Pearl Side

Maurolicus muelleri

This is a moderate-size species, reaching a maximum length of about 65 mm SL (Grey, 1964). Although the distribution of M. muelleri is worldwide (Briggs, 1960), it was rare in the Ocean Acre area and was completely absent from discrete-depth samples.

Four specimens were taken in open nets in July and September, all during crepuscular periods. In July, two postlarvae, 12 and 14 mm SL, were taken in a tow at 0–250 m and a subadult, 18 mm SL, came from 0–1425 m. The lone September capture, a subadult 18 mm SL, was taken at 0–100 m.

Williams and Hart (1974) collected eggs of M. muelleri from March to October in the open North Atlantic (59°N, 19°W). Eggs were taken at depths of 100–500 m and temperatures of 8.8°–10.0° C. Most studies have reported spawning to occur near the edge of the continental shelf (Robertson, 1976, and papers cited by him). The presence of two postlarvae in our study area and the report of two smaller larvae (7–8 mm) from Beebe's 8-mile cylinder (Beebe, 1937) suggest that M. muelleri spawns in deep water near Bermuda.

Habitat Segregation and Resource Partitioning

Table 22 shows the maximum standard lengths, depths of concentration, and peak spawning seasons for the four most abundant sternoptychids. Sternoptyx diaphana is the deepest dwelling species and is almost completely separated from the other three in the water column. Although the spawning season of S. diaphana completely overlaps those of the other sternoptychids, its postlarvae are found primarily between 550 and 750 m, much deeper than postlarvae of the other species. Thus, S. diaphana does not compete for any resource with any other member of the family.

The two congeners, Argyropelecus hemigymnus and A. aculeatus, differ strikingly in size and have different (though overlapping) depth distributions, especially at night. Probably they have very different diets. In both the eastern North Atlantic (Merrett and Roe, 1974) and the Caribbean-central Gulf of Mexico region (Hopkins and Baird, 1977), stomach contents of A. hemigymnus were dominated by copepods, while those of comparable size A. aculeatus contained mostly ostracods; larger specimens of A. aculeatus from the Caribbean-Gulf region also contained significant numbers of larger prey items, such as amphipods, molluscs and tunicates, all of which were rare or absent in A. hemigymnus.

A. hemigymnus and Valenciennellus tripunctulatus are similar in size and morphology, have nearly identical diel vertical distributions, exhibit similar diel changes in pigmentation (Badcock, 1969), and even share the same feeding period: afternoon and evening (Merrett and Roe, 1974; Hopkins and Baird, 1977). However, in the eastern North Atlantic, Merrett and Roe (1974) found important differences in the diets of these two fishes, with species of the copepod genus Pleuromamma predominant in the stomachs of V. tripunctulatus. Furthermore, Hopkins and Baird (1977), who compared specimens of both species from the same haul in the eastern Gulf of Mexico, found a greater diversity of prey in the stomachs of A. hemigymnus, which contained a much higher percentage of small (<2 mm) prey items. In addition, our postlarvae of A. hemigymnus were abundant only in late summer (corresponding to the spring-summer breeding peak) and were concentrated at 401–450 m in all seasons. In contrast, postlarvae of V. tripunctulatus were scarce in the Ocean Acre area, as might be expected in a year-round spawner, and appeared in discrete-depth samples only in winter, at 301–350 m. Thus, these two very similar species probably compete little or not at all.

In summary, Ocean Acre Sternoptychidae appear to partition their resources primarily by habitat segregation in the vertical plane. Where depth distributions overlap or coincide, competition probably is reduced or eliminated by differences in feeding selectivity, which may or may not be related to differences in SL.