Nannobrachium isaacsi (Wisner 1974)

13-14 AO photophores; 9-10 tooth patches on the lower limb of the second gill arch; 34-36 lateral line organs (Ref. 36121).

Oceanodromous. Migrating within oceans typically between spawning and different feeding areas, as tunas do. Migrations should be cyclical and predictable and cover more than 100 km.

Dorsal spines (total): 0; Dorsal soft rays (total): 14 - 16; Analspines: 0; Analsoft rays: 17 - 20; Vertebrae: 35 - 36

High-oceanic, mesopelagic species. Found at 550-750 m during the day, at 40-325 m during the night, while maximum abundance is at 100m (Ref. 4479).

Nannobrachium isaacsi (Wisner, 1974)

Lampanyctus micropterus Brauer, 1906 [in part] [2 of 3 syntypes, fide Hulley, 1981].

Lampanyctus isaacsi Wisner, 1974:14–17 [original description, eastern equatorial Atlantic].—Nafpaktitis et al., 1977:201–203 [description, distribution, figure].—Backus et al., 1977:267, 275, 277 [zoogeography].—Paxton, 1979: 14 [holotype and paratypes].—Hulley, 1981:204–206 [description, Atlantic distribution].—Kinzer and Schulz, 1985:314, 316 [depth distribution, central equatorial Atlantic].—Rubies, 1985:578, 584 [Benguela Current, off Namibia, gill raker number low—questionable identification].

Lampanyctus (Lampanyctus) isaacsi.—Bekker, 1983:87–89, 193, 200 [key, description, distribution].

COMPARATIVE DIAGNOSIS.—Nannobrachium isaacsi (Figure 24) can be distinguished from all other species of Nannobrachium by the position of the VO2 photophore forward above the VO1. It can be further separated from the species of the Cuprarium group by the combination of characters in Table A10 and from all species of Nannobrachium by the combination of characters in Table 1.

DESCRIPTION.—Counts are based on up to 25 specimens from the eastern Atlantic and are given in Tables A2–A8, A10.

Proportions: Given in Table 20.

Fins: Origin of anal fin before vertical from middle of dorsal-fin base. Pectoral fin barely reaching vertical from PO5, its rays rather weak, flexible. Base of adipose fin above end of anal-fin base.

Luminous Organs: PLO –1 photophore diameters below lateral line. PO4 slightly higher than level of PVO2 and behind vertical from PO3. VLO usually touching lateral line. SAO2 before vertical from AOa1. AOa1 slightly depressed; AOa series slightly concave downward. AOp1 above end of anal-fin base. Pol2 well before vertical from origin of adipose-fin base. Prc nearly continuous with AOp; Prc2–3–4 usually forming straight line at angle of about 120° with line connecting Prc1–2. Supracaudal and infracaudal scales rather weakly developed, sometimes with single separated scale preceding infracaudal gland. No secondary photophores found.

Size: Nannobrachium isaacsi apparently reaches a maximum size in excess of 130 mm. The largest specimen examined herein was 125 mm. Nafpaktitis et al. (1977) reported a maximum size of 130 mm, whereas Hulley (pers. comm., 1980) gave the maximum size as 132 mm among the 179 specimens he examined.

Material: 99 (16–125 mm) specimens were examined.

VARIATION.—Nannobrachium isaacsi is found principally in the eastern tropical Atlantic (Figure 16), although Nafpaktitis et al. (1977) reported a few specimens from the western tropical Atlantic off eastern Brazil, which they regard as waifs. Based on the findings of Nafpaktitis et al. (1977), Hulley (1981; and pers. comm., 1980), and the material examined herein, N. isaacsi may have a disjunct distribution, with the two centers separated by a gap in the Gulf of Guinea. It is reported to occur between 550 and 750 m during the day and between 40 and 325 m at night (Nafpaktitis et al., 1977). No geographic variation was evident. As material in better condition becomes available, however, specimens from the two apparent centers of distribution should be carefully compared for possible differences.