INTRODUCTION

Eumicrotremus soldatovi Popov, 1930 belongs to the Cyclopteridae family of lumpsuckers (Soldatov and Lindberg, 1930), which currently includes twenty-four fish species common in the Arctic and Far Eastern waters (Voskoboinikova et al., 2020). As a rule, its representatives are benthic forms. Among them, only a few species (Cyclopterus lumpus L., 1758, Aptocyclus ventricosus (Pallas, 1769), E. soldatovi, and E. asperrimus Tanaka, 1912) move to the pelagic habitats to different extent during the feeding period, approaching coastal shallow waters only for spawning (Orlov, 1993; Mel’nikov, 1995; Kudryavtseva, 2008; Orlov and Tokranov, 2008; Antonenko et al., 2009; Solomatov and Orlov, 2018). This is owing to largely the partial reduction of the skeleton, the presence of extensive subcutaneous gelatinous tissue and low-density muscles (Kudryavtseva, 2008), as well as the ability to distend the stomach greatly and fill it with water or air. When studying the morphology of E. soldatovi, we assumed that the structural features that distinguished it from the other lumpsucker species were especially due to its bentho-pelagic lifestyle (Mel’nikov, 1995). However, both morphological and molecular reconstructions of the relationships of lumpsuckers revealed not only the isolation of E. soldatovi from other species of the genus Eumicrotremus, but also its intermediate position between them and the genera of generalized lumpsuckers (Voskoboinikova et al., 2020). A number of morphological differences between E. soldatovi and other representatives of the genus Eumicrotremus and its isolated position in the morphological and molecular cladograms indicate the possibility to establish genera status for E. soldatovi.

MATERIALS AND METHODS

The study was carried out on the material from the fund collection of the Zoological Institute of Russian Academy of Sciences (ZIN RAS).

ZIN no. 22003—holotype, Sea of Okhotsk, 57°28′ N 148°5′ E, June 27, 1914, depth 90 m, collector Hydrographic Expedition to the Eastern Ocean; ZIN no. 22031—1 spc., vessel Kapitan Bering, Sea of Okhotsk, from the Iona Island, August 27, 1908, collector B.D. Geineman; ZIN no. 25231—1 spc., R/V Plastun, Sea of Okhotsk, August 13, 1932, collector I.A. Polutov; ZIN no. 25233—6 spc., fishing vessel Askold, Sea of Okhotsk, 57°02′ N 141°40′ E, September 5, 1932, depth 147 m, collector M.N. Krivobok; ZIN no. 49017—7 spc., Novodrutsk, Sea of Okhotsk, 55°58′5″–55°52′6″ N 140°04′5″–140°05′ E, depth 145–140 m, mid-water trawl RT 118/620, October 13, 1987, collector V.V. Fedorov; ZIN no. 49018—1 spc., Novodrutsk, Sea of Okhotsk, 59°59′4″–59°59′2″ N 140°04′5″–140°05′ E, depth 200–0 m, oblique tow, trawl no. 329, mid-water trawl RT 118/620, September 28, 1987, collector V.V. Fedorov; ZIN no. 53838—1 spc., R/V Zodiak, Sea of Okhotsk, 58°32′ N 148°19′ E, depth 128 m, trawl no. 14 m, August 18, 2001, collector M.V. Nazarkin; ZIN no. 53839—1 spc., R/V Zodiak, Sea of Okhotsk, 59°11′ N 148°03′ E, depth 71 m, trawl 16 m, DT, 18.08.2001, collector M.V. Nazarkin; ZIN no. 53840—1 spc., R/V Zodiak, Sea of Okhotsk, 58°31′ N 149°49′ E, depth 122 m, trawl 20 m, DT, August 19, 2001, collector M.V. Nazarkin; ZIN no. 53841—1 spc., R/V Zodiak, Sea of Okhotsk, 58°30′ N 150°15′ E, depth 123 m, trawl 21 m, DT, August 19, 2001, collector M.V. Nazarkin; ZIN no. 54357—3 spc., Sea of Okhotsk, 57°53′ N 152°08′ E, depth 203–214 m, trawl no. 27, July 26, 2006, collector M.V. Nazarkin; ZIN no. 54370—1 spc., fishing vessel Kapitan Melamud, Sea of Okhotsk, 57°54′ N 152°09′ E, depth 206–215 m, shrimp trawl, trawl no. 409, October 8, 2006, collector M.V. Nazarkin; ZIN no. 54753—1 spc., fishing vessel Kapitan Melamud, Sea of Okhotsk, Pritauiskii region, 57°57′ N 151°45′ E, depth 216 m, trawl no. 56, July 31, 2006, collector M.V. Nazarkin; ZIN no. 54754—1 spc., fishing vessel Kapitan Melamud, Sea of Okhotsk, Pritauiskii region, 57°57′ N 151°33′ E, depth 222–224 m, bottom trawl, trawl no. 100, August 7, 2006, collector M.V. Nazarkin; ZIN no. 55983—1 spc., fishing vessel Kapitan Melamud, Sea of Okhotsk, Pritauiskii region, 57°52′ N 152°04′ E, depth 223–226 m, July 11, 2006, collector M.V. Nazarkin; ZIN no. 56265—1 spc., R/V TINRO, Sea of Okhotsk, Shelikhov Gulf, 59°6′ N 158°5′ E, depth 147–145 m, trawl no. 159, April 30, 2014, collector O.A. Maznikova; ZIN no. 56266—6 spc., R/V TINRO, Sea of Okhotsk, 56° N 139° E, depth 115 m, trawl no. 284, May 22, 2014, collector O.A. Maznikova; ZIN no. 56267—1 spc., R/V TINRO, Sea of Okhotsk, north-western Sakhalin, 52° N 143°8′ E, depth 53–50 m, trawl no. 313, May 27, 2014, collector O.A. Maznikova; ZIN no. 56361—1 spc., R/V TINRO, northern Sakhalin, 52° N 143°8′ E, depth 53–50 m, trawl no. 313, May 27, 2014, collector O.A. Maznikova.

Morphometry was studied according to the standard methods applied in previous works (Voskoboinikova and Nazarkin, 2015). The structure of the axial skeleton and the number of rays in unpaired fins were studied using radiographs. The scheme of the seismosensory system is given in accordance with the work of Mandritsa (2001). The morphological terminology is mainly followed the previous publications (Lindberg and Legeza, 1955; Ueno, 1970). The abbreviations used in the text: SL—standard body length; D, A, P, V, C—dorsal, anal, pectoral, pelvic and caudal fins, respectively; canals of the seismosensory system: CSO—supraorbital, CIO—infraorbital, CT—temporal, CPM—preoperculo-mandibular.

Proeumicrotremus gen. nov.

Type species Eumicrotremus soldatovi.

The grammatical gender is masculine.

Etymology. The genus name is formed by the Latinized Greek words pro (first, in front) and Eumicrotremus (eu—own, mikros—small, trema—sucker).

Composition. One species Proeumicrotremus soldatovi (Fig. 1).

Fig. 1.
figure 1

Proeumicrotremus soldatovi: a—ZIN no. 22003, holotype juv. SL 28 mm; b—ZIN no. 55983, adult specimen SL 113 mm; designation of rows of plaques: cp—circumpectoral, if—interfin, so—supraorbital, da—superior ventral.

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Diagnosis. The body is not high, its height in pre-spawning fish does not exceed 46% SL. Head length is short, ~33% SL. D1 is high, triangular. The length of the D1 base is two or more times shorter than the distance between D1 and D2. The posterior ends of D2 and A locate far from the base of C. Three to four bony plaques and five free pterygiophores between D1 and D2. Two to five very small flattened bone plaques in the interorbital space between the supraorbital rows on the head. The circumpectoral row consists of seven to eight bony plaques, three to six of which are closely spaced to each other in a horizontal row above the pectoral fin, and the remaining one or two plaque(s) are separated from below by a noticeable gap. On the abdominal surface, there are two horizontal rows (upper and lower) of several (three to five) bone plaques closely spaced to each other. All bone plaques are radially striated. Their spines are poorly developed, except for the central one, usually curved caudally. The CIO has five pores. The CPM pores are located directly on the skin surface or at the apex of very short skin tubes, the height of latter is several times less than the pore diameter.

Comparative notes. Proeumicrotremus has a number of common features with Eumicrotremus: the plan of the structure of the external armour (among the Eumicrotremus species, it is absent only in the dwarf species E. jindoensis and E. uenoi (Lee et al., 2017)), the location of the canals of the seismosensory system on the head, the presence of lateral line pores on the body. At the same time, Proeumicrotremus differs from Eumicrotremus by the absence of rows of bone plaques in the interorbital space between the supraorbital rows, a greater number of bone plaques between D1 and D2 (3–4 versus 0–1–2), the horizontal arrangement of the upper part of the circumpectoral row and the upper abdominal row of bone plaques, as well as by the backward-curved form of bone plaques (in the Eumicrotremus species, they are vertical) (Fig. 2). Proeumicrotremus has five pores in the CIO versus one or two in the Eumicrotremus species (Fig. 3). The new genus is also distinguished from the Eumicrotremus species by a significant maximum body length (SL 26 cm (Mel’nikov, 1995) versus the largest of 18 cm in the genus Eumicrotremus observed in E. orbis (Voskoboinikova et al., 2020)), lower body height (38–46 versus 42–70% SL) and smaller head length (33 versus 35–50% SL), as well as by the posterior margins of rays D2 and A far not reaching the base of rays C, widely separated dorsal fins, and by relatively short base and triangular shape of D1 (Popov, 1931; Lindberg and Legeza, 1955). Most likely, these features are associated with the benthopelagic lifestyle of Proeumicrotremus, contributing to an increase in the speed of the fish movement. Despite its seeming sluggishness, an individual of this species moved away very quickly from the underwater vehicle, as noted by Orlov (1993). In comparison with other Cyclopteridae, Proeumicrotremus is similar to the most generalized representative of lumpsuckers Cyclopterus by the horizontal rows of bone plaques on the sides of the body (upper part of the circumpectoral row and upper abdominal row), which are homologous in location and shape to the middle and abdominal rows in C. lumpus (Ueno, 1970; Voskoboinikova et al., 2016; Voskoboinikova et al., 2020); the upper ends of the bone plaques bent backward (Popov, 1931); the presence of five pores in the periorbital canal of the seismosensory system; poor development or absence of cutaneous tubules of the pores of the preoperculo-mandibular canal (well expressed in Eumicrotremus species) (Lindberg and Legeza, 1955; Ueno, 1970). The new genus is primarily distinguished from other generalized genera of lumpsuckers, which include Aptocyclus, Cyclopsis, and Lethotremus, by the presence of external armour of the head and trunk, as well as by the peculiarities of the location of unpaired fins and pterygiophores of the dorsal row: far apart located D1 and D2, the presence of five free pterygiophores between by these fins, noticeable distance between the base of C and the posterior ends of D2 and A. Summarizing the analysis of the features of Proeumicrotremus, we can conclude that it occupies an intermediate position between the generalized genera of lumpsuckers and Eumicrotremus in the system of the Cyclopteridae family.

Fig. 2.
figure 2

The first bone plaque of the postorbital row of Proeumicrotremus soldatovi.

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Fig. 3.
figure 3

Seismosensory system of the head of Proeumicrotremus soldatovi ZIN no. 55983; canals: CSO—supraorbital, CIO—infraorbital, CT—temporal, CPM—preoperculo-mandibular, CC—coronal commissure; (⚫)—free neuromasts, (×)—intracanal neuromasts.

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