Description
provided by Deep sea Fishes of the World LifeDesk
Dorsal-fin rays 8; anal-fin rays 11; pectoral-fin rays 10–11; total gill rakers (rarely 18) 19–23 (rarely 24); vertebrae (rarely 36) 37–38. Dwarf species rarely exceeding 38 mm SL; trunk very long and narrow; its depth at origin of anal photophores three or more times into greatest body depth, subcaudal photophores well separated from anals; dorsal spine medium-to-high, its height often exceeds its length; post-temporal spines well developed; postabdominal spines fused to form a single spine complex; lower preopercle spine directed ventrally, the upper posterio-dorsally; jaws medium; teeth small to minute; gill rakers long and numerous; in preservative abdominal region dark, trunk pigmentless except in definite patches along midline and above anal and subcaudal photophore groups.
Two pigment forms designated form "A" and form "B" occur over much of the species range. Form A is characterized by distinct and clearly defined body pigmentation, while in form B, the body pigmentation is quite diffuse. This pigment difference is not a function of size or sex, is intermediate in few individuals, and both forms do occur syntopically. A morphometric analysis of three sympatric populations in several characters and meristics, plus measurements from one or the other pigment forms from other areas by Baird (1971) failed to show any significant difference between sympatric populations. In addition, there was no consistent sorting out over the range of variability of any one form in any character. Analysis of diurnal depth distributions revealed a marked sorting out of pigment types with the A form most numerous during the day while the B form was predominantly caught at night.
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- Kenaley, Christopher
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- Kenaley, Christopher
Biology
provided by Fishbase
Oceanic (Ref. 4739) and mesopelagic, mainly at 250-650 m (Ref. 4054). Depth range from 100-700 m (Ref. 4462) and from 460-1082 m in the eastern Ionian Sea (Ref. 56504). Adults make marked vertical migrations (Ref. 4739). Found singly or in small groups (Ref. 4739). An opportunistic feeder at dusk on calanoid copepods, small fishes, etc (Ref. 4739). Sexual dimorphism with regard to body size (Ref. 5168), the males being slightly smaller than the females (Ref. 8966). Oviparous, with planktonic eggs and larvae (Ref. 35838).Occurs between 200 and 700 m depth during the day, concentrated between 350 and 550 m, and between 100 and 650 m at night, preferring a depth between 150 and 380 m Ref. 47377). Also Ref. 58302.
- Recorder
- Astrid Jarre-Teichmann
Comprehensive Description
provided by Smithsonian Contributions to Zoology
Argyropelecus hemigymnus
Argyropelecus hemigymnus is broadly distributed in the Atlantic, Pacific, and Indian oceans. It is particularly abundant in the Gulf of Mexico, the eastern and western North Atlantic, and the Caribbean Sea (Baird, 1971). This distribution corresponds to the North Atlantic temperate, North Atlantic subtropical, Gulf of Mexico, and Atlantic tropical faunal regions proposed by Backus et al. (1977).
This is a medium-size sternoptychid, ranging from 6 to 37 mm SL in the Ocean Acre area. Our largest specimens are near the maximum size recorded for the spiecies, which rarely exceeds 38 mm (Baird, 1971). A. hemigymnus was the most abundant sternoptychid in the Ocean Acre area in late summer and was second in abundance in winter and late spring (Table 5). A total of 2870 specimens was captured in the 14 cruises; 1428 were taken during the seasonally paired cruises, including 733 in discrete-depth samples, with 653 of these from noncrepuscular tows (Table 4).
DEVELOPMENTAL STAGES.—Sex and/or developmental stage were determined for 2831 specimens of A. hemigymnus. Postlarvae were 6–9 mm (mean = 7.5 mm) and juveniles 7–15 mm (mean = 10.0 mm). It was not possible to determine sex in these two stages. Subadult females were 10–29 mm (mean = 16.5 mm) and adults 21–33 mm (mean = 26.1 mm). Males were difficult to stage due to the gradual and subtle changes in the testes with maturation. On the basis of microscopic examination and sizes of subadult and adult females, an arbitrary demarcation between subadult and adult males was drawn between 14 and 15 mm SL. On this basis, subadult males were 10–14 mm (mean = 13.0 mm) and adults 15–25 mm (mean = 17.0 mm). The largest females were 5–8 mm longer than the largest males in the seasonally paired cruises. In addition, mean female size for the paired cruises (21.3 mm) was about 5.6 mm longer than mean male size for these cruises (15.7 mm). Thus there is a sexual dimorphism in size, with females being larger.
REPRODUCTIVE CYCLE AND SEASONAL ABUNDANCE.—Argyropelecus hemigymnus appears to breed continuously throughout the year, with an extended peak in spawning from April through September. This is in agreement with Jespersen (1915) and to some extent with Sanzo (1931), who stated that the breeding period lasts throughout the year in the Mediterranean. Apparently both sexes reach maturity at about one year of age, with most adults dying shortly after spawning. In our collection, several females that were staged as subadults due to the condition of their ovaries were well into the size range usually associated with adults. These specimens may have spawned previously and were approaching a second spawning.
The onset of peak spawning is indicated by an abundance of ripe adults in April (cruise 6). The extended nature of this peak period is indicated by the relative abundance of postlarvae, juveniles, and small subadults in late summer (Table 10). Most spawning seems to be completed by fall since very few adults were taken in November (cruise 1) and December (cruise 5). The presence of some adults and small, young individuals in each season suggests that some spawning occurs year round.
In late summer all life history stages were abundant (Table 10). Postlarvae were more numerous than in any other season of the year, accounting for 27.1 % of the catch. They had an average SL of 7.5 mm. Although their age at this time is difficult to estimate, the fact that members of the genus Argyropelecus undergo a protracted metamorphosis (Ahlstrom, 1974) indicates that they were probably spawned several weeks prior to capture. Juveniles, too, were more abundant in late summer than in any other season. They were only slightly larger than postlarvae, averaging 9.9 mm, and were probably about 6–8 weeks old at time of capture. Subadults were also small, averaging 13.6 mm. Presumably these individuals were spawned sometime in the spring and would have matured and spawned the following spring. The abundance of small individuals in late summer clearly indicates that spawning activity had been heavy in the previous months, and the presence of some adults (13.8% of the catch) indicates that some spawning continues into the fall. The scarcity of adults in November and December suggests postspawning mortality of most adults.
By winter the composition of the catch had changed considerably. Subadults were by far the most abundant stage (Table 10). These represent growth of the very abundant late summer postlarvae and juveniles. Presumably they would have continued to develop over the winter and early spring and probably would have spawned in late spring or summer. The growth of these abundant winter subadults would account for the abundance of adults seen in April (cruise 6). During winter, postlarvae and juveniles were at their lowest seasonal abundance (Table 10). Postlarvae were especially scarce, accounting for only 1.5% of the catch, but their presence indicates that some spawning occurs in the winter months, and the presence of juveniles suggests some late fall-early winter spawning. While the abundance of adults was only slightly lower than in other seasons, they were relatively small, averaging only 19.7 mm. These individuals probably were spawned in the preceeding late spring or summer and would have reached sexual maturity the following winter or spring.
In late spring, small juveniles predominated, accounting for 55.7% of the catch (Table 10). Their small size (mean 10.0 mm) indicates that they were spawned six to eight weeks prior to capture, probably at the beginning of the peak spawning season in late April or early May. Postlarvae, only slightly smaller than the juveniles, probably were spawned several weeks prior to capture. Subadults were at their lowest seasonal abundance. These probably represent continued growth of winter juveniles, and they would have matured and spawned in the upcoming late summer months. The decline in subadults from winter to late spring is almost certainly due to development of winter subadults into late spring adults. Following this logic, a high abundance of late spring adults would be expected, and in fact adults accounted for 27.4% of the catch. Catch per unit effort, however, was not greatly different than in other seasons. This is probably due to loss of adults through postspawning mortality.
Seasonal changes in abundance of the different life history stages indicate a one-year life history for most A. hemigymnus. For example, postlarvae spawned in late summer would reach the juvenile stage by early fall and the subadult stage by the winter months. These would continue to develop over the winter, spring, and early summer months, reaching sexual maturity and spawning in late summer. Most individuals would then die.
SEX RATIOS.—No significant difference between total numbers of males and females was indicated at any of the three seasons (Table 11). In actual numbers, more males than females were captured in winter (164:137) and late spring (109:93), and more females than males in late summer (130:97). Subadult females were significantly more numerous than subadult males in all seasons, and adult males significantly outnumbered adult females in all seasons. Although these data indicate that sex ratios within stages are different, they must be viewed with caution, due to the somewhat arbitrary delineation between subadult and adult males. Furthermore, all subadult females were probably recognized as such, while some subadult males may have been staged as juveniles due to the similarity between very small testes and immature gonads. Probably sex ratios are not different than 1:1.
VERTICAL DISTRIBUTION.—In the seasonally paired cruises, A. hemigymnus was caught from <50–1550 m during the day and from <50–1250 m at night (Table 12). Of the Of the 278 discrete-depth specimens captured at night, 232 (83.5%) were caught from 351–550 m (Table 12). Twelve of the 367 discrete-depth, noncrepuscular specimens captured during the day, 322 (87.7%) were from 351–600 m. Of the remaining 45, 6 were taken shallower than 350 m. These included 1 late summer postlarva, 1 winter juvenile, 3 subadults (1 in each season), and 1 late summer adult. Probably all of these were contaminants from previous tows. Twenty-eight specimens representing all life-history stages and seasons were taken from 601–750 m during the day. These captures indicate that some specimens occupy slightly greater depths. Below 750 m, 11 specimens were captured. These included juveniles (winter and late summer), subadults (winter and late summer), and adults (all three seasons). All of these were considered contaminants, since in each case the previous tow had fished at the preferred depths of A. hemigymnus.
Of the 278 discrete-depth specimens captured at night, 232 (83.5%) were caught from 351–550 m (Table 12). Twelve of the remaining 46 specimens were taken shallower than 350 m. Of these, 4 were shallower than 150 m and probably represent contaminants from previous tows. The remaining 8 specimens came from depths of 251–350 m, and their presence indicates some upward scattering of the species to slightly shallower depths. Below 550 m, 34 specimens were captured. Twenty-three of these were taken below 750 m and were probably contaminants. The remaining 11 were captured from 551–750 m, and these probably represent a scattering of individuals to slightly greater depths.
The vertical distribution of A. hemigymnus in the Ocean Acre area is similar to that in other localities. Goodyear et al. (1972) gave the daytime depth range of Mediterranean specimens as 150–800 m, with peak concentrations from 300–500 m. Nighttime ranges were from 70–600 m, with peak concentrations from 235–600 m. Badcock (1970) stated that near the Canary Islands the species is concentrated at 450–650 m during the day and 300–450 m at night. Baird (1971), reporting on a variety of locations, gave daytime depth ranges from 200–700 m, with peak concentrations at 350–550 m. Night depth range was given as between 100 and 650 m, with the peak concentration from 150–380 m. In the eastern North Atlantic, Badcock and Merrett (1976) found that this species was distributed from 300–600 m during the day and from 200–600 m at night.
Although the daytime depth distributions of all life-history stages overlap broadly, there is some indication of stage stratification during the day in this species, with older and larger individuals occurring slightly deeper. Similar observations have been made by Jesperson and Taaning (1926), Badcock (1970), Goodyear et al. (1972), and Badcock and Merrett (1976). There is also an indication that some life-history stages are found deeper in late summer and shallower in winter. In late summer, when postlarvae were most abundant, they were found from 401–700 m, but were abundant only from 451–500 m (Table 12). The paucity of postlarvae taken during the winter and late spring prevents accurate descriptions of their preferred depths during these seasons, but they were generally 50–100 m shallower than in late summer. Juveniles occurred over a wider range of depths than postlarvae in all seasons, and the depths at which they were abundant were generally slightly deeper. In winter, juveniles were found from 301–500 m and were abundant at 401–500 m. In late spring, juveniles were quite abundant from 351–550 m, but seemed to be concentrated at 351–450 m. These depths (351–450 m) overlap those of postlarvae. In late summer, the range of depths inhabited by juveniles during the day was quite wide (401–700 m), but juveniles were concentrated from 451–600 m. This is approximately 100–150 m deeper than their winter distribution and 50–100 m deeper than their late spring distribution. Although the maximum abundance of both postlarvae and juveniles occurred at 451–500 m in late summer, the abundance of juveniles and the scarcity of postlarvae from 501–600 m indicates that juveniles are distributed slightly deeper than postlarvae. Subadults were captured from 351–700 m during the day in winter, but their maximum abundance was at 451–500 m. During this season subadults and juveniles were abundant at similar depths. While subadults were captured from 301–750 m in late spring, too few specimens were taken to allow speculation on their preferred daytime depths. In late summer, subadults were captured from 451–750 m. The shallow end of this range is about 100 m deeper than the range of other seasons, indicating that subadults occupy deeper depths during late summer. This is further substantiated by their relatively deep strata of maximum abundance (451–600 m). Although both juveniles and subadults were abundant within this range, the maximum catch rate for subadults was 150–200 m deeper than that of juveniles. Adults were captured from 351–700 m during the day in winter, with the maximum rate at 451–500 m. Both juveniles and subadults were also abundant at this depth. In late spring adults occupied a deeper range (401–750 m), and were abundant from 501–650 m. This is 50–150 m deeper than their preferred winter depths and deeper than the preferred depths of the other life-history stages present at this time. The late summer depth distribution of adults was similar to that of late spring. Once again there was a tendency for adults to be found deeper than the other life-history stages.
Nighttime depth distributions of the different life-history stages overlap broadly in all seasons (Table 12). There is a tendency for all life-history stages, with the possible exception of postlarvae, to be found deeper in late summer, and also a tendency for larger individuals to be found deeper than small individuals. Postlarvae were taken from 351–450 m in winter and from 401–450 m in late spring. Although they were found slightly deeper in late summer (401–500 m), their maximum catch rate was at 401–450 m. Thus it appears that postlarvae occupy about the same depths year-round. Juveniles were taken from 351–550 m in winter, with the maximum catch rate at 401–450 m. Their range in late spring was somewhat shallower (301–500 m), and individuals were abundant throughout this range. In late summer juveniles were widely distributed from 351–900 m, but the maximum catch rate was at 451–500 m. This was 50–100 m deeper than depths of maximum catch rate in both late spring and winter. Subadults were taken from 251–550 m in winter. Within this range they were abundant from 401–550 m. A slightly deeper range of depths was occupied in late spring (301–650 m), but the preferred depths (451–500 m) were similar to those of winter. In late summer subadults were taken from 451–1050 m. This range was deeper than in other seasons, and the depths of maximum abundance (501–550) were about 50 m deeper as well. The nighttime depth ranges inhabited by adults in winter (251–550 m), late spring (401–750 m), and late summer (401–700 m) all overlapped broadly (Table 12). Depths of maximum abundance, however, changed seasonally. In winter, adults were abundant from 501–550 m, which was 50–100 m deeper than in late spring. In late summer, adults were abundant from 451–650 m, which was deeper than in other seasons.
All life-history stages of A. hemigymnus appear to undergo slight diel vertical migrations (Table 12). In late summer, when postlarvae were abundant, the range of depths inhabited by this stage was shallower at night. This, along with the observation that the depth of maximum abundance was about 50 m shallower at night, strongly indicates that postlarvae undergo a short diel vertical migration. Similar day-night differences were seen among juveniles. In each season the range of depths inhabited at night was shallower than the daytime range. Furthermore, the depths of maximum abundance were 50–100 m shallower at night, indicating a diel vertical migration. In late summer and winter when subadults were abundant, the depths of maximum abundance at night were 50–100 m shallower than during the day.These day-night differences indicate that subadults undergo a diel vertical migration. Among adults, the range of depths inhabited during night and day did not change significantly in any season. Depths of maximum abundance, however, were 50–150 m shallower at night in late spring and late summer, indicating that adults undergo a diel vertical migration in these seasons. This pattern was not evident in winter.
PATCHINESS.—Argyropelecus hemigymnus appears to be a patchily distributed species both day and night. Although these patches, or clumps, typically involve more than one life-history stage, one stage usually dominates.
At night, significant CD values were seen in late spring (351–400 m) and in late summer (401–450, 451–500 m). The late spring clumping was dominated by juveniles, but some subadults were also present. In late summer the 401–450 m interval was dominated by postlarvae and juveniles but contained some adults. The 451–500 m interval at this time was strongly dominated by juveniles, although some members of all other life-history stages were present. The nighttime patchiness is within the preferred nighttime range of depths.
During the day, significant CD values were seen in late spring (351–400 m), late summer (451–500, 501–550, 551–600 m), and winter (351–400, 451–500 m). The late spring depth interval contained a few postlarvae and subadults but was composed primarily of juveniles. In late summer there was a correlation between the depth of patchiness and the dominant life-history stage. The shallowest depth interval (451–500 m) contained mostly postlarvae; the next deepest interval (501–550 m) was composed mainly of both juveniles and subadults, while the deepest interval (551–600 m) was dominated by subadults. In winter, patchiness was indicated at two depths during the day (351–400, 451–500 m). The shallower of these contained all life-history stages except juveniles. The deeper contained all life-history stages except postlarvae and was strongly dominated by subadults. Each of the daytime intervals was within the preferred daytime depth distribution of this species.
While adults were minor contributors to several of the depths where patchiness was indicated, none of these depths were dominated by adults. This may indicate that adults do not aggregate to the same extent as younger individuals. An alternative explanation would be that adults, because of their larger size, are better able to avoid the approaching net.
NIGHT:DAY CATCH RATIOS.—The ratios of total night to day discrete-depth catch rates were slightly different from 1:1 in each season (Table 13). In winter, slightly more fish were caught per unit effort at night, but in late spring and late summer this trend was reversed. The greatest differences in catch ratios were seen among postlarvae in late spring and late summer, when day catches were much higher. This apparent difference is difficult to explain, since in postlarvae no diel difference in patchiness was indicated. It also seems improbable that net avoidance, if it occurs at all in these small individuals, would change from day to night. Among juveniles the night:day catch ratios were not greatly different. This was also the case with subadults except in late summer when nearly twice as many fish per unit effort were taken during the day. This discrepancy may be due to patchiness, since subadults appear to occur in patches during the day but not at night. Among adults, the only real difference in catch ratios was seen in winter, when more fish per unit effort were taken at night. Since adults apparently are not patchily distributed, this difference is difficult to explain.
Although no pattern emerges from these night:day ratios, Badcock (1970) and Badcock and Merrett (1976) have found that catch rates of this species generally were lower at night. They suggested that this may have been due to increased activity at night, presumably associated with feeding, which occurs from late afternoon through early night (Merrett and Roe, 1974).
- bibliographic citation
- Gibbs, Robert H., Jr. and Krueger, William H. 1987. "Biology of midwater fishes of the Bermuda Ocean Acre." Smithsonian Contributions to Zoology. 1-187. https://doi.org/10.5479/si.00810282.452
