Melamphaes pumilus Ebeling 1962

Head with sharp angle in lateral margin. Preopercle with angle smooth, rounded and slightly emarginate (Ref. 37108).

Dorsal spines (total): 3; Dorsal soft rays (total): 14 - 16; Analspines: 1; Analsoft rays: 7 - 9; Vertebrae: 27 - 28

fisheries: of no interest

Melamphaes pumilus

The most abundant melamphaid at all seasons in the Ocean Acre area, this species is confined to the western gyre of the North Atlantic, west of 25°W between 10° and 45°N, but mostly between 20° and 40°N. It is a diminutive species, rarely exceeding 22 mm SL; its maximum size is 24 mm. It was represented in the Ocean Acre collections by 2604 specimens; 1675 were taken during the seasonally paired cruises, including 747 in discrete-depth samples, with 638 of these from noncrepuscular tows (Table 147).

DEVELOPMENTAL STAGES.—Postlarvae were characterized by the absence of pigment and small size (10 mm or less). Juveniles ranged from those just developing adult coloration (9–10 mm) up to 15 mm; their gonads were small and located posterodorsad to the terminal enlargement of the intestine (rectum). Subadults were 14–18 mm and showed progressive enlargement of the gonads that was obvious in both sexes, but moreso in females; the gonads were mainly lateral to the rectum and extended anteriorly beyond the rectal enlargement. Adults were 18–24 mm. Adult females had large eggs, and the ovaries extended well forward, occupying most of the coelom. In adult males the testes were larger than in subadults, but the size difference was less obvious than for ovaries. External sexual dimorphism was not evident in any of the stages.

REPRODUCTIVE CYCLE AND SEASONAL ABUNDANCE.—Melamphaes pumilus appears to have a one-year life cycle. It apparently breeds throughout the year, with a peak in spring and early summer (Table 169). This species was most abundant in winter, when subadults and adults comprised most of the catch, least abundant in late spring, and intermediate in abundance in late summer, when postlarvae and juveniles were at their greatest abundance (Table 148).

All stages occurred at all seasons, indicating year-around spawning. The peak of spawning probably is in spring and early summer, as indicated by the large number of postlarvae and juveniles caught in July and September (Table 149; includes nondiscrete samples) and the greatest abundance of these stages in the late summer in the paired cruises (Table 148). The ovaries of adult females taken throughout the year had fairly large eggs, 0.25–0.35 mm in diameter, but three adult females captured in March and June possessed eggs twice as large (0.6–0.7 mm). These eggs were swollen and had a much thicker translucent area surrounding the yolk than those of most adult females. Presumably these females were caught just prior to spawning, reinforcing the suggested time of peak spawning.

In winter, over 95% of the population consisted of subadults and adults, with adults about twice as abundant as subadults (Table 148). By late spring, the few winter postlarvae and juveniles had become juveniles and subadults, respectively, and almost 70% of the sampled population consisted of adults. Adults, however, were less abundant at this time than in winter, which may be attributed to mortality following spawning. Postlarvae probably were small and undersampled. In July and September (Table 149) juveniles appear in abundance, and subadults were more numerous. These probably represent mainly the recruits from the spring spawning. At the same time, adults have become still less abundant, due to continuing mortality. Continued into the fall and winter, this sequence of events should result in juveniles becoming less abundant as they become subadults, and both subadults and adults increasing in abundance. The data in Table 149 suggest that juveniles are, indeed, the most abundant stage in October–November (almost 65% of the total) and subadults the most abundant stage in December (almost 64%). This would lead to the predominance of adults in winter, already noted.

The seasonal progression of developmental stages and the fact that adults compose a majority of the catch only in winter and spring indicate a one-year life cycle. Further evidence was provided by otoliths from 10 of the largest adults (22–23 mm) from different seasons, all of which have only the initial opaque central ring surrounded by a translucent area, indicating that they are less than a year old. The central ring is present in the otoliths of juveniles, but the translucent belt surrounding it is narrower than that of subadults and adults.

SEX RATIOS.—Table 150 shows the numbers of each sex for each sampling period. During no period was there a significant difference in the number of juvenile males and females. A trend toward males being more abundant than females is seen in subadults, although only one sample showed a statistically significant difference. Adult males outnumber adult females at all times except in April, which was represented by a relatively small sample, and in August, when sampling was done with the large Engel trawl (EMT). If the two discordant samples of adults are ignored, and adult males do, indeed, outnumber adult females, a differential mortality during reproduction would explain the phenomenon. Females might die soon after spawning, but males might not die for some time, possibly spawning several times during the season.

VERTICAL DISTRIBUTION.—This species is most abundant from 51 to 400 m at night and from 551 to 1300 or 1350 m during the day (Table 151). It is rare in the upper 50 m at night, and only one subadult male was taken at the surface in a crepuscular sample in July. Diel migratory behavior is indicated clearly in juveniles, subadults, and adults.

Both stage- and size-stratification are apparent both day and night. Postlarvae were taken at 51–100 m during the day and between 33 and 300 m at night, most of them between 51 and 150 m. The latter depths correspond to the maximal seasonal thermocline. Larger postlarvae were taken at 551–750 m, to which depths they descend prior to transformation to the juvenile stage. The possibility of diel vertical migration by some of the deeper postlarvae cannot be ruled out, but we suspect that it does not occur. That vertical migration begins in juveniles is suggested by two small transitional juveniles that had not migrated upward, but were caught in late summer between 851 and 1000 m at night (Table 151). Later developmental stages and larger sizes tend to occupy greater depths (Table 151), a fact noted by Ebeling (1962). This is most obvious at night because of the greater number of specimens caught. Each succeeding stage inhabits a more extensive range of depths, is most abundant at a greater depth, and extends to greater maximum depths than the preceding stage. There is, however, considerable overlap in vertical ranges, with adults occurring throughout most or all of the ranges of the other stages and sometimes exceeding both their upper and lower limits. This is illustrated well by the late spring data and by the night data for the other two seasons (Table 151).

Depths occupied by males and females appeared to be identical at any stage.

PATCHINESS.—Patchiness of distribution appears to be characteristic of all stages of M. pumilus. Significant clumping at night was indicated in winter at 151–200 m, 301–400 m, and 501–600 m; in late spring at 51–100 m and 201–250 m; and in late summer at 51–150 m and 451–500 m. The only significant clumping during the day was in late spring at 951–1000 m. At 7 of the 11 50-m intervals for which clumping was indicated, the seasonal maximum catch rate occurred for at least one developmental stage, and two other intervals had catch rates very near the maximum for one stage.

With one exception, all stages appear to have patchy distributions at all seasons. In late summer, adults were apparently randomly distributed; no clumping was indicated from 151 to 400 meters at night, at which depths almost all adults were taken. Most adults are probably in a postspawning condition in late summer, with death imminent, and their normal tendency toward aggregation presumably is lost under such conditions.

NIGHT:DAY CATCH RATIOS.—The ratios of night to day discrete-depth captures, using interpolation in unsampled depth intervals, were 5.6:1 in winter, 1.2:1 in late spring, and 2.9:1 in late summer (Table 152). Adults and subadults are responsible for most of the catch rates, but postlarvae and juveniles generally show the same propensity to be more abundant in night samples.

The nearly equal ratio in late spring suggests that factors other than enhanced daytime avoidance alone may be responsible for the predominance of night over day captures. One possibility is the constriction of the vertical ranges at night, while another is seasonal differences in clumping.

Constriction of the vertical range would enhance the chances that either randomly distributed or aggregated populations would be sampled. The degree of constriction of the vertical range at night increases in the same seasonal order as the night:day catch ratios. If all postlarvae and those small juveniles that remain deep at night are ignored, the vertical ranges of all other stages are included within that of adults both day and night (Table 151) with two exceptions. At night, adults were taken 100 m shallower than juveniles and subadults in winter, and in late summer both juveniles and subadults were taken 100 m shallower than adults. As defined, the night vertical range was 53% of the daytime range in winter, 86% in late spring, and 69–81% in late summer (shallow juveniles and subadults made up a significant part of the catch at this time).

The effect of night constriction of the vertical range is magnified for adults, which were taken in greatest abundance in only part of the night vertical range. In winter, most adults were taken in a 100 m stratum (150 m with interpolation) within a vertical range of 400 m; in late spring most were taken in 300 m within a 600-m breadth; and in late summer most were taken in 250 m within a 550-m breadth. Considering both total vertical ranges and adult concentrations, the compression is greatest in winter, when the night:day ratio also is greatest, next greatest in late summer, and least in late spring, which has the lowest ratio (Table 152).

The presence of aggregations of M. pumilus during the day, indicated as being uncommon, nevertheless may involve a significant part of the population that was not adequately sampled. The only instance of significant daytime clumping was at one 50-m depth interval in late spring, the only season when the night:day catch ratio approached 1:1. If the catch rate for adults alone in this interval had been 1.0, as in both adjacent sampled (not interpolated) intervals, instead of 7.2 (Table 151), the ratio would have been 1.9:1 instead of 1.2:1, approaching that for late summer.

Net avoidance remains a possible contributor to the night:day imbalance. If M. pumilus is inactive during the day, and rests in a vertical position, as Barham (1971) noted in several myctophids, this would enhance the likelihood that they would dart out of the net's path. No direct observations of M. pumilus have been made, however, and this possibility is conjectural.

Western Atlantic: between 40° and 10°N.

Found to depths of 2380 m.

nektonic