New Records of Rare Psychrolutid Fishes (Psychrolutidae) in the Sea of Okhotsk

Two rare species of psychrolutid fish (Psychrolutidae) were found in the Sea of Okhotsk. Psychrolutes dolganovi was found at a depth of 142 m near Urup Island (southern Kuril Islands); earlier the species was known only from the holotype collected off the Pacific coast of Iturup Island. This species is recorded in the Sea of Okhotsk for the first time. P. pustulosus was collected at depths of 205–400 m along central part of Kuril Islands and in the northwestern part of the Sea of Okhotsk at depths of 240–294 m. A key to the species of the genus Psychrolutes inhabiting the northern Pacific Ocean is presented.


INTRODUCTION
According to Nelson (Nelson, 1982;Jackson and Nelson, 1998), the genus Psychrolutes includes 12 species, more than half of them inhabit the North Pacific (Nelson, 1982;Fricke, 1990;Mecklenburg et al., 2002;Nakabo, 2002;Parin et al., 2014). P. dolganovi Mandrytsa, P. paradoxus Günther, P. pustulosus (Schmidt), P. phrictus Stein et Bond are known from the waters of the Sea of Okhotsk (Mandritsa, 1993;Fedorov, 2000;Sheiko and Fedorov, 2000). Among the listed species, only P. phrictus reaches a total length (TL) of 70 cm and is considered a common species, while the maximal TL of other species does not exceed 120 mm. This is the reason why they are relatively rarely caught by trawls and dredges and, therefore, their morphology and zoogeography remain poorly studied. Recently redescribed pimpled sculpin P. pustulosus has been studied the most completely (Nazarkin et al., 2014). The range of this species covers the northern Sea of Japan and the Sea of Okhotsk, including the southern Kuril Islands (Yabe et al., 1995;Nazarkin et al., 2014). It is reported that this species is found in the waters of the western Kamchatka, the northern and central Kuril Islands (Sheiko and Fedorov, 2000), but in fact these data have not been confirmed, and the material has obviously not been preserved. Dolganov's sculpin P. dolganovi (Mandrytsa) is another representative of the family, apparently the rarest in the northern Pacific Ocean, known from the moment of description by a single specimen SL 35.5 mm caught near the southern Kuril Islands. This study describes new finds of these two rare representatives of the family Psycholutidae in the Sea of Okhotsk.

MATERIALS AND METHODS
New data on the distribution and ecology of P. dolganovi and P. pustulosus were obtained during two expeditions in the Sea of Okhotsk in August-September 2013 onboard the R/V Akvaresurs (A) (fishing bottom trawl 27.1/24.4) and in June-August 2019 onboard the R/V Akademik Oparin (AO) (beam trawl).
The following abbreviations are adopted in the text: SL-standard length; D, A, P, V, C-dorsal, anal, pectoral, ventral, and caudal fins, respectively; canals of the seismosensory system: CSO-supraorbital, CIOinfraorbital, CPO-postorbital, CT-occipital, CPMpreoperculomandibular, СLL-trunk, CC-coronal pore; ZIN-collection of the Zoological Institute of the Russian Academy of Sciences (St. Petersburg, Russia), MIMB-Museum of the National Scientific Center for Marine Biology of the Far Eastern Branch of the Russian Academy of Sciences (Vladivostok, Russia). The number of vertebrae and rays in unpaired fins was calculated from radiographs made using a Faxitron MX-20 X-ray machine.

RESULTS AND DSCUSSION Psychrolutes dolganovi (Mandrytsa)-
Dolganov's sculpin ( D e s c r i p t i o n. The body has three distinct wide dark stripes against a light background, one of which descends from D1 to the base of P, the second, from D2 to A, the third is located on the caudal fin (Figs. 1a,1b). A dark wide transverse stripe runs along the distal edge of the light pectoral fin. A narrow dark stripe stretches through the eye orbit, without reaching the eye, from its antero-inferior margin to the supraposterior one. There are 12 main unbranched rays in C. The peritoneum is light (Fig. 1c). There are small supraorbital barbels.
M e r i s t i c c h a r a c t e r s : D X 17, A 13, P 18, V I 3, C 8 + 6 + 6 + 5; gill rakers 0 + 7; branchiostegal rays 7 (3 anterior rays are longer than following four rays); number of vertebrate with urostyle 14 + 21 = 35; 14 pairs of epipleuralia (two anterior pairs are thicker than following). Pleuralia are absent. 13 pores in СLL.
On the upper part of the head, the number of pores is reduced: there are no pores in the frontal and occipital commissures, in CT and CSO, between the nasal and frontal bones, and in the area of the CIO and CPM junction. Number of pores as in type specimen: CPM 8, CIO 5, CSO 1.
L i v e c o l o r a t i o n. The upper part of the head and the occipital region, up to the beginning of the dorsal fin, are white with a barely noticeable yelloworange tint in the snout area, covered with small brown specks. The area of the operculum membranes is light yellow. The background color of the body is yellowishorange, the body is crossed by two wide dark brown stripes. The first one descends obliquely from the 2nd-8th ray of D1 to the upper part of the base of P, covering the bases of several upper rays. The second stripe covers rays 12-20 of D2, from where it extends to the entire anal fin. A dark brown stripe runs along the distal edge of P. Caudal fin is dark brown in front. Distal parts of rays P, D2, A, and C are white. The coloration of the head of specimen MIMB 40773 is somewhat different from that described for holotype ZIN 50136. A narrow longitudinal dark stripe passes through the eye backward from the 4th pore of the CIO. Another similar stripe runs transversely through both eyes to the 6th pore of the CPM. At the intersection in the eye area, both stripes form a cruciform pattern, which is absent in the type specimens. In the fixed specimen, the yellow-orange tones of color have disappeared; dark stripes are preserved.
N o t e s. Specimen MIMB 40773 generally corresponds to the original description, but has no tubercles on the skin at all, probably due to the dimensional variability of this trait (the holotype is larger). P. dolganovi differs from the representatives of the genus Psychrolutes inhabiting the northern Pacific Ocean by the combination of a unique coloration of three contrasting dark stripes on a light background of the body (two on the body and one on the caudal fin), a special coloration of the head, and the presence of supraorbital barbels. It is most similar to P. paradoxus and P. pustulosus by the structure of the caudal fin (the number of main rays, the shape and relative position of the bones of the hypural complex, and the shape of the processes of the second pre-caudal vertebra) (Jackson, Nelson, 1998;Nazarkin et al.,    (c) M e r i s t i c c h a r a c t e r s (mean and standard deviation are given in brackets). D1 VII-VIII (7.3 ± 0.49), D2 17-19 (18.1 ± 0.83), A 15-16 (15.4 ± 0.51), P 15-17 (15.9 ± 0.49), V I 3, C 4-5 + 7 + 7 + 4-5; gill rakers 2-4 + 10-14 (14.2 ± 1.48), branchiostegal rays 7, vertebrae (with urostyle) 11-13 + 19-22 = 32-34 (32.9 ± 0.69). Very large pores in CLL 10-13 (11.0 ± 1.08). All head canals usually have pores, their number is maximum in genus Psychrolutes: CSO 2, CC 0-1 (0.9); CIO 7-8 (7.1 ± 0.26), CPO 2-3 (2.9 ± 0.26); CT 0-3 (1.1 ± 1.06), CPM 9 (two pores on the chin). The diameter of some of the pores of the CPM and CLL, located closer to the base of C, is not less than the pupil diameter.
C o l o r a t i o n. In live specimens, the upper part of the body and fins are pink, sometimes with an admixture of gray; the belly and several lower rays of P are light white-pink, the peritoneum is dark gray (Fig. 2). Fixed specimens are light gray or white with dark gray peritoneum.
N o t e s. The fish from the northern Sea of Okhotsk have less pronounced skin tubercles compar-ing to the individuals from the waters of the Kuril Islands: in the first, the tubercles are located lower and less frequently. There is a dimensional variability of the cutaneous tubercles, which are well developed on the head, belly, and rays of unpaired fins in large specimens in comparison with small ones, in which the cutaneous tubercles are very weakly developed and are noticeable only at the base of P (MIMB 40774-SL 34 mm) or absent (MIMB 40776-SL 41 mm). D i s t r i b u t i o n a n d e c o l o g y. New finds expand the range of the species in the northwestern Sea of Okhotsk almost to Iona Island, and in the waters of the Kuril Islands to northward to Raikoke Island. In the waters of the central Kuril Islands at station no. 22, P. pustulosus was accompanied by Artediellus ingens Nelson, B. lysimus The identification table of the species of the genus Psychrolutes, inhabiting the northern Pacific Ocean from the southeast Japan to the Pacific coast of North America is presented below, compiled on the basis of original and literary data (Jordan, Starks, 1895;Nelson, 1980Nelson, , 1982Nelson et al., 1985;Fricke, 1990;Mecklenburg et al., 2002;Nakabo, 2002;Nazarkin et al., 2014).

TAXONOMIC KEY OF THE PSYCHROLUTES SPECIES INHABITING THE NORTHERN
PART OF THE PACIFIC OCEAN 1 (10) Body coloration monotonous or almost monochromatic, except for the distal part of the rays of P, the coloration of latter may differ slightly from the main one; there are no distinct contrasting dark stripes on the body; some specimens reach SL 600 mm.

(5)
The skin is covered with small tubercles (may be absent in fish SL < 40 mm); barbels on the skin of the head are absent; membranes D and A pass to base of C, caudal peduncle not pronounced; P 14-18; SL < 100 mm.

(4)
There are large pores in the coronal and occipital commissures; the posterior pore of the trunk canal is at the base of C; the pectoral fin reaches the beginning of the base of A; gill rakers 10-18 (Sea of Okhotsk and the northern part of Japan; 208-620 m).

FUNDING
The expedition to the Kuril Islands was organized with the financial support of the Ministry of Science and Higher Education of the Russian Federation and supported by grant no. 13.1902.21.0012, agreement no. 075-15-2020-796.

COMPLIANCE WITH ETHICAL STANDARDS
Conflict of interests. The author declares that he has no conflict of interests.
Statement on the welfare of humans or animals. All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.

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