Figure 1, 2. Table 1.
Holotype. SAIAB81939, 140.8 mm SL, Indian Ocean, Mozambique Channel, RV Dr Fridtjof Nansen, bottom trawl, Station M 40, 25°12’48”S, 35°59’54”E, 116-120m, P.C. Heemstra & E. Heemstra, 8 October 2007.
Paratypes. SAIAB 209565, 6: 121.8-147.8 mm SL, same data as holotype. [Tissue No.HM07-610, Genbank (COI) paragenetype: MT 468885; HM07-611, Genbank (COI) paragenetype: MT 468886].
Diagnosis. Pectoral-fin rays ii,14 (ii,13-15); gill rakers 14 (13-15); body moderately elongate, depth 3.0 (2.9- 3.2) in SL; head length about equal to body depth, depth 1.0 (1.0) in head length; diameter of eye about equal to or a little more than snout length, 1.0 (0.8-0.9) in snout; lower margin of eye tangent to a line from tip of snout to upper pectoral fin base; suborbital shallow, its lower edge emarginate, least depth 2.6 (2.4-2.6) in eye diameter; imaginary line extended upward from posterior edge of suborbital reaching dorsal profile about 5-6 scale rows before origin of dorsal fin; pectoral fins moderately long, reaching to about level of vent; pelvic fins very long, reaching to between level of vent and origin of anal fin; upper lobe of caudal fin produced into short filamentous extension.
Description. Dorsal-fin rays X,9; anal-fin rays III,7; pectoral-fin rays ii,14 (ii,13-15); lateral-line scales 46 (46- 49); transverse scale rows 3½/8½; gill rakers 14 (13-15).
Depth 3.0 (2.9-3.2) in SL; head 3.2 (3.0-3.2) in SL; head length about equal to body depth, depth 1.0 (1.0) in head; snout short, bluntly rounded in profile, 3.3 (3.3-3.6) in head; eye 3.1 (2.8-3.1) in head; eye moderately large, its diameter about equal to or a little more than snout length, 1.0 (0.8-0.9) in snout; lower margin of eye tangent to a line from tip of snout to upper pectoral fin base; interorbital width 1.2 (1.4-1.5) in eye; least depth of suborbital 2.6 (2.4-2.6) in eye; ventral margin of suborbital emarginate, its posterior edge more or less straight; an imaginary line extended upward from posterior edge reaching dorsal profile about 5-6 scale rows before origin of dorsal fin; margin of preopercle finely denticulate; caudal-peduncle depth 1.2 (1.0-1.2) in peduncle length; dorsal-fin length 1.9 (1.7-1.9) in SL; dorsal spines 8-10 longest, 1.6 (1.5-2.2) times length of first dorsal spine; dorsal rays 6-8 longest, 1.2 (1.1-1.3) times length of longest dorsal spine; anal-fin length 5.1 (4.8-5.0) in SL; first anal spine 1.6 (1.5-1.8) in second; second anal spine 1.2 (1.1-1.3) in third; pectoral fins moderately long, reaching to about level of vent, length 1.2 (1.1-1.2) in head; pelvic fins very long, reaching to between level of vent and origin of anal fin, length 1.2 (1.1-1.3) in head; pectoral-fin length 1.0 (0.9-1.2) in pelvic-fin length; caudal fin forked, upper lobe slightly longer than lower, second and third principal caudal rays produced into long filamentous extension (broken in some specimens).
Maxillary reaching to anterior third of eye; jaw teeth small, slightly recurved canines, in narrow tapering bands in both jaws; upper jaw with anterior outer row of 3-4 pair of larger recurved canines.
Colour in life (Fig. 1): head and upper part of body pinkish; cheeks and body with golden reflections; pinkishmauve mid-laterally; silvery on belly; suborbital whitish; preopercle and opercle silvery with golden reflections, traces of iridescent mauve on cheeks behind subopercle and lower limb of opercle; eye reddish-orange; dorsal fin translucent pale blue, upper margin of fin with yellow edge and a yellow band from just above base of first spine, extending along middle of fin, broadening behind last dorsal spine; anal fin translucent pale blue, with row of yellow spots on lower one third of fin membrane behind third anal spine; caudal fin pale reddish, uppermost three rays sulphur yellow, this colour extending onto caudal filament; pectoral fins transparent pinkish; pelvic fins translucent, yellow on base of fin and axillary scale.
Colour in alcohol (Fig. 2): body pale brown, lacking traces of colour.
Etymology. Named elaine for Elaine Heemstra, who first brought this species to the attention of the senior author, in recognition of her contribution to Western Indian Ocean ichthyology over many years. The specific epithet is to be used as a noun in apposition. The common name Elaine’s threadfin bream is proposed for this species.
Comparisons. Nemipterus elaine n. sp. appears to be most similar morphologically and genetically to N. randalli Russell, 1986 (Figure 3, 6; Table 2), a species that is widespread in the Indian Ocean. The two species, however, are readily distinguishable on the basis of fresh colour: N. elaine n. sp. is distinct from N. randalli in having the upper caudal rays, caudal filament and outer edge of dorsal fin bright yellow (versus red in N. randalli). In preserved specimens, all traces of colour are lost and the two species are more difficult to separate: body shape is very similar and suborbital depth is shallow in both species, but in N. elaine n. sp. the paired fins are shorter, the pectoral fins reaching to about level of vent, and pelvic fins reaching to between level of vent and origin of anal fin (versus pectoral fins reaching to or just beyond level of origin of anal fin, and pelvic fins reaching to or beyond level of origin of anal fin in N. randalli). Two other species of Nemipterus in the Western Indian Ocean— N. japonicus (Bloch, 1791) (Fig. 4) and N. zysron (Bleeker, 1856) (Fig. 5) also have yellow caudal filaments, but N. japonicus has a much deeper suborbital (suborbital depth 1.0- 1.9 in eye, versus 2.4-2.6 in N. elaine n. sp.), and N. zysron has a more slender body (body depth 3.8-4.6 in SL, versus 2.9-3.2 in N. elaine n. sp.).
Genetics. Based on a 648 nucleotide COI sequence alignment, the midpoint-rooted NJ tree (Fig. 6) revealed N. elaine n. sp. to form a unique cluster, divergent from N. randalli and the other included species (N. bipunctatus (Valenciennes, 1830), N. japonicus, N. peronii (Valenciennes, 1830) and N. zysron). Among the examined species, N. randalli was the most closely-related to N. elaine n. sp., separated by a mean K2P sequence divergence of 5.2% (Table 2), clearly exceeding intraspecific divergences within each of the included species. The remaining species were more divergent from N. elaine n. sp., with mean divergences ranging from 13.6 to 20.0%.
Distribution.Nemipterus elaine n. sp. is so far known only from a single location in the southwestern Indian Ocean off the coast of southern Mozambique. Specimens were trawled from the outer continental shelf in depths ranging from 116-120 m (Johnsen et al. 2008), an area accessible only to deep water shrimp trawl fisheries in Mozambique (C. Maunde, pers. com. 2019).