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Abstract. Branchotenthes octohamatus sp. n. (Monogenea: Hexabothriidae) is described from the gills of the southern fiddler ray, Trygonorrhina fasciata Müller et Henle (Elasmobranchii: Rhinobatidae), off Adelaide, South Australia. It is distinguished from the type species, Branchotenthes robinoverstreeti Bullard et Dippenaar, 2003, by producing eggs that are joined end to end forming a chain, in the morphology of the male copulatory organ that has a pronounced constriction in duct diameter between proximal and distal regions, the possession of a thin muscular layer surrounding the proximal part of the male copulatory organ and distal region of the vaginae, and by the absence of a raised process on the shaft of the hamulus. An amended generic diagnosis is provided and the reliability of sperm duct number as a generic character is discussed. The oncomiracidium of B. octohamatus is also described and is the first monogenean to be described with only eight hooklets in the larval haptor. This discovery of eight hooklets may be important for higher-level monogenean evolutionary hypotheses.The Hexabothriidae Price, 1942 includes polyopisthocotylean monogeneans exclusively parasitic on the gills of chondrichthyan fishes (the sharks, rays and chimaeras). Since the first hexabothriid was discovered by Kuhn (1829), over 60 species have been described from almost as many host species. Currently however, the taxonomy of the Hexabothriidae is in a state of confusion. Boeger and Kritsky (1989) have undertaken the only comprehensive familial revision. They recognized 13 genera but declared many species incertae sedis, suggesting these species as likely representatives of presently unrecognized genera. Recently, Bullard and Dippenaar (2003) proposed Branchotenthes to accommodate B. robinoverstreeti from the gills of the bowmouth guitarfish, Rhina ancylostoma Bloch et Schneider (Rhynchobatidae) in the Indian Ocean. A combination of characters was used to differentiate Branchotenthes from other hexabothriid genera but the possession of dilated, glandular-walled 'vasa efferentia' was considered the most distinct feature.Selection of appropriate characters to determine species relationships is undoubtedly difficult for the Hexabothriidae. Studies of larval morphology have proven valuable in the clarification of taxonomic issues within some monogenean families (e.g. for Monocotylidae see

The southern fiddler ray, Trygonorrhina fasciata (Rhinobatidae), is parasitized by 3 monogenean (platyhelminth) species from 3 families on 3 different sites of the host : Calicotyle australis (Monocotylidae) from the cloaca, Pseudoleptobothrium aptychotremae (Microbothriidae) from the skin and Branchotenthes octohamatus (Hexabothriidae) from the gills. Cues that promote egg hatching were investigated for each species and the behaviour of their larvae was also documented. Eggs were laid by parasites in vivo and maintained at 22 xC. Three different egg hatching and host finding strategies were discovered. Calicotyle australis eggs hatched spontaneously with a strong diurnal rhythm that is likely to be under circadian control. The larva is ciliated, photo-responsive and can survive for up to 24 h at 22 xC after hatching. Pseudoleptobothrium aptychotremae may have a ' bet-hedging ' strategy. Some eggs hatched spontaneously and rhythmically. However, since the hatching success was low, it is possible that other eggs require a different cue provided by the host. The larva is also ciliated but shows no photo-response and was observed to remain active for <4 h at 22 xC after hatching. Branchotenthes octohamatus has a ' sit-and-wait ' strategy that depends on mechanical disturbance to stimulate hatching. The larva is unciliated, shows no photo-response but may survive for more than 2 days at 22 xC after hatching. The implications of hatching strategy, larval behaviour and morphology in the goal to find a host are discussed for each species.

The adult of Pseudoleptobothrium aptychotremae Young, 1967 (Monogenea, Microbothriidae) is redescribed from the dermal denticles of the southern fiddler ray, Trygonorrhina fasciata (Rhinobatidae) collected off Adelaide, South Australia. This is a new host and locality record. The anatomy of the larva is described from observations of live larvae and the presence of six needle-like spicules in the larval haptor is confirmed. The development of P. aptychotremae is also described.