Holsworthotrema, Martin & Huston & Cutmore & Cribb, 2019

HOLSWORTHOTREMA View in CoL GEN. NOV.

Diagnosis: Body elongate, subcylindrical. Forebody short, less than one-fifth body length. Tegument smooth in forebody, textured with fine projections in hindbody. Oral sucker unspecialized, terminal, opening antero-ventrally, roughly spherical. Ventral sucker larger than oral sucker, roughly spherical, pedunculate, papillate with 7–9 small, round papillae either side of aperture. Prepharynx very short. Pharynx unspecialized, smaller than oral sucker, subspherical. Oesophagus short, constricted. Intestine bifurcates at approximately 45° in forebody. Caeca thin, unite with excretory vesicle to form uroproct close to posterior extremity. Testes two, ellipsoidal, smooth, tandem, usually separate, intercaecal, situated more than 50% of body length from anterior extremity; post-testicular zone less than 25% of body length. Cirrus-sac well developed, exceptionally long, highly sinuous, extends well into hindbody, occupies more than 33% of body length. Internal seminal vesicle long, sinuous, broad, narrowing anteriorly. Pars-prostatica indistinct. Ejaculatory duct exceptionally long, tightly sinuous. Cirrus sometimes everted. Genital atrium short, simple. Genital pore sinistro-submedial, bifurcal or immediately post-bifurcal, small, often indistinct. Ovary smooth, roughly spherical, medial, anterior to and separated by short distance from anterior testis, situated about 50% of body length from anterior extremity. Seminal receptacle uterine. Laurer’s canal present, short, opening dorsal to ovary, widening proximally into small, receptacle-like chamber. Oviduct short. Oötype prominent. Uterus sinuous, preovarian, intercaecal, ascending dorsal to cirrus-sac. Metraterm thin to indistinct. Mehlis’ gland prominent, extensive, antero-dorsal to ovary. Vitellline reservoir smaller than and antero-dorsal to ovary. Vitelline follicles numerous, regular, restricted to hindbody, distributed laterally and ventrally, sparse dorsally, interrupted at level of ovary and each testis. Eggs operculate, tanned, unembryonated in utero, without polar protuberances or filaments. Excretory vesicle tubular, extends to level of ovary. Excretory pore terminal or dorso-subterminal. In the digestive tract of herbivorous, marine fishes of the genus Kyphosus ( Kyphosidae ) in southern Australian waters.

Type-species: Holsworthotrema enboubalichthys sp. nov.

Other species: Holsworthotrema chaoderma sp. nov.

ZooBank registration LSID: http://www.zoobank. org/ urn:lsid:zoobank.org:act:30C5014D-2AF3-447E-900A-5861F640397A

Etymology: The new genus is named in honour of and in gratitude to Dr William (Bill) and Carol Holsworth. The Holsworth foundation has generously supported Ph. D. candidatures of SBM and DCH, has directly enabled this study, and has contributed to various other student projects undertaken at the Marine Parasitology Research Laboratory, The University of Queensland.

R e m a rk s: T h e t w o n e w s p e c i e s f o r w h i c h w e propose Holsworthotrema occur in sympatry with S. longistipes in phylogenetically related and ecologically similar host fishes. They are clearly closely related to that species, differing at 9, 16 to 17, and 8 to 9 base-positions in the ITS 2, 28S and 18S rDNA regions, respectively. The two new species also share several morphological features with S. longistipes , including an elongate body, short forebody, specialized tegument, pedunculate ventral sucker, long cirrus-sac extending well into the hindbody, long seminal vesicle, entire ovary, preovarian uterus, uterine seminal receptacle and vitelline follicles restricted to the hindbody and not reaching to the level of the posterior margin of the ventral sucker. However, several important morphological distinctions are also apparent such that these two new species should be allocated to a distinct genus. Most importantly, the caeca form a uroproct in the new species but terminate blindly in S. longistipes , a distinction always considered of generic significance among the Opecoelidae ( Cribb, 2005) , and usually also among digeneans more generally. Additionally, in the new species the uterus runs dorsal vs. ventral to the cirrus-sac, the cirrus-sac and ejaculatory duct are much longer and prominently sinuous, the ventral sucker is larger than the oral sucker vs. smaller, the ventral sucker peduncle is relatively short and broad vs. exceptionally long and thin, and the ventral sucker has papillae.

The two new species are also similar morphologically to another stenakrine concept, Pseudopecoelina Yamaguti, 1942 . Species of that genus do have a uroproct and essentially differ from the new species only in that the ventral sucker lacks papillae and is sessile vs. pedunculate, and in that the cirrus-sac and ejaculatory duct, although long and tubular, are never as long as in the new species and are simple vs. strongly sinuous. There has been no suggestion of any tegument specializations in species of Pseudopecoelina , although none has been examined with SEM. Species of Pseudopecoelina are known only from shallow-water carnivorous fishes in the Tropical Indo-West Pacific. The type-species, P. dampieriae Yamaguti, 1942 , is known only from the firetail dottyback Labracinus cyclopthalmus (Müller & Troschell) ( Perciformes : Pseudochromidae ) [as Dampieria hellmuthi (Bleeker) ] in Japanese waters (Yamaguti, 1942). Therefore, it seems likely that the two new species will prove to be more closely related to S. longistipes than to species of Pseudopecoelina .