Enneapterygius nanus (Schultz, 1960)

[English name: Pygmy Triplefin]

Figures 4–6, 12; Tables 4, 6, 7

Tripterygion nanus Schultz, 1960: 288, fig. 112 (original description; type locality: Rujoru Island, Eniwetok Atoll, Marshall Islands).

Enneapterygius nanus: Myers, 1991: 204, unnumbered fig. [Enewetak, Marshall Islands (based on Schultz, 1960)].

Holotype. USNM 142233, male, 15.8 mm SL, lagoon reef at Rujoru island, Eniwetok Atoll, Marshall Islands, 2 June 1946, L. Schultz.

Paratypes. 4 specimens (16.4–17.9 mm SL). USNM 142234 *, 2 males, 16.4–17.6 mm SL, same data as holotype; USNM 142238, 2 males, 17.5–17.9 mm SL, tidepool at Enybarbar island, Rongelap Atoll, Marshall Islands, 18 June 1946, L. Schultz .

Non-type specimen. NSMT-P 54443, male, 14.3 mm SL, Eroj island, Majuro Atoll, Marshall Islands, 0.5 m, 6 Aug. 1982, K. Matsuura .

Diagnosis. A species of Enneapterygius (Fig. 12) with the following combination of characters: 12 or 13 (modally 12) second dorsal-fin spines; 9 or 10 (9) third dorsal-fin rays; I, 16 or 17 (16) anal-fin rays; ii–iv (ii) + 4–6 (6) + vii = 15 pectoral-fin rays (ii + 6 + vii = 15 in holotype, iii + 5 + vii = 15 in 2 specimens, iv + 4 + vii = 15 in single specimen); 12 or 13 pored lateral-line scales; 19–21 (20 or 21) notched lateral-line scales; 31–33 (33) scale rows in longitudinal series; 1 or 2 (1½ or 2) scales above 1st pored lateral-line scale; 2 or 2½ (2) scales below 1st notched lateral-line scale; 8 circumpeduncular scales; mandibular pore formula 3 + 1 +3; head relatively short, its length 28.3–30.1% (mean 29.5%) of SL; upper jaw relatively long, its length 9.8–11.2% (10.4%) of SL; 1st spine of first dorsal fin shorter than that of 2nd dorsal fin, its length 8.2–10.5% (9.8%) of SL; anterior nostril tentacle unbranched; orbital tentacle small, leaf-shaped, the edge slightly rounded, its length less than or subequal to half of pupil diameter; body translucent, lower half and ventral surface of head, including pelvic-fin base, black in nuptial males; no black pigmentation on pectoral-fin base; caudal fin transparent, sometimes with faint brownish bars; body plain and pale in females; pectoral-fin rays with no pigmentation.

Distribution. Enneapterygius nanus is currently known only from the Marshall Islands (Schultz, 1960; Myers, 1991; this study) (Fig. 4).

Remarks. Enneapterygius nanus was originally described as Tripterygion nanus based on 41 specimens from the Marshall Islands (Bikini, Eniwetok, Rongerik, Rongelap, and Kwajalein atolls). Subsequently, Myers (1989) transferred the species to Enneapterygius, adding a record from the Mariana Islands. However, Myers (1989) ’s description and a sketch were based entirely on Schultz (1960). Although Myers (1989, 1991) included an underwater photograph of E. nanus taken in Guam, that individual was identified here as Enneapterygius sp. 3 sensu Motomura et al. (2010). In addition, Fricke (1994, 1997) recorded E. nanus from widespread western Pacific localities, ranging from Taiwan and Thailand, south to New Caledonia, and east to American Samoa. However, meristics and measurements of E. nanus given by Fricke (1994, 1997) matched poorly those of the E. nanus type specimens examined in the present study. Examination of the specimens identified as E. nanus in this study revealed that those from the Marshall Islands tended to have fewer anal-fin rays (16 or 17 rays) [vs. 17–19 (modally 19) rays in specimens from other areas] and mandibular pores (7 pores in total; mandibular pore formula 3 + 1 + 3) (vs. 7–9 pores; mandibular pore formula 3 + 1 + 3, 3 + 2 + 3, and 4 + 1 + 4). Fricke’s (1994, 1997) description of the mandibular pore formula of E. nanus as 3 + 2 + 3, contradicts that of the Marshall Islands’ specimens, suggesting that the former was based on a different, misidentified species. In fact, it is likely that E. nanus is endemic to the Marshall Islands, as no reliable distributional records of the species from other localities exist.

Enneapterygius nanus is similar to E. olivaceus n. sp. in having 12 or 13 (modally 12) second dorsal-fin spines, 12 or (13) pored lateral-line scales; 19–21 (19 or 20) notched lateral-line scales, 1–2 (2) scales above 1st pored lateral-line scale, 2 or 2.5 (2) scales below 1st notched lateral-line scale, mandibular pore formula 3 + 1 + 3, long upper jaw, its length 9.5–13.1%, and leaf-shaped orbital tentacle with rounded edge (Fig. 5A, C; Tables 1, 4, 6, 7). However, E. nanus differs from E. olivaceus n. sp. in shorter lengths of head length and 1st spine of the first dorsal fin, orbital tentacle shape, and the pigmentation of pectoral-fin (See E. olivaceus n. sp. Remarks.).