Systematic Parasitology 53: 227–233, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 227 Thaumamermis zealandica n. sp. (Mermithidae: Nematoda) parasitising the intertidal marine amphipod Talorchestia quoyana (Talitridae: Amphipoda) in New Zealand, with a summary of mermithids infecting amphipods George Poinar Jr 1 , A. David M. Latham 2 & Robert Poulin 2 1 Department of Entomology, Oregon State University, Corvallis, OR 97331, USA 2 Department of Zoology, University of Otago, PO Box 56, Dunedin, New Zealand Accepted for publication 30th April, 2002 Abstract A new nematode, Thaumamermis zealandica n. sp. (Mermithidae; Nematoda), is described parasitising the in- tertidal marine amphipod Talorchestia quoyana Milne-Edwards (Talitridae) from the coast of South Island, New Zealand. The new species is characterised by the degree of dimorphism of the spicules, the arrangement of the genital papillae, the shape and length of the vagina and the anteriorly-placed amphids. This is the first known marine host of a member of the family Mermithidae. Published records of mermithid nematodes from amphipods are presented. Introduction The family Mermithidae Braun, 1883 constitutes a unique group of nematodes, which parasitise a wide range of invertebrates, especially insects (Poinar, 1983). Crustaceans are unusual hosts for mermithids and are limited to terrestrial members of the Isopoda (see Poinar, 1981) and fresh-water representatives of the Amphipoda, with hosts in the families Gammari- dae (see Rubstov & Bekman, 1979; von Linstow, 1893), Corophiidae (see Bacesco, 1948) and Hyalelli- dae (see Camino, 1989) (Table 1). We describe here a new species of mermithid parasitising an intertidal ma- rine amphipod, Talorchestia quoyana Milne-Edwards (Talitridae), from the coastal sands of South Island, New Zealand. Materials and methods Amphipods were collected during November, 2001 on Long Beach, just north of Dunedin, South Island, New Zealand, from under tidal debris along the strand-line. The amphipods were hand-picked from the sand; only larger individuals were taken because these are more likely to harbour mermithids (Poulin & Rate, 2001). Hosts were maintained in the laboratory in moist sand at room temperature and under a natural photoperiod. Post-parasitic juvenile mermithids that emerged from their hosts within 10 days of capture were used for the following description. They were maintained in wet sand until they moulted and initiated mating, then were killed in hot water (75 ◦ C), fixed in 5% forma- lin and processed to glycerine for taxonomic studies. Microscopic observations and photographs were made on living and dead nematodes with a Nikon Optiphot microscope. In the following quantitative description, all measurements are in micrometres unless otherwise specified. The first figure represents the mean value and is followed by the range in parentheses. Mermithida Rubstov, 1978 Mermithidae Braun, 1883 Thaumamermis Poinar, 1981 Diagnosis (amended from Poinar, 1981). Medium to large-sized nematodes with adult cuticle lacking cross- fibres, but post-parasitic cuticle with fine to extremely