Hygophum reinhardtii (Lütken 1892)

Hygophum reinhardtii

This medium-size myctophid grows to 45 mm in the study area, to 61 mm elsewhere (Hulley, 1981). A questionably bipolar subtropical species (Backus et al., 1977), it is most abundant in the eastern subtropics of the Atlantic Ocean (Nafpaktitis et al., 1977). It is uncommon in the study area, never being among the twenty most abundant lanternfish. The Ocean Acre collections contain 106 specimens; 65 were caught during the paired seasonal cruises, 51 of these in discrete-depth samples, of which 37 were in noncrepuscular tows (Table 23).

DEVELOPMENTAL STAGES.—The single postlarva was 12 mm, juveniles were 12–24 mm, subadults 24–45 mm, and adult males 32–45 mm. Most juveniles 12–15 mm could not be sexed, but all those greater than 18 mm could be. Subadult females had infracaudal luminous tissue, and subadult and adult males supracaudal luminous tissue. There was no apparent sexual dimorphism in size. Some subadult females larger than 40 mm had slightly enlarged ovaries with occasional ova up to 0.3 mm in diameter, but no females had many eggs of this size.

REPRODUCTIVE CYCLE AND SEASONAL ABUNDANCE.—Hygophum reinhardtii may or may not be a breeding resident of the study area. The parent population apparently spawns over most of the year, with a peak in fall. Judging from the maximum size attained in the study area, the life span is about one year. Abundance, although never great, is at a maximum in winter, intermediate in late spring, and least in late summer (Table 74).

Because 13–15 mm juveniles mostly were captured in oblique and crepuscular discrete-depth tows, they were not included in the reckoning of abundance. These small fish were taken over most of the year, appearing in greatest number in late summer, when they accounted for half of all H. reinhardtii taken. The maximum of juveniles in winter (Table 74) is due mostly to fish 20–24 mm. Collections made in June contain no juveniles of any size, which is puzzling. Fish 15 mm or smaller were taken in April and July when most of the small catch (5 of 6 specimens) was 13–15 mm.

Fish 30 mm and larger were taken in low numbers and were not abundant at any season. They were about equally numerous in winter and late spring collections (4 and 5 specimens, respectively), and only one was taken in late summer. The small number of large fish taken in late summer apparently was an accurate reflection of the population structure, for even the larger Engel trawl captured only 19 specimens larger than 30 mm. Several subadult females larger than 40 mm in the Engel collections were the most sexually developed fish taken. The seasonality of both small (13–15 mm) and large fish (greater than 30 mm) suggests a spawning peak in spring, and that spawning occurs most of the year with only a short interruption in late winter or early spring. This protracted breeding season is similar to that for H. reinhardtii found near Hawaii (Clarke, 1973). Clarke also noted that the species matures at a size of approximately 33 mm. However, none of the females that size or larger in the Ocean Acre collections were ripe and only a few showed sexual development. This lack of adult and ripe females casts some doubt on the reproductive status of the species near Bermuda. However, the presence of 13–15 mm juveniles and the 12 mm postlarva suggest that spawning occurs nearby if not in the area.

Abundance was greatest in winter, when juveniles from the spawning peak were best sampled, and decreased until the late summer low was reached (Table 74). Abundance probably was greatest in the fall soon after the spawning peak.

SEX RATIOS.—Females were more numerous in the collections than males at all seasons, with ratios of 2.1:1 in winter, 4.0:1 in late spring and 1.2:1 in late summer (Table 75). Sample sizes in late spring and late summer were too small (less than 10 specimens) to be statistically meaningful. The difference in winter was significant; it was due mostly to juveniles, but subadults also contributed. The difference for juveniles was not significant and the number of subadults was too small to be meaningful.

VERTICAL DISTRIBUTION.—By day eight fish were taken in discrete-depth tows; six in winter and one each in the other two seasons. Seven were taken at 701–850 m, and the remaining one at 1051–1100 m (Table 76). Night catches accounted for 29 fish; 22 in winter, 2 in late spring, and 5 in late summer. In winter fish were taken from 0–200 m and at 551–600 m with slight concentrations at the surface and 101–150 m, in late spring at the surface and 201–250 m, and in late summer at 90 m, 151–250 m, and 851–900 m.

Apparently most fish undergo diel migrations, as only one fish was taken near diurnal depths by night. Migrants included fish 15–43 mm.