Hysterothylacium Ward and Magath, 1917

Arai, Hisao P. & Smith, John W., 2016, Guide to the Parasites of Fishes of Canada Part V: Nematoda, Zootaxa 4185 (1), pp. 1-274 : 134

Hysterothylacium Ward and Magath, 1917

Synonyms: Thynnascaris Dollfus, 1933 ; Contracaecum (Thynnascaris) Dollfus 1935 ; Contracaecum (Erschovicaecum) Mozgovoi, 1951 ; Contracaecum (Simplexonema) Kreis, 1952 nom. nud.; Iheringascaris Pereira, 1935 ; Contracaecum (Acollaris) Araujo, 1970

Generic diagnosis (after Deardorff & Overstreet 1981). Raphidascaridinae . Body elongate, reaching greatest width near midbody. Cuticle with annulations moderately or weakly defined or lacking (presumably unreported). Cuticular alae distinct or indistinct. Lips approximately equal in size, usually wider than long, bearing transparent cuticular flanges on lateral margins: flanges with or without indentations; internal pulp usually pedunculated; dorsal lip with two lateral doubled papillae; subventral lips with amphid, adjacent mediolateral doubled papilla, and single lateral papilla. Dentigerous ridges absent. Interlabia present. Interlabial grooves present or absent. Ventriculus almost spherical; ventricular appendix sac-like or cylindrical, with septum dividing structure into two equal longitudinal pouches; intestinal caecum usually shorter than ventricular appendix . Excretory system with excretory pore at or near nerve ring level, with duct extending to at least left lateral cord. Rectal glands present or absent (presumably unreported). Spicules similar, alate, equal or slightly unequal in length. Gubernaculum absent. Vulva anterior to midbody. Uterus didelphic, opisthodelphic. Tail conical, tip with or without spines. Medioventral pre-cloacal organ usually distinct on pre-cloacal fold. Phasmids usually distinct. Parasites of marine, estuarine and fresh-water fishes.

Comments: Gopar-Merino et al. (2005) claimed that 61 species of Hysterothylacium have been recognized; however, there are more than 61species if “morphospecies”, “sibling species” and “cryptic species” are taken into account. According to Rye & Baker (1992) most Hysterothylacium species occur in the alimentary tract of marine fishes rather than fresh-water ones. In common with some other aquatic ascaridoids, species of Hysterothylacium have evolved remarkable flexibility in completing the life-cycle. For example, Luque et al. (2007) reported L4 of Hysterothylacium sp. in the haemocoel of amphipods, Paracorophium excavatum , from New Zealand waters. Furthermore, several of the adult males showed precocious sexual development, having undergone the 3rd and 4th (final) moults in the amphipod host. Moreover, H. haze also departs from the life-cycle known for many other species. Yoshinaga et al. (1989) studied natural and experimental infections of H. haze in gobies Acanthogobius flavimanus from Tokyo Bay, concluding that the life-cycle may be direct and involve invertebrates as paratenic hosts; this life-cycle might represent an example of extreme precocity in which the fish intermediate host has also become a final host (Anderson 1998).

Like Contracaecum and Phocascaris (see above) the genus Hysterothylacium is defined by characteristics of the adults, and agreeing again with Moravec (2013) it is difficult to assign larvae to any one species. It is not possible to design a key for their reliable identification, and the following keys are for the identification of the adults only. Feeding experiments to rear larvae to adults and the application of molecular markers to identify species may be solutions to this and similar problems. Several taxa are considered and, where possible, described, their names having appeared in the relevant Canadian literature. Only two Hysterothylacium species (** H. analarum and ** H. brachyurum ) are known from Canadian fresh-water fishes. While four species (** H. aduncum , H. corrugatum , H. incurvum and H. reliquens ) have been reported from Canadian marine fishes, only ** H. aduncum is abundant, having been recorded from a wide variety of fish species. In contrast, there is only one report, that of Hogans et al. (1983), for each of the other three Hysterothylacium species. All of the specimens were recovered from the stomachs of swordfish, Xiphias gladius , collected from the “northwest Atlantic Ocean”. The coordinates for the collection site(s) are not given but it may be assumed that these were within Canadian waters, so the descriptions are given below. Swordfish are oceanic travellers that enter Canadian waters usually about June and leave in October and November. They move through a great range of depths, feeding on various fish species and shortfin squid, Illex illecebrosus ( Scott and Scott 1988) . It is possible, therefore, that infections with the three species of Hysterothylacium were acquired by swordfish at locations remote from Canadian waters. It is worth noting that Hogans et al. (1983) deposited representative specimens of each nematode species (and of the other helminths collected) at a public museum and provided catalogue numbers; details may be found in the original paper. While two other species of Hysterothylacium , H. magnum and H. melanogrammi , have been recorded from Canadian marine fishes the species are of doubtful validity, as discussed below.