Introduction

The gobiid genus Priolepis Valenciennes in Cuvier and Valenciennes 1837, comprising 36 valid, small-sized (up to 50 mm standard length) species, is widely distributed around shallow rocky and coral reefs in tropical to temperate waters of Indo-Pacific and Atlantic Oceans (Allen et al. 2018; Fricke et al. 2021; Senou et al. 2021). Further, 11 species have been known from southern Japan, excluding three unidentified or undescribed ones (Nogawa and Endo 2007; Hoese and Larson 2010; Fujiwara et al. 2020a, b; Senou et al. 2021). Although most inhabit shallow water, a few have been recorded from greater depth (e.g., Priolepis goldshmidtae Goren and Baranes 1995: two types were collected at a depth of 400 m in the Red Sea, and an additional one at 120 m from off Okinawa Island, Japan by Fujiwara et al. 2020a).

The genus is characterized by the gill opening extending anteroventrally below or just anterior to the vertical limb of the preopercle, 0–20 predorsal scales, denticles (usually; odontoid process) on the medial surface of the outer gill rakers of the first gill arch, vertical bars (usually) on the head and body sometimes with darkened borders, cephalic sensory canals and associated pores absent, and a transverse row of cheek sensory-papillae between the posterodorsal eye region and upper longitudinal row absent (Winterbottom and Burridge 1989, 1992; Senou et al. 2021). Members of the genus are divided into three species grades by the sensory papillae patterns on the cheeks and the presence or absence of predorsal scales (Winterbottom and Burridge 1993a, b).

A single specimen of a previously unknown species of Priolepis, sharing characteristics of the Priolepis profunda species grade (sensu Winterbottom and Burridge 1992), was collected from an artificial reef established for 2.5 years in ca. 100 m depth off Kashiwa-jima Island, Kochi, southern Japan. However, its unique coloration, the combination of fin ray and scale counts, and cephalic sensory papillae patterns were distinct from those of the congeners. Accordingly, the specimen is described herein as a new species.

Materials and methods

Sample collection. The specimen of Priolepis was collected from an artificial reef “Kaisou-kun” (individual units each comprising a cement block with a plastic basket filled with oyster shell; Ocean Construction Co., Ltd.; Fig. 1a). The reef was established on the sandy bottom off Kashiwa-jima Island, Kochi in ca. 100 m depth (Fig. 1b) on 1 July 2017, and retrieved on 22 January 2020. Curatorial procedures followed Motomura and Ishikawa (2013). Collection abbreviations: BSKU (Department of Biological Science, Faculty of Science and Technology, Kochi University); KBF (Kuroshio Biological Research Foundation); ZUMT (Department of Zoology, the University Museum, the University of Tokyo).

Fig. 1
figure 1

Photograph of an artificial reef unit “Kaisou-kun” (cement block with plastic basket filled by oyster shell); a after retrieval; b underwater view of a different unit (arrowed) established in the same area, captured from GoPro HERO 4 Session video

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Observations. Counts and notation for cephalic sensory canals and papillae follow Akihito (1984) and Miller (1986), i.e., anterior transverse interorbital papillae (ATI) and posterior transverse interorbital papillae (PTI). Measurements (rounded to the nearest 0.1 mm) followed Winterbottom and Burridge (1992), being made on the left side using digital calipers. All measurements were shown as % of standard length (SL), followed by seven on the head as % of head length (HL). The fresh color description was based on color photographs of the holotype.

Priolepis duostella sp. nov.


(New English name: Double-stared reefgoby; new standard Japanese name: Houseki-irezumihaze)


(Figs. 2, 3; Tables 1, 2)

Fig. 2
figure 2

Fresh (top) and preserved (middle) conditions of the holotype of Priolepis duostella sp. nov., BSKU 129609, 28.8 mm SL, female, collected from artificial reef off Kashiwa-jima Island, Kochi, Japan, and fresh condition of P. akihitoi (bottom), KBF-I 1423, 47.4 mm SL, male, collected off Kashiwa-jima Island, Otsuki, Kochi, Japan

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Fig. 3
figure 3

Dorsal (top), lateral (middle), and ventral (bottom) views of head of Priolepis duostella sp. nov., holotype, BSKU 129609, showing cephalic sensory papillae. Dots represent sensory papillae (rows identified by italic letters and numerals). AN and PN indicate anterior and posterior nostrils, respectively

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Table 1 Proportional measurements of Priolepis duostella sp. nov. and P. akihitoi
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Table 2 Selected characters of fresh coloration in 12 species of Priolepis profunda grade
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Holotype. BSKU 129609, 28.8 mm SL, female, off Kashiwa-jima Island, Otsuki, Kochi, Japan (32°46ʹN, 132°36ʹE), 22 Jan. 2020, collected from retrieved artificial reef established on sandy bottom at ca. 100 m.

Diagnosis. A species of the Priolepis profunda grade with the following combination of characters: presence of well-developed sensory papillae rows on cheek and predorsal scales. This new species can be clearly distinguishable from congeners by its distinctive coloration, comprising two black blotches, each crossed by a single vertical white bar, on caudal fin, six white bars on body, the 2nd and 4th curved, the 3rd bent at middle, 2nd continuous with anteriormost oblique stripe of first dorsal fin, four white stripes on head with an oblique stripe from posteriormost point of eye to uppermost point of fourth stripe. This species also differs from congeners by combination of following characters: dorsal-fin segmented rays 9; predorsal scales 17; and ATI 3 and PTI 4 or 5 in distinct rows.

Description. Dorsal-fin rays VI-I, 9; anal-fin rays I, 8; pectoral-fin rays 20 (21 on right side); pelvic-fin rays I, 5; branched caudal-fin rays 7 + 6; upper unbranched caudal-fin rays 8; lower unbranched caudal-fin rays 8; longitudinal scale counts 26; anterior transverse scales above anal-fin origin 12; posterior transverse scales above anal-fin origin 11; posterior transverse scales below second dorsal-fin origin 11; predorsal scales 17; scales around caudal peduncle 12; gill rakers on first arch 5 + 14; branchiostegal rays 5; vertebrae 10 + 16. Proportional measurements given in Table 1.

Body moderately elongate, slightly compressed. Body depth greatest at middle of dorsal-fin base. Head slightly depressed, width 96.9% of depth. Caudal peduncle moderately long, height 47.2% of length.

Snout short, rounded. Eye positioned dorsolaterally, large, dorsal rim bulging slightly above dorsal outline of head. Interorbital space wide. No dermal ridge on mid-line of nape. Mouth terminal, strongly oblique, forming an angle of ca. 60° with body axis. Lower jaw slightly projecting beyond upper jaw. Posterior end of jaws below anteriormost margin of eye. Anterior nostril at end of long tube, extending to upper lip when depressed anteriorly. Posterior nostril at end of short tube without dermal flap, located just before eye.

Anterior tip of tongue concave, free from mouth floor. Posteroventral margin of lower lip interrupted at symphysis, no mental flap on chin. Gill membrane attached to isthmus, gill opening large, anteroventral margin below midpoint between posterior margin of eye and preopercle. Posterior margin of preopercle smooth. Outer gill rakers on first arch long, slender, with small denticles.

Upper jaw with outer row of well separated canine-like teeth; anterior half with inner row of canine-like teeth; 5 or 6 indistinct rows of dense tiny conical teeth comprising a band between outer and inner rows. Lower jaw with outer row of canine-like teeth along anterior half; inner row of well separated canine-like teeth; 4 or 5 indistinct rows of dense tiny conical tooth comprising a band between outer and inner rows. Vomer and palatine without teeth; very tiny well separated fleshy lobes on roof of oral cavity.

Second dorsal fin higher than first dorsal fin; fourth segmented ray of second dorsal fin longest. Interdorsal space narrow without fin membrane, its length 1.9% of SL. All segmented rays of dorsal- and anal-fins branched, posteriormost rays branched from base. Anal-fin origin located below first dorsal-fin segmented ray. Fourth segmented ray of anal fin longest, shorter than longest dorsal-fin segmented ray. Second dorsal and anal fins not reaching to caudal-fin base when depressed. Pectoral fin semi-elliptic, middle rays longer than upper and lower rays, reaching posteriorly to vertical through anal-fin origin. Uppermost point of pectoral-fin base slightly anterior to vertical through first dorsal-fin origin, and slightly posterior to vertical through anteriormost point of pelvic-fin base. Pelvic fin without frenum; connecting membrane between pelvic fins poorly developed, its length 26.6% of length of fifth segmented ray; all pelvic-fin rays branched, fourth ray longest, its posterior tip reaching anus when depressed.

Ctenoid scales on posterodorsal region of head, upper opercle, lateral body surface, and abdomen. Cycloid scales on pectoral-fin base and prepelvic area. Body scales deciduous, increasing gradually in size posteriorly. Snout, interorbital space, cheek, lower two-third of opercle, ventral surface of head, and pectoral-fin axilla without scale.

No sensory canals or pores on head. Cephalic sensory papillae patterns illustrated in Fig. 3. Rows of papillae comprising simple series or single units, not forming complex rows. Six well developed transverse papillae rows on cheek. Uppermost points of rows 1 to 4 slightly separated from ventral margin of eye. Those of rows 5 and 6 located beside posteroventral margin of eye. Lower point of row 5 connected with row b. Lowermost point of row 2 connected with anteriormost point of row d, but lowermost points of rows 3 and 4 isolated. Row b long, slightly oblique, reaching to posterior margin of preopercle, connecting with row i. Rows q1 and q2 both comprising a single papilla. Each papilla in row i on ventral surface of lower jaw widely spaced, larger than those of row e. Anterior transverse interorbital papillae (ATI) and posterior transverse interorbital papillae (PTI) in distinct rows, comprising 3 and 5 (4 in right side) papillae, respectively, on each side, continuous across mid dorsum. Longitudinal rows r with 5 papillae before ATI. Transverse rows of papillae on lateral body surface absent. Three longitudinal rows of papillae on caudal fin, extending from posterior margin of scaled area to 5th, 8th, and 10th branched fin-ray, respectively.

Coloration when fresh. Body brownish-yellow with six vertical white bars; first straight, through pectoral-fin base; second gently curved posteriorly, uppermost part originating at middle of first dorsal-fin base; third somewhat bent at middle, extending between second dorsal- and anal-fin origins; fourth gently curved posteriorly, extending between two-thirds length of second dorsal- and anal-fin bases, respectively, dorsally continuous with anteriormost oblique stripe on second dorsal fin; fifth and sixth straight, in middle of caudal peduncle and on caudal-fin base, respectively. Head yellowish-orange, with four vertical white stripes; first to third stripes on cheek, first and second running through eye, third through posterior margin of eye; fourth through middle of opercle. An oblique stripe from posteriormost margin of eye to uppermost point of fourth stripe. Nape darkish. Iris gold. First dorsal fin brown with three white stripes; first oblique, lowermost point continuous with uppermost point of third bar on body; second slightly oblique, absent from interspinous membrane, anteriormost point at two-thirds height of dorsal-fin anterior margin, posteriormost point on membrane between last dorsal-fin spine and body; third slightly oblique, absent from interspinous membrane, anteriormost point at four-fifths height of dorsal-fin anterior margin, posteriormost point on membrane between fifth and last dorsal-fin spines. Second dorsal fin yellowish-brown with three white stripes; first oblique, lowermost point continuous with uppermost point of fourth stripe on body; second oblique, wavy, absent from interspinous membrane, anteriormost point on spine tip posteriormost point near base of last dorsal-fin segmented ray; third slightly oblique, absent from interspinous membrane, anteriormost point on tip of first segmented ray of second dorsal-fin, posteriormost point on tip of last dorsal-fin segmented ray. Outer margin of second segmented ray whitish. Anal fin reddish-brown; outer margin whitish. Pectoral fin semi-translucent reddish-orange. Pelvic fin reddish-orange; outer margin whitish. Caudal fin yellow with two distinct black eye-sized blotches, each crossed by white vertical bar; outer margin of fin whitish, bordering a faint orange band. Preserved specimen retained white bars on body and stripes on head, oblique stripes on both dorsal fins, and distinct black blotches on caudal fin.

Etymology. The specific name of the new species, duostella, means “two stars” in Latin, in reference to the two distinct black blotches on the caudal fin.

Distribution. Priolepis duostella sp. nov. is known only from the holotype collected off Kashiwa-jima Island, southwestern Shikoku, southern Japan. The specimen was collected from an artificial reef unit, set on a sandy bottom area with small rocks, in a depth less than ca. 100 m depth (Fig. 1b).

Remarks. Priolepis duostella sp. nov. has two distinct black blotches on the caudal fin, not appeared in any congeners. The new species was identified as a member of the P. profunda species grade (sensu Winterbottom and Burridge 1992) due to the presence of well-developed cheek papillae and predorsal scales. Of the 11 species previously recognized in this species grade, the new species is unique in having not only the blotches on the caudal fin but also the 2nd to 4th white bar on the body bent or curved (Table 2). Although P. duostella sp. nov. is most similar to P. akihitoi Hoese and Larson 2010, both sharing black blotches on the upper side of caudal fin (absent in all other species in the grade), the former can be clearly distinguished from the latter in having six white bars on the body (vs. eight), the 2nd to 4th bars on the body being curved (vs. all bars straight), the 3rd and 4th white stripes on the head connected by an oblique white stripe (vs. parallel, not connected), black blotch on upper side of caudal fin crossed by a single vertical white bar (vs. two bars), dorsal-fin segmented rays 9 (vs. 9 or 10, usually 10), predorsal scales 17 (vs. 17–25, more than 21 in the Japanese specimens), and ATI 3 and PTI 4 or 5 in distinct rows (vs. ATI 1 or 2 and PTI 1 or 2 in indistinct rows) (Hoese and Larson 2010; present study; Table 1). Although the holotype of P. duostella (28.8 mm SL) is smaller than the comparative specimens of P. akihitoi (33.9–47.4 mm SL), both species seem to be distinguishable by the following proportional characters: a shorter second dorsal-fin base (19.5% SL in P. duostella vs. 21.5–22.5% SL in P. akihitoi), narrower head (56.5% HL vs. 61.4–67.6% SL), larger eye (31.4% HL vs. 26.1–30.3% HL), and greater interorbital width (10.9% HL vs. 5.3–7.8% HL). Additional specimens of the new species are required to establish that these differences are the diagnostic characters of the species. Further, the new species somewhat resembles Priolepis anthioides (Smith 1959) known from the western Indian Ocean by having the similar counts except for pectoral-fin rays (20 or 21 vs. 18 in P. anthioides) and predorsal scales (17 vs. 19). However, P. anthioides is shown as vivid red without white body stripes in the original description, clearly differing from P. duostella (Smith 1959; present study). The collecting depth (ca. 100 m) of the new species is close to the known range (38–95 m) of Priolepis winterbottomi Nogawa and Endo 2007 (Nogawa and Endo 2007; Fujiwara et al. 2020b), but the former differs from the latter in having body stripes on dorsal fins (vs. absent in P. winterbottomi), curved bars on body (vs. straight), two black blotches on caudal fin (vs. absent), 20–21 pectoral-fin rays (vs. 18–19), 26 longitudinal scale counts (vs. 28–29), and 4–5 PTI (vs. 12) (Nogawa and Endo 2007; Fujiwara et al. 2020b; this study).

The holotype of P. duostella sp. nov. was collected from the sandy bottom area in ca. 100 m depth, a region commonly termed “twilight zone” (e.g., Pyle et al. 2018). Although the bycatches of bottom trawl fisheries often provide an opportunity for collecting fish specimens from such depths, the commercial targeting of larger species makes difficult the catching of small fishes, such as gobies. In the present study, the specimen was obtained from an artificial reef unit filled by oyster shell, which had been established for 2.5 years. A large number of benthonic organisms, such as crabs (including hermit crabs), shrimps, sea stars, brittle stars, polychaetes, and shells were also obtained from the artificial structure, providing an insight into the ecosystem inhabitants of the twilight zone. The collection of a distinct and tiny new fish species from such a structure illustrates the potential of the latter in researching the biodiversity of twilight zone depths.

Comparative materials. Priolepis akihitoi: BSKU 104016, 41.4 mm SL, male, off Okino-shima Island, Sukumo, Kochi, Japan, 15 m depth; BSKU 118729, 44.6 mm SL, male, off Issai, Otsuki, Kochi, Japan, 20 m depth; KBF-I 456, 33.9 mm SL, female, off Nishidomari, Otsuki, Kochi, Japan, 28 m depth, 28 July 2019; KBF-I 1423, 47.4 mm SL, male, off Kashiwa-jima Island, Otsuki, Kochi, Japan, 30 m depth, 11 Feb. 2021. Priolepis cincta (Regan 1908): KBF-I 0030, 36.7 mm SL, male, off Okino-shima Island, Sukumo, Kochi, Japan, 18 m depth, 6 May 2019; ZUMT 55215, 37.0 mm SL, tidepool at Nagasakibana, Ibusuki, Kagoshima, Japan, 8 Sept. 1959.