Rhinogobius mizunoi Suzuki & Shibukawa & Aizawa 2017, sp. nov.

Rhinogobius mizunoi sp. nov.

(Japanese name: Ruri-yoshinobori)

(Figs 1–6, Table 2)

Rhinogobius brunneus (not of Temminck & Schlegel, 1845): Hayashi in Masuda et al., 1984: 270 (in part: Cobalt type).

Rhinogobius sp. CO; Mizuno in Kawanabe & Mizuno, 1989: 592 (Japan exclusive of east Hokkaido and the Ryukyu Islands); Akihito et al., 2002: 1252 (Japan from Hokkaido southward to Kyushu and Cheju Island, Korea); Suzuki et al., 2004: 454 (Japan from Hokkaido southward to Kyushu and Cheju Island, Korea); Akihito et al., 2013: 1456 (Japan from southwestern Hokkaido southward to Kyushu and Cheju Island, Korea).

Holotype. SPMN-PI 3196, male, 67.2 mm SL, Satogawa River, Ochiai-gawa tributaries, Inouzawa-gawa water system, Ochiai, Shimoda, Izu Peninsula, Shizuoka Prefecture, Japan, 34°43'10.9"N 138°56'57.2"E, collected by Y. Kitahara and K. Kano, 22 August 2013.

Paratypes. Five specimens (male and four females, 51.6–77.5 mm SL), collected with holotype: KPM-NI 41338, female, 77.5 mm SL; OMNH-P 40617, male, 73.6 mm SL, a cleared and stained; OMNH-P 40618, female, 61.9 mm SL; OMNH-P 40619, female, 55.2 mm SL; OMNH-P 40620, male, 51.6 mm SL.

Digital images examined. KPM-NR 78800 & 91331, lower reaches of Okitsu-gawa River, Shizuoka Prefecture, Japan, 26 August 2006, 1 m depth, photographed by K. Uchino.

Diagnosis. Rhinogobius mizunoi is distinguished from all congeners by having the following unique combination of characters: 13–18 predorsal scales; 33–35 longitudinal scale series; 8 or 9 transverse scale series; 10+16=26 vertebrae; first dorsal fin elongate in male, its distal tip reaching to base of fourth branched ray of second dorsal fin when adpressed; when alive or freshly-collected, cheek with several pale sky spots; caudal fin lacking distinct rows of dark dots; a pair of dark brown blotches at caudal-fin base in young and females.

Description. In the following description of meristics, data from the holotype have an asterisk, and the frequency of each count is given in parentheses following the relevant count. Dorsal-fin rays V-I, 8 (1) or VI-I, 8* (5); anal-fin rays I, 8* (5) or I, 9 (1); pectoral-fin rays 18 (1), 19* (4), or 20 (7); pelvic-fin rays I, 5* (12); segmented cauda-fin rays 9+8* (6); branched caudal-fin rays 7+7* (5) or 8+7 (1); longitudinal scales 33 (1), 34* (2), or 35 (3); transverse scales 9* (5) or 10 (1); scales between origin of dorsal and pectoral fin 9 (1), 10* (3), 11 (1), or 12 (1); predorsal scales 13 (1), 14 (2), 15 (1), 16* (1), 18 (1); P-V 3/II II I I 0/9* (6); vertebrae 10+16=26* (6); gill rakers 3+7 (1).

Proportional measurements are given in Table 2. Body slender, almost cylindrical anteriorly, compressed posteriorly. Head moderately large, slightly depressed. Eye large, located dorsolaterally on head on a vertical line a little before midway between snout tip and posterior margin of preopercle. Interorbital relatively wide, slightly wider than pupil diameter. Cheek somewhat fleshy. Lips thick and fleshy; upper lip slightly protruding beyond lower lip; gape oblique, forming an angle of about 20 degrees with body axis; rear margin of lower jaw extending to and slightly beyond a vertical through anterior margin of eye in female and male, respectively. Anterior naris a short tube without skin flap at its tip; posterior naris a round pore with low rim, closer to anterior naris than to eye. Gillopening extending anteriorly to a vertical through posterior margin of preopercle. Gill membranes broadly attached to isthmus. No fleshy papilla- or finger-like projections on lateral margin of shoulder girdle. Tongue free from floor of mouth, with rounded anterior margin. Mental flap on chin weakly developed, nearly rectangular. Genital papillae is cone-shaped and oval-shaped in male and female, respectively.

In the following description of the fins, data for the holotype (male) are given first, followed by data for the paratypes in parentheses where different. First dorsal fin originated 1.5 times the width of the orbit-diameter behind from uppermost of pectoral-fin base, falcate (nearly semicircular in females); second spine longest but not filamentous; when adpressed, rear tip of first dorsal fin extending to base of fourth branched rays of second dorsal fin (not to origin of second dorsal fin in females). Second dorsal fin originated an orbit-diameter width behind from end of first dorsal-fin base (1.3 orbit-diameter behind in a female); second dorsal fin lower than first dorsal fin (equal to first dorsal fin in a female); all segmented rays of second dorsal fin branched; seventh branched ray longest (second, third, fourth, fifth or sixth in females); when adpressed, rear tip of second dorsal fin reaching to procurrent rays of caudal fin (not reaching to caudal fin in females); posteriormost base of second dorsal fin at a vertical through posteriormost of anal-fin base. Anal fin originated below base of first branched ray of second dorsal fin (below base of second branched ray in one, between first and second branched ray bases in four); anal fin lower than second dorsal fin; all segmented rays branched; seventh branched ray longest (sixth in a male, fourth in a female, fourth to sixth in two females); rear tip not reaching to caudal fin when adpressed. Pectoral fin elliptical; rear tip reaching to a vertical through interspace between base of sixth spine and posterior end of base of first dorsal fin (to a vertical through base of sixth spine of first dorsal fin in two females). Pelvic fins fused medially via a frenum (between spines) and connecting membrane (between innermost rays), forming a rounded disc (longitudinally-elongate oval in two, transversely-elongate oval in one); rear tip reaching to a vertical through base of fifth spine of first dorsal fin (to a vertical through interspace between bases of third and fourth spines in a male, between bases of fourth and fifth spines in a male, between bases of second and third spines in three females); round membranous lobe developed around distal tip of pelvic-fin spine; first branched ray of pelvic fin longer than pelvic-fin spine; fifth pelvic-fin ray divided into four primary branches just above its base (Fig 1B). Caudal fin elliptical (round fan-shaped in four).

Abbreviations: SL: Standard Length; D1: First dorsal fin; D2: Second Dorsal fin; HL: Head length

Scales on body moderately large and ctenoid posteriorly, small and cycloid anteriorly. Base of pectoral fin with some small cycloid scales. Belly with small cycloid scales. A small cycloid scale between genital papillae and origin of anal fin in alizarin-red stained paratype (OMNH-P 40617). Anterior part of predorsal area naked. Predorsal squamation with trifurcate anterior edge and anterior extension of middle series extending to beyond above through canal pore H' (Fig. 1A). The other part of head and prepelvic area naked.

Teeth in both jaws conical, inwardly curved; upper jaw with 3–4 rows of medium-sized teeth anteriorly; middle and posterior parts of upper jaw with an outer row of eight large teeth, and anterior half of central part with an inner row of medium-sized teeth; anterior half of lower jaw with an outer row of nine large teeth, and two to three inner rows of medium-sized teeth, central part with an inner medium-sized tooth, posterior part with an outer large tooth.

Cephalic sensory systems are illustrated in Fig. 2. Nasal extension of anterior oculoscapular canal with terminal pore B' located above anterior naris. Anterior interorbital sections of anterior oculoscapular canal separated, with paired pore C and a single pore D. Pore E present just behind posterior edge of eye. Lateral section of anterior oculoscapular canal with anterior pore F and terminal pore H'. Posterior oculoscapular canal with two terminal pores K' and L'. Gap between anterior and posterior oculoscapular canals slightly smaller than length of posterior oculoscapular canal. Preopercular canal present, with three pores M', N, and O'. Sensory-papillae row a oblique and uniserial, composed of loosely-arranged papillae, extending anteriorly to a vertical through anterior margin of eye. Row b longitudinal, extending anteriorly to a vertical through middle of eye; its length slightly longer than eye diameter. Rows c and d composed of denselyarranged papillae, extending posteriorly to, or slightly behind, a vertical through posterior margin of eye. Row cp comprising a single papilla. Row f comprising a paired papillae. Anterior end of row oi well separated from a vertical row ot.

Coloration of male (Figs. 3 & 4). Freshly-collected coloration of male holotype is as follows. Ground color of head and body light bluish gray and light bluish gray to yellowish gray, respectively. Ventral side of belly white. Cheek with 10 or more small pale-sky spots and a deep pink network pattern. Snout with a vivid red oblique stripe between eye and anterior one fourth of upper lip; several irregular-shaped, short vivid-red lines/spots on dorsal surface of snout and interorbital space; dorsoanterior margin of cheek edged by a narrow deep-pink line; upper part of operculum with two oblique vivid red broken stripes. Nape and occipital region with several irregular-shaped, essentially longitudinal or oblique vivid-red stripes. Branchiostegal membrane tinged with orange ventrally, without any distinct markings. Many of scale pockets on body edged by deep yellowish pink. Dorsum of body with four saddle-like, large light-gray blotches; anteriormost one below base of first dorsal fin, middle one below base of second dorsal fin, and the other two on caudal peduncle. Fin membranes grayish white. Vertical fins with deep pink rays; distal margins of dorsal, anal, and caudal fins white; anal fin whitish basally. Base of pectoral fin white with two deep yellowish pink crescent-shaped marks. All reddish, yellowish, and bluish colorations faded after preservation in alcohol (Fig. 4C).

Coloration of female (Fig. 5). Freshly-collected coloration of female resembles that of male, except as follows. Ground color of head, body and fins more yellowish. Cheek spots pale, less in number. Caudal-fin base with a pair of diagonal dark-brown blotches, forming an incomplete, bold C-shape. All reddish, yellowish, and bluish colorations faded after preservation (Fig. 5C).

Coloration when alive (Fig. 6). Coloration when alive in underwater photographs resembles freshly-collected coloration, except as follows. Cheek with more pale-sky spots in number. Many of body scales with pale sky spots, and deep yellowish pink of scale pockets much more indistinct. Lower lip dull pale sky in male. Dorsal side of body with about six (5–7) dark gray saddle-like blotches anteriormost one on nape, middle 3–4 below dorsal fins, and posterior 2–3 on caudal peduncle. Mid-lateral side of body with a longitudinal series of eight large dark-gray blotches.

Distribution. Known from middle reaches or mountain torrents of freshwater streams in Japan (western Hokkaido southward to southern Kyushu) and Cheju Island, Korea (Akihito et al., 2013); sometimes landlocked in freshwater reservoirs (e.g., Mizuno, 1989).

Etymology. The new species is named after Dr. Nobuhiko Mizuno, the former professor of Ehime University, Japan, in honor of his great contribution to our knowledge of the ecology of freshwater fishes in Japan, particularly gobies of Rhinogobius.

Comparison. Rhinogobius is currently known as the most species-rich freshwater goby genus, comprising 72 described, valid species (Table 1). As indicated by Chen & Shao (1996) and Suzuki et al. (2015), the genus Rhinogobius is divided into two distinct groups; one comprises only a single species R. similis [referred as Rhinogobius giurinus (Rutter, 1897), a junior synonym, by many authors including Chen & Shao, 1996; see Suzuki et al., 2015], whereas the other includes all the remaining species. Rhinogobius similis differs from the other congeners by having large ctenoid scales on the nape (vs nape naked or with cycloid scales in the others) and several short transverse rows of sensory papillae on the cheek (vs no distinct transverse rows of sensory papillae on cheek) (Suzuki et al., 2015). The new species R. mizunoi clearly belongs the latter group, here is treated as within the Rhinogobius brunneus complex, following Chen & Shao (1996).

The Rhinogobius brunneus complex is divided into two distinct groups. One, comprising 37 species (Huang et al., 2016), has almost always 27 or more vertebrae, whereas the others, including 25 species, has lower count of vertebrae (25–27, almost always 26) (e.g., Chen & Kottelat, 2005; Wu & Chen, 2008; Li & Zhong, 2009; Akihito et al., 2013); no information about the vertebral counts are known in the other nine congeners. Rhinogobius mizunoi, having 26 vertebrae, is readily distinguished from the first group.

Abbreviations: D1, first dorsal fin; D2, second dorsal fin; *, segmented ray base of D2; **, of D2.

These 35 congeners having low or unknown vertebral counts are listed in Table 3 with selective characters. Rhinogobius mizunoi can be distinguished from all others but three species (viz., R. candidianus, R. fluviatilis, and R. nantaiensis) by having the following combination of characters: 13–18 predorsal scales; first dorsal fin elongate in male, extending posteriorly well beyond origin of second dorsal fin (reaching to base of fourth segmented ray in the holotype) when adpressed; no distinct vertical dark lines or rows of dark spots on caudal fin (Regan, 1908a; Aonuma & Chen, 1996; Chen & Shao, 1996; Wu & Chen, 2008; Suzuki & Chen, 2011).

Rhinogobius candidianus and R. nantaiensis differ from R. mizunoi in having 12–14 and 11–13 transverse scales, respectively (vs 8–9 in R. mizunoi) (Aonuma & Chen, 1996; Chen & Shao, 1996). Rhinogobius fluviatilis (Fig. 7), a sympatric species in temperate Japan, is also distinguished from R. mizunoi by having: when alive or freshly-collected, no sky spots on the cheek (vs present in R. mizunoi); a distinct dark spot at upper base of pectoral fin (vs two deep yellowish pink crescent-shaped marks on the pectoral-fin base; a distinct vertical dark band along the caudal-fin base (vs a pair of diagonal dark brown blotches at caudal-fin base) (Suzuki & Chen, 2011; Akihito et al., 2013).

Several sky spots on the cheek appear to be unique for R. mizunoi amongst the described species of Rhinogobius, although the fleshly-collected coloration in detail of some non-Japanese species is hitherto unknown. An undescribed species, Rhinogobius sp. MO of Mizuno (1989) from the Ryukyu Islands, is known to have bright sky spots on the cheek, but has several vertical rows of dark spots on the caudal fin (e.g., Akihito et al., 2013).

Remarks. Rhinogobius mizunoi was one of the best known unidentified species of the genus in Japan. Due to previous confusion in taxonomy of the Japanese species of Rhinogobius (see, e.g., Suzuki & Chen, 2011; Suzuki et al., 2015), its identification was left unresolved. Mizuno (1972) first recognized this as a color morph of R. brunneus sensu Takagi (1962) from western Shikoku of Japan; he named it as “Nise-o-gata” (meaning “false large morph”) in Japanese vernacular, because the size of the adult fish is similar to “Kokusyoku-o-gata” (meaning “blackened large morph”), the other color morph named by him, the “Kokushoku-o-gata” is currently identified as a distinct species R. fluviatilis (see Suzuki & Chen, 2011). The “Nise-o-gata”, however, renamed as “Ruri-gata” (meaning “cobalt morph”) by Mizuno (1976) in reference to many pale sky spots on cheek and body. Subsequently, Mizuno (1989) regarded the morph as a distinct species based on ecological, ethological, and/or molecular genetics evidences, and recognized it as an unidentified species with the provisional name Rhinogobius sp. CO (the “CO” is an abbreviation of “Cobalt”) or "Ruri-yoshinobori", a Japanese vernacular meaning “a goby of Rhinogobius in cobalt”. Since then, all of the Japanese ichthyologist used the Mizuno’s (1989) provisional name for this species (e.g., Akihito et al., 2013).