Family GYRODACTYLIDAE Van Beneden & Hesse, 1863

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Family GYRODACTYLIDAE Van Beneden & Hesse, 1863

Introduction

The Gyrodactylidae is a large family comprising in excess of 400 species parasitic on the fins and skin of teleost fishes. It has been predicted there may be >20,000 species worldwide. Gyrodactylids occupy a special position among the Monogenea, because of the unique blend of reproductive adaptations displayed by its viviparous species. Their reproductive strategies have lpromoted the viviparous gyrodactylids as excellent models of parasite reproduction for parasitological teaching, and it is possible that with further advances in molecular biology, these worms will be used as models in comparative developmental biology. One species of gyrodactylid, Gyrodactylus salaris, is a major pathogen of salmonids in the Northern Hemisphere .

Gyrodactylids reproduce either by viviparity or oviparity. Oviparous species, of which there are few, have only ever been recorded from South American fishes; viviparous species make up the vast bulk of the described species. All viviparous forms combine a unique form of viviparity and polyembryony to reproduce. Each adult worm gives birth to a single offspring at a time. At birth, the offspring has fully mature somatic tissues, and a mature female system that already carries the embryonic tissues of its own progeny.

Viviparous gyrodactylids are transmitted by contagion, and no dispersive larval form is known. Adult worms are best observed on freshly euthanised fish, using a dissecting microscope with incident or transmitted light. The worms are minute and care is needed to examine the host surface these tiny parasitic flatworms. Gyrodactylids are most common on the body and fins of fish. The parasites can be identified by careful examination of the hard sclerotised tissues of the haptor and male reproductive system. For this, worms are best mounted in an ammonium picrate glycerine solution (Malmberg 1970) or lactic acid and the coverslips ringed with nylon lacquer.

The tiny size (< 500 microns) of gyrodactylids and, in wild fish populations, a relatively low prevalence and intensity of infection, means that parasite populations can be cryptic. Undoubtedly, species have been overlooked in previous surveys. In some parts of the world, gyrodactylids are known from squid (Europe) and aquatic toads (South Africa). In Australia, the Gyrodactylidae are currently represented by two subfamilies, three genera and 13 species but this is very likely a huge underestimate of the actual fauna on fish in Australia.

Three species of Gyrodactylus, G. bullatarudis Turnbull, 1956, G. kobayashi & G. macracanthus Hukuda, 1940, were intruduced into Australia via the freshwater aquarium trade (Dove & Ernst 1998; Fletcher & Whittington 1998).

Diagnosis

Small worms, with a simple haptor, bearing 16 hinged hooklets, a pair of ventral hamuli and 2 ventral bars. Spike sensilla usually present at anterior extremity of body. Pharynx digitiform. Male system with a single testis, 2 seminal vesicles. Vagina absent. Mehlis gland large in oviparous forms. Viviparous or oviparous. Parasites of teleost fishes.