Morphometric and molecular descriptions of three new species of Hysterothylacium (Nematoda: Raphidascarididae) from Australian marine fish.

Three new species of Hysterothylacium Ward & Magath, 1917, including H. australe, H. kajikiae and H. brucei, from Australian marine fish are described and illustrated by light microscopy followed by genetic characterization of their first and second internal transcribed spacers (ITS-1 and ITS-2, respectively). This is the first study reporting ITS sequence data for adult Hysterothylacium spp. in Australia, which provides an insight into the identification of some of the Hysterothylacium larval types. Alignment of ITS sequences of these species with Hysterothylacium larval types previously reported in Australia showed that fourth-stage Hysterothylacium larval type XI from Seriola lalandi and third-stage Hysterothylacium larval type X from Sphyraene novae-hollandiae are identical with ITS sequences of H. australe, suggesting that these fish are natural intermediate/paratenic hosts of H. australe.