BATHYCREADIINAE, Martin & Huston & Cutmore & Cribb, 2019

Martin, Storm Blas, Huston, Daniel Colgan, Cutmore, Scott Christopher & Cribb, Thomas Herbert, 2019, A new classification for deep-sea opecoelid trematodes based on the phylogenetic position of some unusual taxa from shallow-water, herbivorous fishes off south-west Australia, Zoological Journal of the Linnean Society 186, pp. 385-413 : 404

BATHYCREADIINAE SUBF. NOV.

Diagnosis: Body elongate-oval to elongate, dorso-ventrally flattened, large. Tegument smooth. Forebody short, occupies less than one-fifth total body length. Oral sucker unspecialized. Ventral sucker larger than oral sucker, sessile to protuberant. Pharynx unspecialized. Prepharynx short. Oesophagus unspecialized. Intestine bifurcates in forebody or dorsal to ventral sucker. Caeca unite to form cyclocoel near posterior extremity. Testes two, smooth, medial, usually separate. Cirrus-sac well developed, extending into hindbody. Seminal vesicle, internal. Pars-prostatica present. Ejaculatory duct long. Common genital atrium simple. Genital pore pre-bifurcal, sinistro-submedial to sinistral. Ovary smooth, medial, smaller than, anterior to and usually separate from testes. Seminal receptacle canalicular. Laurer’s canal present. Uterus preovarian. Vitelline follicles restricted to hindbody, extending anteriorly beyond ovary, sometimes reaching posterior margin of ventral sucker. Eggs operculate, unembryonated in utero. Excretory vesicle tubular, extends to level of ovary. Excretory pore terminal. In demersal and bathydemersal, mesopelagic, northern Atlantic marine fishes of Gadiformes ( Lotidae , Macrouridae , Moridae , Phycidae )

Type and only genus: Bathycreadium Kabata, 1961 .

Remarks: The Bathycreadiinae is proposed for Bathycreadium . Analyses of molecular data available for B. brayi demonstrate that it is phylogenetically distinctive among the deep-sea and freshwater clade and consistently resolves as sister to the freshwater taxa ( Fig. 1). Bathycreadium currently comprises six species, five of which are known only from mesopelagic gadiform fishes in the north Atlantic, collected from depths between 340 and 665 m [no depth reported for B. elongata (Maillard, 1970) Bray, 1973 ]. These species agree closely in morphology, most significantly in that in each the caeca form a cyclocoel. Among opecoelids with a canalicular seminal receptacle, only one other genus, Nicolla Wisniewski, 1934 , is defined for species with a cyclocoel. The two concepts are distinguished by the presence vs. absence of vitelline follicles in the forebody. Nicolla comprises both freshwater and marine species. Sequence data generated from specimens identified as the type-species, N. skrjabini (Iwantitzky, 1928) Dollfus, 1960 [= N. ovata (Pigulewsky, 1931) Wisniewski, 1934 ], a freshwater form, indicate that it resolves external to all other freshwater plagioporine taxa, thus providing a link between the freshwater genus concepts and that of Bathycreadium . Nicolla skrjabini is separated by relatively long branch lengths from the remaining freshwater taxa represented in the analyses, but B. brayi is separated by even greater distinction ( Fig. 1). Moreover, like the other freshwater taxa, species of Nicolla have a short excretory vesicle, reaching only to the testes, as opposed to the ovary in species of Bathycreadium . Therefore, considering the ecological, morphological and phylogenetic distinctions relative to the freshwater taxa, we think it is most informative to recognize Bathycreadium in a separate subfamily.

The sixth species of Bathycreadium , B. mullii Abdel-Gaber et al., 2018 , probably does not belong in the genus. It was recently described based on specimens recovered from Mullus surmeletus ( Perciformes : Mullidae ) in the south-east Mediterranean ( Abdel-Gaber et al., 2018), a frequently reported combination for both Opecoeloides furcatus (Bremser in Rudolphi, 1819) Odhner, 1928 and Poracanthium furcatum Dollfus, 1948 , two elongate opecoeline taxa in which the caeca form a uroproct in the former and unite and open via a common anus in the latter.