Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Platyhelminthes
Class Monogenea

Common names: flatworms, flukes, monogeneans

Overview

The Monogenea are generally ectoparasites of fish species, attaching to the external surface, fins, gills, eyes and oral cavities of the host animal. A few species can infect the internal tissues such as the blood vessels or digestive tract, and several species can parasitise amphibians and freshwater turtles. Monogeneans are easily distinguished from the Trematoda and Cestoda by the haptor, the posterior attachment organ. Adults can reach a few centimetres in length, but most species are only a few millimetres long when mature.

Distribution and diversity

There are approximately 5000 species of monogeneans described worldwide, with 318 species in 125 genera and 28 families listed in the most recent checklist of species found in Australia.

Life cycle

Monogeneans are usually strictly host specific, and have a single host lifecycle. Adult monogeneans are hermaphroditic and lay their eggs directly into the water, from which a free-swimming larva will hatch. The larva must infect a host to complete the lifecycle.

Feeding

Many species of monogeneans are able to move freely around on the surface of their fish host, eating mucus and epithelial cells, whilst a few species will remain attached to a single site. Monogeneans attach to the host with the posterior haptor, but another attachment and feeding organ is found anteriorly, associated with the mouth and pharynx.

Ecology

Monogeneans parasitise marine, brackish and freshwater fishes, and as such are found in a wide variety of aquatic habitats.

Discocotyle sagittata parasitising the gills of a salmon in Canada
Image credit: Chris Blanar
used under a Creative Commons Attribution-NonCommercial license