Systematic Parasitology 43: 1–6, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 1 Scomberomorocotyle munroi n. g., n. sp. (Scomberomorocotylinae n. subf.), a thoracocotylid monogenean from Scomberomorus munroi (Scombridae) off Australia and Papua New Guinea Klaus Rohde & Craig J. Hayward Division of Zoology, University of New England, Armidale NSW 2351, Australia Accepted for publication 1st June, 1998 Abstract Scomberomorocotyle munroi n. g., n. sp. is described from the gills of Scomberomorus munroi, a Spanish mack- erel from the coasts of northern Australia and southern Papua New Guinea. The genus belongs to the suborder Gastrocotylinea because a pair of basal accessory sclerites is present in the clamps. However, the worm does not belong to any of the eight gastrocotylinean families as they are currently recognised. The worm appears to be a member of the Thoracocotylidae, in that the male copulatory organ has relatively weakly developed spines, and that the haptor is one-sided with two rows of clamps. However, the worm differs from all thoracocotylids in that the clamps lack the characteristic lateral rib-like thickenings. To accommodate the new genus and species, the diagnosis of the Thoracocotylidae Price is amended to include worms lacking ribs in their clamps, and a new subfamily, the Scomberomorocotylinae n. subfam., is erected; a key to the four subfamilies which we recognise as valid is provided. Introduction A total of 18 species of Spanish mackerels (Scombero- morus spp., fam. Scombridae) occurs in warm coastal seas around the world (Collette & Nauen, 1983). To date, the gills of these fishes are known to be infected with representatives of two families of monogeneans that belong to the suborder Gastrocotylinea Lebe- dev, 1972. The first of these families, Gotocotylidae Yamaguti, 1963, belongs to the superfamily Gotocoty- loidea Yamaguti, 1963; nine of its members infect Spanish mackerels (see review by Hayward & Ro- hde, 1999a). The other family, the Thoracocotylidae, belongs to the superfamily Gastrocotyloidea (Price, 1943) Lebedev, 1972, and 10 of its species are said to infect Spanish mackerels (see the treatise on the Gastrocotylinea by Lebedev, 1986). During the examination of representatives of all 18 species of Scomberomorus for their monogenean parasites, a new gastrocotylinean belonging neither to the Gotocotylidae nor the Thoracocotylidae (as they are currently recognised) was discovered amongst the material collected from Scomberomorus munroi Col- lette & Russo. This host, also known as the Australian spotted mackerel, is relatively restricted in its distrib- ution, occurring only along the coasts of the northern half of Australia and off southern Papua New Guinea (Collette & Nauen, 1983). This host species is also believed to form a monophyletic species-group of its own within the genus Scomberomorus (see Collette & Russo, 1985). Materials and methods Specimens of all 18 species of Scomberomorus were examined. The gills of fishes were removed and examined for ectoparasites with a dissecting micro- scope. Specimens of a new gastrocotyloid worm were found only on one species, S. munroi; these hosts are all in the collection of the Smithsonian Insti- tution,Washington DC (USNM). The worms were stained in Grenacher’s carmine alum, dehydrated in an alcohol series and mounted in Canada balsam. The worms were drawn with the aid of a drawing tube and body parts measured with a calibrated eye-piece