Monogeneans: Monogenea

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.