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The gyrodactylids are best known as a large (over 400 species described), homogenous group of viviparous monogeneans, highly specialised to give birth to fully grown embryos which already contain an embryo developing inside them.    They are characterised by the presence of 16 articulated marginal hooks, a single pair of hamuli joined by a dorsal bar, and a shield-like ventral bar.  The adaptation for viviparity has meant  extensive modification to internal anatomy,  which confused students of the group from the earliest days (von Nordmann, 1823).  On the other hand, the peculiar nature of viviparity, with one embryo developing inside another, caught the imagination of 19th Century embryologists, who undertook a series of first –rate microscopical studies of the phenomenon, which they considered to illustrate the germ-line theory (e.g. Wagener, 1860; Kathariner, 1904; Gille, 1914) current at that time.    The mechanism remains poorly understood (Cable & Harris, 2002), but retains relevance today for the light it can shed on stem cell totipotency in higher organisms.

The family Gyrodactylidae  also contains a number of egg laying genera.  The first to be described was  Phanerothecium caballeroi (see Kritsky & Thatcher, 1977), but, as the type series lacked eggs, the significance of the discovery was overlooked, and the genus was assumed to be viviparous.  Harris (1983) described the egg laying Ooegyrodactylus farlowellae and placed it in its own family, the Oogyrodactylidae, but it is now generally considered (Boeger & Kritsky, 1994) that the viviparous genera form a specialised clade within the Gyrodactylidae, a family containing both egg-laying and viviparous genera.
Examination of the larva of Ooegyrodactylus farlowellae (see Harris, 1983) made it clear that the viviparous genera are highly progenetic, retaining a larval morphology despite being  reproductively mature.  However, it is also clear that the group as a whole possess characters which are unusual amongst Monogenea.  These include the articulated marginal hooks (seen elsewhere in the Acanthocotylidae)  and the spike sensilla on the cephalic lobes (which may be unique amongst the Monogenea, or may additionally be present in Enoplocotyle – see Kearn, 1993).    Additionally, the larva of the egg-laying genera is an unciliated crawlaway, similar to that seen in the Acanthocotylidae and the Udonellidae.   Although not conclusive, these characters tend to link the Udonellidae, the Gyrodactylidae and the Enoplocotylidae (and maybe the Acanthocotylidae) into a  clade within the lower Monopisthocotylea.

The Gyrodactylidae currently contains 7 oviparous genera and 25 viviparous genera, although the validity of several of the latter is questionable (Bakke et al., 2007).  The oviparous forms are all South American, mostly parasitic on loricariid catfishes.   Most viviparous gyrodactylids belong to the genus Gyrodactylus or closely allied genera, and some are significant fish pathogens (e.g. G. salaris, G. cichlidarum and others).   These have a greatly simplified reproductive anatomy, with most of the internal space taken up by the developing embryo.  All are protogynous (female system develops first), and insemination is achieved by hypodermic impregnation.  A number of other viviparous genera, however, especially those from tropical Africa and South America, show greater diversity of form, and may be significantly different in biology.

Bakke, T.A., Cable, J. & Harris, P.D. (2007). The Biology of Gyrodactylid Monogeneans.  Advances in Parasitology 64, 162-377.
Boeger, W.A. & Kritsky, D.C. & Belmont-Jegu, E. (1994). Neotropical Monogenoidea. 20.  Two new species of oviparous Gyrodactylidea (Polyonchoinea) from loricariid catfishes (Siluriformes) in Brazil, and the phylogenetic status of  Ooegyrodactylidae Harris, 1983.  Journal of the Helminthological Society of Washington 61, 34-44.
Cable, J. & Harris, P.D. (2002).  Gyrodactylid developmental biology: Historical review, current status and future trends.  International Journal for Parasitology 32, 255-280.
Gille, K. (1914).  Untersuchungen uber die Eireifung, Befruchtung und Zellteilung von Gyrodactylus elegans v. Nordmann.  Archiv Zellforschung 12, 415-456.
Harris, P.D. (1983).  The morphology and life cycle of the oviparous Oogyrodactylus farlowellae gen et sp. Nov. (Monogenea, Gyrodactylidea).  Parasitology 87, 405-420.
Kathariner, L. (1904). Ueber die Entwicklung von Gyrodactylus elegans v. Nrdm.  Zoologischer Jahrbucher, Suppl. 7, 519-550.
Kearn, G.C. (1993).  A new species of the genus Enoplocotyle (Platyhelminthes, Monogenea) parasitic on the skin of the moray eel Gymnothorax kidako in Japan, with observations on hatching and the oncomiracidium.  Journal of Zoology 229, 533-544.
Kritsky, D.C.  & Thatcher, V.E.  (1977). Phanerothecium gen. nov. and Fundulotrema gen. nov. two new genera of viviparous Monogenea (Gyrodactyloidea) with a descriptionof P. caballeroi sp. Nov and a key to the subfamilies and genera of the family.  In Exerta Parasitologica in Memoria del doctor Eduardo Caballero y Caballero, pp53-60, Mexico Instituto di biologie.
Wagener, E.D. (1860).   Ueber Gyrodactylus elegans v. Nrdm.  Archiv fur Anatomie, Physiologie und Wissenschaftliche Medizin, Leipzig 768 -797.


Monogenean Pictures