A new genus of viviparous gyrodactylid (Monogenea) from the Great Barrier Reef, Australia with descriptions of seven new species

Five pairs of posterior gland cells surround the posterior terminations of the gut. The male copulatory organ is a muscular, non-eversible bulb with several spines around the distal opening. Species of Acanthoplacatus have a bilateral excretory system consisting of six pairs of fl ame cells and a pair of excretory bladders.

Seven new species are described: Acanthoplacatu s adlardi sp. nov. and A. amplihamus sp. nov. from Siganus punctatus (Forster, 1801), A. brauni sp. nov. from S. corallinus (Valenciennes, 1835), A. parvihamus sp. nov. from S. vulpinus (Schlegel and Mueller, 1845), A. puelli sp. nov. from S. puellus Schlegel, 1852, A. shieldsi sp. nov. from S. lineatus (Valenciennes, 1835) and A. sigani sp. nov. from S. fuscescens (Houttuyn, 1782).

Species can be discriminated by shape and size of the hamuli, marginal hooks and ventral bar and by male copulatory organ sclerite morphology. Three species (A. brauni sp. nov., A. shieldsi sp. nov. and A. sigani sp. nov.) were assessed for seasonal variation of sclerite size. Ten of thirteen morphological characters showed seasonal variation in size for at least one of the species.

The characters were longer in winter except dorsal root tissue cap width. Only one character, marginal hook length, showed signi fi cant seasonal variation for all three species. Species of Acanthoplacatus were observed to attach using only the marginal hooks and the role of hamuli in attachment is unclear.

The dorsal fin of the host is the preferred site for most species but the anal fin, caudal fin and body surfaces are preferred by some species. Prevalences for species range from 57 to 100%.