A new species of Trileptium (Nematoda, Thoracostomopsidae) from a sandy beach in southeastern Australia, with a key to species and observations on geographical distribution

Thoracostomopsidae are common predatory marine nematodes found in littoral and sub-littoral sands. The buccal cavity is characterised by a mandibular complex and onchia (teeth) forming grasping jaws. Three constituent subfamilies and genera are distinguished by variations in the development of these structures. Of the three subfamilies, Thoracostominae have been found in sub-littoral habitats and have not been reported from Australia. A number of species from several genera of Enoplolaiminae have previously been recorded from southeastern Australian beaches. In this paper, Trileptium australis n. sp. belonging to the third subfamily, Trileptiinae, is described from one of these beaches. In this genus, weakly developed mandibles and one, or three, well developed onchia are situated closer to the mouth opening than in Enoplolaiminae, and the lips have a lower profile. Important specific diagnostic characters of Trileptium australis n. sp. are the possession of three onchia, absence of cuticular annulation, presence of a pre-cloacal supplementary organ, thin strongly curved spicules with mid-ventral pegs and a gubernaculum that loops over the spicules. A dichotomous key for the 11 described species, based on male characters, is provided. The geographical distribution of all the described species is reviewed. One species, T. subterraneum has a very wide geographical distribution, but other species appear to be restricted to one sea or ocean.