Class MONOGENEA

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Introduction

Monogenea (Phylum Platyhelminthes) are common ectoparasitic flatworms of marine, brackish water and freshwater fishes. Together with parasitic copepoda, Monogenea are the most diverse assortment of ectoparasites of fishes. Mongeneans generally live on the external surfaces including the 'skin' or general body surfaces, fins, the head, gills, eyes and oral and branchial cavities. Some monogenean species parasitise internal sites with openings to the exterior such as the nasal tissue, urogenital system (including the cloaca and rectal gland) and sometimes the body cavity. Exceptionally, a few species infect the digestive tract, heart musculature or blood vessels. Despite these exceptions, the Monogenea are widely referred to as ectoparasitic flukes of fish which distinguishes them from the Digenea that are principally endoparasitic or internal flukes (Whittington & Chisholm 2008).

The unique and characteristic morphological feature of the Monogenea that easily distinguishes this class from their wholly parasitic flatworm relatives, the tapeworms (Eucestoda), digeneans and aspidogastreans (collectively the Trematoda), is the posterior attachment organ known as the haptor. The haptor is the main attachment organ used by monogeneans to attach to their hosts. The huge diversity of haptor structure and its adaptability enables attachment to a range of different host substrates and is a major reason for the evolutionary success of Monogenea and their radiation across aquatic vertebrate hosts. The range of fish groups parasitised by monogeneans includes primitave jawless fishes (Agnatha), chimaeras, sharks and rays (collectively the cartilaginous fishes or Chondrichthyes), lungfish (Dipnoi) and the bony fishes (Teleostei). Monogeneans also parasitise aquatic and semi-aquatic tetrapods in the Amphibia (frogs, toads and urodeles) and Reptilia (freshwater turtles) (Whittington & Chisholm 2008).

Monogena means 'single generation' which reflects the fact that these parasitic flatworms have a single-host life cycle with no intermediate host, unlike tapeworms and digeneans. Adult monogenans usually lay their eggs (oviparous) directly into the water. When the eggs hatch, a free-swimming ciliated or crawling unciliated larva (= oncomiracidium) must find and infect its fish host in order to complete the life cycle. Monogeneans are generally strictly host specific (i.e. one species of Monogenea may infect only a single fish species), although exceptions exist (Whittington & Chisholm 2008).

In this checklist the Australian monogeneans are divided among two subclasses: Monopisthocotylea and Polyopisthocotylea. This division is based largely on the site (= microhabitat) parasitised, the diet and related factors (e.g. gut pigmentation), parasite mobility and haptor morphology. A total of 28 families are represented, comprising 126 genera and 316 species. For most monogenean species, available host data are included plus primary references for the names. For major taxa, references are given to the most recent reviews.

Some authorities worldwide use an ordinal system for the Monogenea but most specialists tend to use families as the most useful and meaningful higher taxonomic grouping. Orders, as stated in Boeger & Kritsky (2001), are included for information only.

Diagnosis References

Boeger, W.A. & Kritsky, D.C. 2001. Interrelationships of the Monogenoidea. pp. 92-102 in Littlewood, D.J.T. & Bray, R.A. (eds). Interrelationships of the Platyhelminthes. London, UK : The Systematics Association, Taylor & Francis 356 pp. [92]

General References

Boeger, W.A. & Kritsky, D.C. 2001. Interrelationships of the Monogenoidea. pp. 92-102 in Littlewood, D.J.T. & Bray, R.A. (eds). Interrelationships of the Platyhelminthes. London, UK : The Systematics Association, Taylor & Francis 356 pp.

Whittington, I.D. & Chisholm, L.A. 2008. Diseases caused by Monogenea. pp. 683-816 in Eiras, J.C., Segner, H., Wahli, T. & Kapoor, B.G. (eds). Fish Diseases. New Hampshire : Science Publishers Vol. 2. [683]

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