MONOGENEANS: Monogenea

PHYSICAL CHARACTERISTICS

Monogeneans (mah-nuh-JEE-nee-uhns) are flatworm parasites that live mainly on fish skin and gills. Parasites (PAIR-uh-sites) are animals or plants that live on or in other animals or plants, or hosts, without helping them and usually harming them. Monogeneans have an organ at the rear of their bodies that holds hooks the worms use for attaching themselves to hosts. The organ holds large hooks, called anchors, and small hooks. Monogeneans live in only one host for their entire life cycle. Monogeneans are one–thirty-second to three-fourths of an inch (1 millimeter to 2 centimeters) long. Large monogeneans tend to be flat and leaf shaped, but the smaller worms are usually cylindrical. These flatworms are colorless and almost clear. When on fish skin some may be almost invisible to the human eye, either because they are clear or because they contain scattered coloring that matches the color of the host's skin. The digestive system of monogeneans consists of a muscular tube used to suck in food and a saclike or branched intestine with no anus (AY-nuhs). Monogeneans make both eggs and sperm.

GEOGRAPHIC RANGE

Monogeneans live all over the world.

HABITAT

Many monogeneans live on or in specific hosts, mainly the skin of freshwater and saltwater fishes. Some species live in the bladders of frogs and toads and the bladders or mouths of freshwater turtles. One species lives beneath the eyelids of a hippopotamus. Another lives on the skin of squids.

DIET

Most monogeneans feed on the skin of their hosts. Some eat blood.

BEHAVIOR AND REPRODUCTION

Many monogeneans move like leeches from their site of first attachment on the host to the site where they mate and lay their eggs. Many can change their location on the host throughout their lives. Some stay in one place. Some skin parasites breathe by wavy movements of their bodies. Some young and adult parasites can swim.

Monogeneans make both sperm and eggs. The male part of the reproductive system is usually first to mature. Fertilization (FUR-teh-lih-ZAY-shun), the joining of egg and sperm to start development, takes place in one of three ways: two worms mate and fertilize (FUR-teh-lyze) each other; one worm fertilizes another, but the favor is not returned; or one worm fertilizes itself. The fertilized eggs are released into the environment and produce infective larvae, which can swim freely by using hairlike fibers that cover their bodies. Larvae (LAR-vee) are animals in an early stage that change form before becoming adults.

The larvae of many monogeneans hatch at a particular time of day, which is often the same time the host is particularly vulnerable to invasion. Hatching may also be triggered by host cues such as chemicals, movement, or shadows. The larvae do not feed until they reach the host, which means that their survival as free-living animals and their chance of infecting a host are limited, usually to a period of several hours. Rather than depositing fertilized eggs in the environment, some monogeneans keep the eggs inside themselves for development and give birth to offspring that are usually full size at birth.

MONOGENEANS AND PEOPLE

In the wild, the number of monogeneans living on an individual host is low, and infestations of these parasites do not usually cause disease. In crowded fish farms, however, parasite populations often increase uncontrollably, and the hosts can be damaged or killed.

CONSERVATION STATUS

Monogeneans are not threatened or endangered.

NO COMMON NAME (Dactylogyrus vastator): SPECIES ACCOUNTS

Physical characteristics: Worms in the species Dactylogyrus vastator (abbreviated as D. vastator) are a little more than one–thirty-second of an inch (1.25 millimeters) long. They have two pairs of hooks. A set on the bottom surface of the worm is smaller than the set at the rear. These worms have three pairs of sticky sacs and four eyespots.

Geographic range: D. vastator lives in the northern parts of North America, Europe, and Asia.

Habitat: D. vastator lives in the gills of carp and goldfish, which live in freshwater.

Diet: D. vastator feeds on gill cells.

Behavior and reproduction: D. vastator worms attach themselves to their hosts with their hooks. The eggs are washed out of the host's gills and sink to the bottom of the water. Larvae emerge in three to five days, depending on water temperature. Larvae drawn into the gill cavity by the water current attach themselves to another host's gills. Some larvae may first attach to a host's skin and then migrate to the gills.

Dactylogyrus vastator and people: D. vastator kills young carp in fish-farming ponds. This problem is significant in areas where carp is bred for food.

Conservation status: D. vastator is not threatened or endangered. ∎

NO COMMON NAME (Polystoma integerrimum): SPECIES ACCOUNTS

Physical characteristics: Adult Polystoma integerrimum (abbreviated to P. integerrimum) worms are about three-eighths of an inch (10 millimeters) long. The attachment organ has six muscular suckers, one pair of large hooks, and sixteen small hooks. There also are suckers around the mouth.

Geographic range: P. integerrimum lives in Europe.

Habitat: P. integerrimum lives in the bladders of common frogs.

Diet: P. integerrimum eats blood.

Behavior and reproduction: Adult P. integerrimum worms do not make eggs while their host frogs are living on land during most of year. When the frogs enter water to spawn in spring, the worms lay their eggs. The larvae that hatch invade the gills of frog tadpoles, or the frog's early water-dwelling stage. When the tadpoles change into frogs, the worms travel through the digestive tract and possibly the skin to the host's bladder, where they mature. A single egg that stays in an adult worm develops and hatches, and the larva stays in the host of its parent, increasing the parasite infection in the host.

Polystoma integerrimum and people: P. integerrimum has no known importance to people.

Conservation status: P. integerrimum is not threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Zimmer, Carl. Parasite Rex. New York: Free Press, 2000.

Web sites:

"The Class: Monogenea." Kansas State University. http://www.ksu.edu/parasitology/classes/625monogene.html (accessed on January 31, 2005).

Reed, Peggy, Ruth Francis-Floyd, and RuthEllen Klinger. "Monogenean Parasites of Fish." University of Florida. http://edis.ifas.ufl.edu/FA033 (accessed on January 31, 2005).