductive cells, and sperm, or male reproductive cells, leave the body through openings in the legs.