Halosaurotrema, Martin & Huston & Cutmore & Cribb, 2019

HALOSAUROTREMA View in CoL GEN. NOV.

Diagnosis: Body pyriform. Tegument smooth. Oral sucker unspecialized, subterminal, subspherical. Ventral sucker unspecialized, larger than oral sucker, transversely ellipsoidal. Prepharynx short. Pharynx unspecialized, slightly smaller than oral sucker. Oesophagus distinct, unspecialized. I n t e s t i n e b i f u r c a t e s i n f o r e b o d y. C a e c a b l i n d, extending near to posterior extremity. Testes two, irregularly lobed, diagonal, contiguous, filling much of hindbody; post-tesicular zone very short, roughly equal to post-caecal zone. Cirrus-sac well developed, overlapping ventral sucker, not extending into h i n d b o d y. S e m i n a l v e s i cl e i n t e r n a l, o c c u p y i n g about one-half cirrus-sac, broad proximally, narrowing abruptly prior to pars prostatica. Pars prostatica narrow, continuous with ejaculatory duct. Common genital atrium simple. Genital pore pre-bifurcal, sinistro-submedial. Ovary irregularly lobed, dextro-submedial, contiguous with anterior testis. Seminal receptacle canalicular, smaller than ovary. Laurer’s canal present, opens dorsal to ovary. Mehlis’ gland present. Uterus preovarian. Vitellarium follicular, distributed length of caeca, becoming confluent dorsally in forebody. Eggs operculate, unembryonated in utero. Excretory vesicle tubular, extends to level of ovary. Excretory pore terminal. In bathypelagic halosaurid fishes (Notocanthiformes) of the north Atlantic.

Type and only species: Halosaurotrema halosauropsi (B r a y & C a m p b e l l, 1 9 9 6) c o m b. n o v. (s y n. Gaevskajatrema halosauropsi ).

ZooBank registration LSID: http://www.zoobank. org/ urn:lsid:zoobank.org:act:3C89E842-E7C6-482D-B24A-21F536D9EF86

Etymology: Halosaurotrema is named for the host family of the only species, the Halosauridae . The name is derived from Greek halos, salt, sauros, lizard and trema for trematode.

Remarks: Gaevskajatrema halosauropsi requires a new genus. It was described based on material recovered from Halosauropsis macrochir (Günther) (Notocanthiformes: Halosauridae ) collected between 1908 and 2570 m deep in the north Atlantic ( Bray & Campbell, 1996); it is the only opecoelid known from a notocanthiform fish. Conversely, G. perezi (Mathias, 1926) Gibson & Bray, 1982 , the type-species, and G. pontica (Koval, 1966) , Machkevsky, 1990, the only other member of the genus, are known from shallow-water wrasses ( Perciformes : Labridae ) from the eastern Mediterranean to the Black Sea and from off Roscoff, north-western France ( Gibson & Bray, 1982). Phylogenetic analyses show that G. perezi and G. halosauropsi are only distantly related, with the type-species resolving among the Opistholebetinae ( Martin et al., 2018b) . The morphological characters considered to be most important for defining Gaevskajatrema are caeca terminating lateral to the testes and vitelline follicles not extending beyond the testes posteriorly. Strictly, G. halosauropsi exhibits both conditions, but not in a form consistent with that seen in G. perezi . In that species, the caeca are short such that the post-caecal zone occupies more than one-quarter of body length, and there is an appreciable post-testicular zone from which the vitelline follicles are excluded. In G. halosauropsi , the caeca and vitelline follicles reach near to the posterior extremity, as is typical for most opecoelids, but the hindbody is relatively short and the testes large, such that the post-testicular zone is roughly equivalent to the post-caecal and post-vitelline zones. Furthermore, in G. halosauropsi the ovary and testes are irregularly lobed vs. smooth and the excretory vesicle extends to the level of the ovary vs. the posterior testis. Thus, we propose Halosaurotrema gen. nov. and H. halosauropsi comb. nov. The new concept is similar to Neolebouria (s.s), but differs in general body shape, the relatively short hindbody and post-testicular zone, and in that the testes are diagonal and the ovary dextro-submedial vs. all gonads medial.