MYZOSTOMIDS: Myzostomida

PHYSICAL CHARACTERISTICS

Myzostomids are typically round in outline and flat. Their bodies measure 0.118 to 1.181 inches (3 to 30 millimeters) in length and are fringed with flexible, needle-thin projections called cirri (si-ri). The upper body surface is smooth, but the lower surface usually has five pairs of flaps used for moving around. Each flap is armed with small hooks to help them attach to their hosts, sea lilies. There are also four pairs of slitlike or suckerlike organs on the underside that probably act as sensory organs. In some species, these organs are greatly reduced in size or absent. Myzostomids feed by extending their throats forward out of the mouth inside out.

GEOGRAPHIC RANGE

Myzostomids are found in all oceans.

HABITAT

Most myzostomids live in warm tropical waters on the bodies of sea lilies, sea stars, and their relatives. A few species are found in the Arctic and Antarctic oceans. They live in shallow waters to depths of over 9,840 feet (3,000 meters).

DIET

Most species use their extended mouthparts to suck up food particles floating in the water around them. A few myzostomid parasites eat the tissues or bodily fluids of sea lilies, sea stars, and brittle stars. Parasites (PAIR-uh-sites) are animals that live in and feed off of the bodies of other animals, often harming the host.

BEHAVIOR AND REPRODUCTION

All myzostomids live on the bodies of sea lilies, sea stars, and brittle stars. Most species live as commensals on the outside of the bodies of sea lilies. Commensals (kuh-MEHN-suhls) are animals that live on or with other animals without harm to either one. Sea lilies and their relatives obtain food by catching floating food particles with short tentacles located along grooves on their arms lined with small hairlike structures called cilia (SIH-lee-uh). When myzostomids want to eat, they simply extend their mouthparts into this groove and suck up water and bits of food into their mouths.

A few species of myzostomids are parasites on sea lilies, sea stars, or brittle stars. They live inside the outer tissues of their hosts, or infest the body cavities, reproductive organs, or digestive systems.

Most species of myzostomids can function as both male and female. Smaller individuals function as males, but as they become larger, they also have female reproductive organs. Later, some older individuals may have only female reproductive organs. Reproduction takes place when one individual briefly comes into contact with another and attaches a sperm packet. Sperm from the packets penetrate the skin and fertilize mature eggs inside the body. The fertilized (FUR-teh-lyzed) eggs are later released into the water. The eggs hatch into free-swimming, unsegmented larvae (LAR-vee) with bands of cilia. Larvae are animals in an early stage that change form before becoming adults.

MYZOSTOMIDS AND PEOPLE

Myzostomids have no direct impact on humans or their activities.

LOOKING FOR A HOME

The strange bodies of myzostomids make them difficult to classify. When they were discovered in 1827, myzostomids were placed with flukes, a kind of flatworm. Later they were classified with a group that contained water bears and crustaceans (krus-TAY-shuns), water-dwelling animals that have jointed legs and a hard shell but no backbone. Today most scientists consider them with the group that includes sand worms, earthworms, and leeches. Studies, including DNA analysis, show that they are most closely related to flat worms.

CONSERVATION STATUS

Myzostomids are not considered endangered or threatened.

NO COMMON NAME (Myzostoma cirriferum): SPECIES ACCOUNT

Physical characteristics: The body of Myzostoma cirriferum is egg-shaped and flat, measuring up to 0.09 inches (2.4 millimeters) in length with ten pairs of cirri around the edge. The tubelike mouthparts reach up to 0.04 inches (1 millimeter) when fully extended. The flaps and other organs are well developed and are located underneath the body.

Geographic range: They are found in the Mediterranean Sea and along the northeastern Atlantic coasts of Europe.

Habitat: This species lives on shallow-water sea lilies.

Diet: Myzostoma cirriferum eats floating particles of food diverted from the food grooves on the arms of the sea lilies.

Behavior and reproduction: Several hundred of these myzostomids may infest a single sea lily.

Mature individuals reproduce year round by attaching sperm packets to one another. Sperm from the packets penetrates the skin and fertilizes eggs inside the body.

Myzostoma cirriferum and people: This species does not directly impact people or their activities.

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

FOR MORE INFORMATION

Books:

Grygier, M. J. "Class Myzostomida." In Polychaetes and Allies: The Southern Synthesis. Fauna of Australia. Vol. 4A, Polychaeta, Myzostomida, Pogonophora, Echiura, Sipuncula, edited by Pamela L. Beesley, Graham
J. B. Ross, and Christopher J. Glasby. Melbourne, Australia: CSIRO, 2000.

Periodicals:

Eeckhaut, I., and M. Jangoux. "Life Cycle and Mode of Infestation of Myzostoma cirriferum (Annelida)." Diseases of Aquatic Organisms 15 (1993): 207-217.

Eeckhaut, I., D. McHugh, P. Mardulyn, R. Tiedemann, D. Monteyne, M. Jangoux, and M. C. Milinkovitch. "Myzostomida: A Link between Trochozoans and Flatworms?" Proceedings of the Royal Society, London, Series B, 267 (2000): 1383-1392.