Acanthurus nigros Günther, 1861

Hanahara, Nozomi, Miyamoto, Kei & Oka, Shin-ichiro, 2021, Northernmost Record of the Surgeonfish Acanthurus nigros (Teleostei: Acanthuridae) from Minamidaitojima Island, Southern Japan, Species Diversity 26 (1), pp. 43-47 : 45-47

Acanthurus nigros Günther, 1861

[New standard Japanese name: Nangoku-kurohagi] ( Figs 1–3 )

Material examined. OCF-P 3701, 116.1 mm SL, Kameike Port , Minamidaitojima Island, Okinawa, Japan (25°48′47″N, 131°13′59″E), 3 m depth, bait fishing, 25 July 2017, collected by S . Oka and K . Miyamoto.

Description. Dorsal-fin rays IX, 25; anal-fin rays III, 25; pectoral-fin rays 16; pelvic-fin rays I, 5; principal caudal-fin rays 16, upper and lower procurrent caudal-fin rays 5; gill rakers 25 (on both left and right sides); vertebrae 9+13.

Body depth, 53.7% SL (1.9 in SL); body width, 17.0% SL; head length, 27.4% SL; snout length, 21.4% SL; orbit diameter, 7.8% SL; interorbital width, 9.4% SL; upper jaw length, 6.4% SL; groove in front of eye length, 4.3% SL; pre-dorsalfin length, 43.1% SL; second dorsal-fin spine length, 11.7% SL; ninth dorsal-fin spine length, 16.2% SL; pre-anal-fin length, 52.5% SL; second anal-fin spine length, 10.2% SL; third anal-fin spine length, 13.4% SL; caudal-peduncle depth, 12.2% SL; caudal-peduncle spine length, 7.4% SL (3.7 in HL); caudal-fin concavity, 7.8% SL (12.8 in SL).

Body deep and compressed. Dorsal profile of head strongly convex in front of eye; interorbital area strongly convex. Mouth small; teeth uniserial, incisiform, denticulate, and close-set, distal end of upper teeth rounded. A shallow groove extending obliquely in front of eyes; nostrils located in front of eye, anterior and posterior nostrils closely aligned; anterior nostril with flaps, anterior flap smaller than posterior flap which completely covers nostril aperture when folded forward; posterior nostril an elliptical opening.

Scales on body ctenoid; scaly sheath at base of dorsal and anal fins; lateral line obscure, approximately following dorsal contour at vertical around dorsal-fin origin and ending at front of caudal-peduncle spine socket.

Dorsal-fin origin above posterior edge of opercle; first spine very short and covered with skin; first soft rays unbranched. Anal-fin origin below eighth dorsal-fin spine origin; first spine very short and covered with skin; first three soft rays unbranched. Upper end of pectoral-fin base slight anterior to a vertical through dorsal-fin origin; third ray longest; upper two and lowermost rays unbranched. Pelvic fins reaching anal-fin origin; first soft ray longest. Caudal-fin slightly emarginate, upper and lower edges not elongate.

Coloration in fresh specimen ( Fig. 1a ). Body and head generally light to dark brown; body with light grayish dots dorsally, irregular and longitudinal light grayish stripes ventrally, narrower than brown interspaces; snout and opercular region with light grayish stripes paralleling outer margin of snout; iris yellow; pupil-sized black blotch at rear base of dorsal and anal fins; dorsal fin dark brown, five to seven faint grayish bands paralleling outer edge; anal fin dark brown, five faint grayish bands paralleling outer edge; pectoral fin yellowish light gray; pelvic fin light brown; caudal fin dark brown; caudal-peduncle spine light brown, its sheath white.

Coloration in preserved specimen ( Fig. 1b ; 10% formalin for 3 years). Body and head generally light to dark brown; body with small dark brown dots dorsally, dark brown stripes on suborbital region; dorsal and anal fins with dark brown bands; pupil-sized black blotch at rear base of dorsal and anal fins; sheath of caudal-peduncle spine light brown.

Distribution. This species has been recorded in a small region of the western Pacific Ocean (Minamidaitojima Island, Yap, Guam, Pagan, Vanuatu, and Great Barrier Reef) and widely in the central Pacific Ocean (except for the Hawaiian Islands and Johnston Atoll) ( Randall et al. 2011; present study) (see also Fig. 2 ).

Remarks. The following morphological characteristics of our specimen closely matched the diagnostic features of A. nigros given by Randall et al. (2011): dorsal rays IX, 25; pectoral rays 16; gill rakers 25; teeth incisiform, denticulate, and close-set, the distal end of upper teeth rounded with 12 upper and 14 lower teeth; body depth 1.9 in SL; light to dark brown body with irregular, longitudinal, dotted stripes narrower than brown interspaces; suborbital and opercular region with stripes paralleling snout outline; pupil-sized black blotch at rear base of dorsal and anal fins; dorsal and anal fins with bands paralleling outer edge.

Acanthurus nigros and A. nigroris are closely similar in coloration: for example, light to dark brown body with irregular, longitudinal, dotted stripes narrower than brown interspaces; suborbital and opercular region with lines paralleling snout; pupil-sized black blotch at rear base of dorsal and anal fins; dorsal and anal fins with bands paralleling outer edge ( Randall et al. 2011). However, A. nigros can be distinguished from A. nigroris by the counts of gill raker (21–25 vs. 26–31 in A. nigroris ), and mitochondrial DNA cytochrome b sequence divergence ( Randall et al. 2011). The counts of gill raker in our specimen (25) agreed with those of A. nigros . In addition, in the maximum likelihood tree based on a partial sequence (754 bp) of the mitochondrial cytochrome b gene, our specimen was placed within the clade of A. nigros , which was clearly separated from the clade of A. nigroris ( Fig. 3 ). Therefore, our specimen was identified as A. nigros .

The number of anal-fin soft rays in our specimen (25) differed from that given by Randall et al. (2011) for A. nigros (22–24, mode 23), and matched with that of A. nigroris (22– 25, mode 24). In some species of Acanthurus , the number of anal soft rays shows relatively high intraspecific variation; for example, 25–28 in Acanthurus lineatus (Linnaeus, 1758) , 19–22 in Acanthurus triostegus (Linnaeus, 1758) , and 23–26 in Acanthurus thompsoni (Fowler, 1923) ( Randall 1956) . Therefore, the differentiation of the number of anal soft rays between our count and that of Randall et al. (2011) was regarded as intraspecific variation. Similarly, the differences between our measurements and those reported by Randall et al. (2011) related to caudal-peduncle spine length (3.7 vs. 2.7–3.5 in HL) and caudal-fin concavity (12.8 vs. 6.7–10.5 in HL) were also considered as intraspecific variations.

Among congeners known from the Japanese waters, A. nigros is similar to A. nigrofuscus in having light to dark brown body with irregular and longitudinal light grayish stripes, dorsal and anal fins with bands paralleling outer edges, and a black blotch at the rear bases of each dorsal and anal fin. However, the fresh specimen of A. nigros can be clearly distinguished from A. nigrofuscus by the presence of the lines parallel to the snout on the suborbital and opercular regions (vs. spots on those regions in A. nigrofuscus ). In addition, these two species are distinguished by the following morphological characters: the size of the black blotch at the rear base of the dorsal fin (larger in A. nigros than in A. nigrofuscus ); the presence or absence of a dark margin around the caudal-peduncle spine (absent in A. nigros vs. present in A. nigrofuscus ); the posterior tips of the dorsal and anal fins (tips more pointed when these fins are elevated in A. nigrofuscus than in A. nigros ); the bands on the dorsal and anal fins more visible in preserved specimens of A. nigros than in specimens of A. nigrofuscus ( Randall 1956; present study).

Acanthurus nigros has been recorded mainly around the islands of the central Pacific Ocean (except for the Hawaiian Islands and Johnston Atoll) ( Randall et al. 2011). Our specimen collected from Minamidaitojima Island represents the northernmost and westernmost record for the species, and the location was more than 1500 km away from the nearest island (Pagan Island, Northern Mariana Islands) previously recorded ( Fig. 2 ). Minamidaitojima Island, oceanic island, is located approximately 300 km east of Okinawa Island in the Ryukyu Islands and is separated from the islands by the Ryukyu Trench. As stated by Koeda et al. (2015), marine fauna around Minamidaitojima Island is still not well understood. In addition to A. nigros , Acanthurus achilles , which are mainly distributed around the islands of the central Pacific Ocean, also inhabit Minamidaitojima Island ( Shimada 2013). The geographical distribution of these species may be important in understanding the origins of the coastal fish fauna of the Daito Islands, including Minamidaitojima Island.

This species has never been listed in the comprehensive publications of Japanese fishes (e.g., Shimada 2013), although Randall et al. (2011) showed specimens of A. nigros collected from Minamitorishima (Marcus) Island in Japanese waters. The proposed new standard Japanese name is a combination of “nangoku”, a Japanese name for tropical region, the distribution area of A. nigros , and “kurohagi”, a common Japanese name for members of the genus Acanthurus .

Comparative materials. Acanthurus nigrofuscus : OCF-P 3702, 147.7 mm SL, Minamidaitojima Island, 25 July 2017;

URM-P 37816, 103.6 mm SL, Okinawa Island , 10 April 1997; URM-P 37817, 110.8 mm SL, Okinawa Island, 10 April 1997 .

Figures

Fig. 1. Fresh (a) and formalin-preserved (b) specimen of Acanthurus nigros, OCF-P 3701, 116.1mm standard length, Minamidaitojima Island, southern Japan.

Fig. 2. Distributional records of Acanthurus nigros. Star and circles indicate present and previous records (Randall et al. 2011), respectively.

Fig. 3. Maximum likelihood tree based on partial sequence of the mitochondrial cytochrome b gene (754bp) of Acanthurus nigros and A. nigroris. Numbers beside the internal branches indicate bootstrap probabilities (>95%) based on 1000 pseudoreplicates.