SEA SPIDERS: Pycnogonida

PHYSICAL CHARACTERISTICS

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.

GEOGRAPHIC RANGE

Sea spiders inhabit oceans worldwide, from warm tropical waters to very cold polar seas. The tropics are warm areas of the world, where the temperature is typically more than 68°F (20°C). Polar regions are the cold areas of the world near the North Pole (Arctic) and South Pole (Antarctic), where temperatures never rise above 50°F (10°C).

HABITAT

Sea spiders usually live near the shore. They are found crawling over seaweeds as well as on sea anemones (uh-NEH-muh-nees), small sea animals with long, thin, armlike body parts called tentacles (TEN-tih-kuhls); corals, the hard skeletons of certain sea creatures; and other animal colonies, or tight groupings of animals, permanently attached to the sea bottom. Some species are found at great depths, up to 23,000 feet (7,000 meters), where they live near hot water springs bubbling up from the sea floor.

DIET

Most sea spiders eat other animals and attack invertebrates (in-VER-teh-brehts), or animals without backbones, that are attached to the ocean bottom, such as corals, clams, and marine worms. A few species feed on red algae (AL-jee), a special group of plantlike ocean life that lacks true roots, stems, or leaves. Other sea spiders feed on bits of plant and animal tissues that build up under colonies of invertebrates. While some species use the teeth at the tip of the proboscis to pierce the bodies of their prey and suck out their juices, others tear their victims apart and feed on the small pieces.

BEHAVIOR AND REPRODUCTION

The habits of most sea spiders are poorly known. Some longer-legged species are good swimmers, but most sea spiders prefer to crawl about colonies of anemones, corals, and other stationary, or unmoving, prey animals, or animals that are their source of food.

Most species have both males and females, but in at least one species each spider has the reproductive organs of both sexes. In the few species of sea spider that scientists have studied, courtship, or the activity meant to attract a mate, is brief. While mating, the male and female are positioned so that they are belly to belly, head to tail.

As the female lets go of her eggs, the male releases sperm into the water over them. He then collects the fertilized eggs into a ball and attaches them to his egg-carrying structures with special "glue." Males usually mate with more than one female and are often seen carrying several batches of eggs, each batch the result of a different mating. Males typically carry the eggs until they hatch. Young seas spiders, or larvae (LAR-vee), usually swim freely in the ocean. Most species gain more pairs of legs as they grow into adulthood, although some hatch from the egg with a complete set of legs.

WHAT'S IN A NAME?

The name Pycnogonida comes from the Greek words pyknos, meaning "thick" or "knobby," and gony, or "knees." These animals are called sea spiders because of their similarity to true spiders. Fossil (FAH-suhl) sea spiders, the remains of animals that lived long ago, date back almost four hundred million years. Even without fossil evidence, scientists studying living sea spiders are convinced that these animals are among the world's oldest groups of animals. There are about one thousand species of sea spiders worldwide. Although sea spiders were first discovered nearly 150 years ago, very little is known about them, especially those species living at great depths in the sea. Their unusual body form has made it difficult for scientists to figure out just how they are related to other arthropods. They are most closely related to horseshoe crabs and spiders and their relatives.

SEA SPIDERS AND PEOPLE

Sea spiders are rarely seen by most people. However, these penny-sized animals are sometimes easily seen in tidal pools, or pools of water that remain after an ocean tide has risen and fallen. Sea spiders are captured and stored in alcohol and other fluids to preserve them and are sold to colleges and universities as study animals.

CONSERVATION STATUS

Sea spiders are not endangered or threatened.

NO COMMON NAME (Colossendeis megalonyx): SPECIES ACCOUNT

Physical characteristics: The body of this sea spider, one of the world's largest, is approximately 0.78 inches (20 millimeters) long, including the long, broadly rounded snout. The leg span is up to 27.5 inches (700 millimeters). Each leg is tipped with a long, slender claw.

Geographic range: This sea spider is found from depths of 10 to 16,400 feet (3 to 5,000 meters) around Antarctica and into the southern Atlantic Ocean, southern Indian Ocean, and southern Pacific Ocean, including the Antipodes Islands off New Zealand.

Habitat: Nothing is known about the spider's preferred living areas.

Diet: This sea spider eats soft corals and small hydrozoans attached to sponges. The hydrozoans are a group of water-dwelling organisms without backbones that includes jellyfish.

Behavior and reproduction: Nothing is known about this sea spider's reproductive habits or other behavior.

Colossendeis megalonyx and people: This species does not interact with people.

Conservation status: This species is not endangered or threatened. ∎

FOR MORE INFORMATION

Books:

Tavolacci, J., ed. Insects and Spiders of the World. New York: Marshall Cavendish, 2003.

Web sites:

"Arthropoda: Chelicerata Sea Spiders." Underwater Field Guide to Ross Island and McMurdo Sound, Antarctica. http://scilib.ucsd.edu/sio/nsf/fguide/arthropoda-2.html (accessed August 17, 2004).

"Introduction to Pycnogonida." University of California, Berkeley, Museum of Paleontology. http://www.ucmp.berkeley.edu/arthropoda/pycnogonida.html (accessed on August 17, 2004).

"Pycnogonida (Sea Spiders)." Earth-Life Web. http://www.earthlife.net/chelicerata/pycnogonida.html (accessed on August 17, 2004).