Larval Hysterothylacium sp. [Nematoda, Anisakidae] and trematode metacercariae from the amphipod Paracorophium excavatum [Corophiidae] in New Zealand

Previously undescribed fourth-stage larvae of anisakid nematodes were found in the haemocoel of the amphipod Paracorophium excavatum (Thomson, 1884) (Corophiidae) in New Zealand. Morphological examination by light microscopy showed that the worms belonged to a species of Hysterothylacium Ward et Magath, 1917, based on several characters including the presence of interlabia, the location of the excretory pore posterior to the nerve ring, and the characteristics of the intestinal caecum and ventricular appendix. Interestingly, several male specimens showed precocious sexual development. This is the first record of fourth larval stage and precocious adult male specimens of Hysterothylacium in an invertebrate host, as well as the first record of anisakid larvae in New Zealand crustaceans. In addition, metacercariae of two trematode species, Coitocaecum parvum and Microphallus sp., are recorded for the first time from the amphipod P. excavatum.