Osteodiscus Stein, 1978

Murasaki, Kenta, Kai, Yoshiaki, Endo, Hiromitsu & Fukui, Atsushi, 2021, Osteodiscus abyssicola, a new snailfish (Cottoidei: Liparidae) collected off the Pacific coast of northern Japan, Zootaxa 5032 (1), pp. 136-142 : 137

Osteodiscus Stein, 1978

Standard Japanese name: Hariban-kusauo-zoku

Osteodiscus Stein, 1978: 24 (original designation, type species Osteodiscus cascadiae Stein, 1978 by monotypy).

Diagnosis. Osteodiscus is distinguished from all other genera by the following combination of characters: nostril single on each side; cephalic pore sizes ranging in size from similar to and much larger than nostril; branchiostegal rays six; pelvic disk present, skeletal, fleshly margin absent, basipterygia and pelvic rays covered only by thin skin, pelvic rays webbed between tips, webbing attenuated posteriorly; pleural ribs absent.

Species included. Four species: Osteodiscus abyssicola sp. nov.; Osteodiscus andriashevi Pitruk & Fedorov, 1990 ; Osteodiscus cascadiae Stein, 1978 ; Osteodiscus rhepostomias Stein, 2012 .

Distribution. All known species are from the Pacific Ocean: O. abyssicola sp. nov. from the western North Pacific, off the Pacific coast of northern Japan, in 4,671 –4,744 m depth; O. andriashevi from the western North Pacific, southern Sea of Okhotsk and off the Pacific coast of northern Japan, in 1,745 –2,108 m depth; O. cascadiae from the eastern North Pacific, off southern British Columbia to central California, in 2,195 –3,500 m depth; O. rhepostomias from the western South Pacific, off southeastern New Zealand, in 2,786 –2,821 m depth ( Stein 1978, 2012; Pitruk & Fedorov 1990; Stein et al. 2006; Murasaki et al. 2021; this study).

Comments. Osteodiscus was diagnosed in part by a deeply notched pectoral fin ( Stein 1978, 2012). However, this character is removed from the emended diagnosis of the genus because O. andriashevi has an unnotched pectoral fin (see Pitruk & Fedorov 1990; Murasaki et al. 2021). Kido (1988) proposed three autapomorphies of Osteodiscus (proximal pectoral radials two; second and third pelvic rays widely spaced; epipleural ribs absent), based on his morphological phylogenetic analysis. However, O. andriashevi , described two years later, has three or four proximal pectoral radials, and epipleural ribs ( Pitruk & Fedorov 1990; Murasaki et al. 2021). While O. rhepostomias has two proximal pectoral radials, the first radial is oddly shaped (an inverted triangle) and unusually large compared to other snailfishes ( Stein 2012). Additionally, it appears that the second and third pelvic rays of O. rhepostomias are not widely spaced (see Stein 2012: fig. 16). Accordingly, the three characters proposed by Kido (1988) have also been deleted from the generic diagnosis.

Although species of Osteodiscus can be clearly diagnosed from all other snailfishes by the emended diagnosis given above, the phylogenetic position of the genus is essentially unknown and its monophyly unproven. Kido (1988) placed Osteodiscus in a basal position to Careproctus Krøyer, 1862 and Paraliparis Collett, 1879 , but his analysis included only a single species of the former, O. cascadiae . In a recent molecular phylogenetic study by Orr et al. (2019), although Osteodiscus recovered a monophyletic group with species of Careproctus from the Southern Ocean, the authors conservatively proposed the informal name “Osteocareprocta” for the monophyletic group, since their analysis was also based only on O. cascadiae . Subsequently, Careproctus laperousei Chernova, Thiel & Eidus, 2020 , characterized by reduced musculature on the pelvic disk (similar to the condition in species of Osteodiscus ), was described from a single specimen collected from the northern slope of the Kuril-Kamchatka Trench, western North Pacific. However, C. laperousei clearly differed from species of Osteodiscus in having a fleshly margin on the pelvic disk and two paired pleural ribs on each posterior abdominal vertebra ( Stein 1978, 2012; Kido 1988; Pitruk & Fedorov 1990; Chernova et al. 2020; Murasaki et al. 2021; this study). Apart from O. cascadiae , specimens of Osteodiscus are very rare, being known only from the types or very limited additional specimens, and no genetic data are available. Clarification of the phylogenetic position and monophyly of Osteodiscus is dependent upon the collection of additional specimens, including those of C. laperousei .