Diaphus Theta Research Articles

北日本海域におけるハダカイワシ科魚類の鰾形態とターゲットストレングス

Swimbladder shapes of 5 myctophid species captured around northern Japan were measured using an X-ray method. The species were classified into swimbladder and swimbladderless fishes according to the presence of gas. All specimens of Lampanyctus jordani had a well-inflated swimbladder. Notoscopelus japonicus and Stenobrachius nannochir had no swimbladder in any body size range examined. Diaphus theta and Symbolophorus californiensis contained swimbladder and swimbladderless fishes in relation to growth. Based on several morphological parameters of the body or swimbladder, the relationships between body length and target strength at 38 and 120 kHz were estimated using theoretical acoustical backscattering models. The estimated values of the target strength showed 10-dB or more differences between bladder and bladderless fishes, even when the fish were of equivalent size. The target strength values of most fishes did not grow proportionally to the square of the body length at both frequencies because of swimbladder allometric growth or the effect of Rayleigh scattering. Our result suggests the importance of swimbladder observation for target strength study of myctophid fishes, because they have various swimbladder conditions, and the characteristics of acoustic backscattering change dramatically with the swimbladder conditions.

Acoustic identification and density estimate of Diaphus theta (myctophidae) off Hokkaido, Japan

Diaphus theta is the most abundant micronektonic fish in the northwestern Pacific. Field acoustic data and biological samples were obtained both day and night off of eastern Hokkaido, Japan in September 2006 to estimate the fish density and diel changes of its vertical distribution. Differences of acoustical target strength (TS) values between at 38 and 120 kHz were applied to identify D. theta echos using field echo data. Theoretical estimation using swimbladder acoustic scattering models showed that the TS difference (TS 120-38 kHz) of larger fishes (over 60 mm) was between −4 and 1 dB; that for smaller fishes (less 60 mm) was less than .4 dB. These values differed from those of other major component species of the deep-scattering layer, such as krill and pollock, suggesting that the echo from D. theta is acoustically identifiable. Diel vertical distributions of D. theta were estimated after identification. Relatively dense schools were observed at around 400-m depth during daytime, although the schools were scattered widely above 100-m depth during nighttime. These results matched those obtained by biological sampling. In the surface layer (above 100 m), the estimated fish densities were about tenfold higher at nighttime than in daytime.

The characteristics of feeding of Diaphus theta (Myctophidae) in the northwestern part of the Pacific Ocean in the Summer-Autumn period

The feeding of Diaphus theta in the zone of the subarctic front and in Pacific waters off the Kuril Islands in November 2001 and in July–August 2002 was studied. Interzonal plankton [euphausiids (mainly Euphausia pacifica), copepods (Metridia pacifica), and amphipods (Themisto pacifica)] dominated in the food bolus. As fish grow, the proportion of copepods decreases, the proportion of euphausiids and amphipods increases in stomachs, and the food spectrum extends. One considerable peak in the feeding (in both the summer and autumn periods) occurring in the nighttime was revealed. The most intensive feeding takes place in the dark time of the day in the epipelagial due to euphausiids and amphipods; in the light time of the day, the feeding activity declines. Fish feed mainly on copepods in the upper pelagial. The daily ration of D. theta was 1.5% of the weight of fish in the summer period and 3.2% in the autumn period.

Larval feeding habits of Diaphus theta, Protomyctophum thompsoni, and Tarletonbeania taylori (Pisces: Myctophidae) in the transition region of the western North Pacific

We examined the larval feeding habits of the 3 dominant myctophids, Diaphus theta, Protomyctophum thompsoni, and Tarletonbeania taylori, in the transition region of the western North Pacific. Feeding incidence for D. theta and T. taylori larvae was higher during the day than at night (mean: 70.7 to 84.4% vs. 2.2 to 4.1%), indicating that they are daytime visual feeders. Larvae of P. thompsoni were also visual feeders, with a daytime feeding incidence of 83.0%. The proportion of nighttime feeding in P. thompsoni gradually increased with development, which suggests faster lar- val adaptation of this non-migrant species to the poor light conditions in the mesopelagic zone. D. theta larvae ≤7.9 mm in body length (BL) fed mainly on copepod nauplii, which were replaced by calanoid copepodites and Oithona spp. in larvae ≥8.0 mm BL. The most important prey for smaller P. thompsoni and T. taylori larvae ≤7.9 mm BL were nauplii and Oithona spp., which were replaced by calanoid copepodites and ostracods in P. thompsoni and copepod eggs in T. taylori with increased development. Competition for prey among the larvae of these 3 myctophids was mostly avoided because of diet and habitat-depth segregation. Interspecific comparisons of feeding habits among the 3 species, in addition to larvae of 3 other myctophids found in the study area, suggested that their feeding strategies were associated with mouth size, i.e. larvae with larger mouths tended to take larger prey rather than a large number of smaller prey. This interspecific difference in feeding strategy is reflected in the diverse morphological characters of myctophid fish larvae.

Distribution patterns of larval myctophid fishes in the transition region of the western North Pacific

We examined the species composition and horizontal distribution of myctophid fish larvae in the transition region of the western North Pacific during the early summer. In total, 4,760 myctophid larvae were collected at 44 stations; 18 species of myctophids from 15 genera were collected, and the 8 most abundant species accounted for >95% of larvae. The distribution patterns of these larvae were well defined by the hydrographic structures of the study area including the Oyashio and Kuroshio fronts, the Subarctic Boundary, and a warm core ring. The horizontal distribution patterns of the eight dominant species were categorized into three groups: northern transition water (Stenobrachius nannochir, Tarletonbeania taylori, and Lampanyctus jordani), southern transition water (Symbolophorus californiensis, Diaphus theta, and Nannobrachium regale), and Kuroshio (Myctophum asperum and Diaphus garmani). The Subarctic Boundary defined the distributions of the northern and southern transition-water groups. The importance of areas of western North Pacific transition water as spawning and nursery grounds for subarctic, transitional, and subtropical myctophid fishes was indicated by the relationship between the horizontal distribution patterns of larvae, juveniles, and adults and the physical oceanographic structures.

Spawning season and migration of the myctophid fish Diaphus theta in the western North Pacific

The spawning season, spawning grounds, and migration of the myctophid fish Diaphus theta were studied in the western North Pacific, based on seasonal sampling and estimation of hatching dates. The peak abundance of larvae was observed in July in the transition waters between the Oyashio and Kuroshio fronts. The spawning season ranged from late March to early September, with a peak from May to July. Larvae and juveniles <40 mm in standard length were distributed in the transition waters, whereas larger individuals were collected in the Oyashio and the Western Subarctic waters. These results indicate that this species undergoes a horizontal spawning migration from the Oyashio and Western Subarctic waters into the transition waters crossing the Oyashio front.

Target strength of mesopelagic lanternfishes (family Myctophidae) based on swimbladder morphology

Abstract This article reports theoretical values of target strength (TS) for mesopelagic lanternfishes based on morphological measurements of their swimbladders. Three species of lanternfishes, Diaphus theta (26.9–77.4 mm standard length (SL)), Symbolophorus californiensis (85.0–108.4 mm SL), and Notoscopelus japonicus (126.0–133.2 mm SL), were examined. After external morphological measurement of the fish body, a specialized “soft X-ray” imaging system was used to map the swimbladders and obtain their morphological parameters. The swimbladder was inflated in D. theta, uninflated in S. californiensis, and was absent in N. japonicus. For D. theta, the swimbladder length does not increase in proportion to the body length, suggesting that the contribution of the swimbladder to acoustic reflection is reduced with growth in this fish. Based on the morphological measurements, the theoretical TS of the fish at 38 kHz was calculated using the approximate deformed-cylinder model (DCM) and the general prolate-spheroid model (PSM). For all three species, the calculations showed about 3 dB difference between the TS indicated by the DCM and PSM. Given that the description of body shape is poor in PSM, the DCM results were adopted for fish without a swimbladder or an empty one. The intercept b20 in the standard formula TS = 20 log SL + b20 was −85.7 dB (DCM) for S. californiensis and −86.7 dB (DCM) for N. japonicus. On the other hand, the PSM model was adopted for D. theta since its swimbladder has too small an aspect ratio to apply the DCM. For D. theta, the relationship between SL and TS is best expressed by TS = 11.8 log SL − 63.5, which implies that its scattering cross-section is not proportional to the square of the body length.

Diets of walleye pollock Theragra chalcogramma in the Doto area, northern Japan: ontogenetic and seasonal variations

Seasonal, ontogenetic and bathymetric variations of diet were examined for walleye pollock Theragra chalcogramma based on a total of 6666 fish collected off the southeastern coast of Hokkaido Island, northern Japan (Doto area) during 1989 to 2000. Walleye pollock depended exclu- sively (>99%) on pelagic prey and showed a clear ontogenetic dietary shift: smaller fish depended mainly upon mesozooplankton such as Neocalanus cristatus and Euphausia pacifica, and larger fish preyed upon pollock (cannibalism), myctophids Diaphus theta and firefly squid Watasenia scintillans. Seasonal variation was also evident. Smaller pollock depended mainly upon N. cristatus during spring then shifted gradually to E. pacifica during other seasons. For larger pollock, major prey shifted from pollock (cannibalism) during spring to micronekton during other seasons. Bathymetric variation was less pronounced, with cannibalism and Themisto spp. being more important in the shallow area (≤150 m). Feeding intensity, measured as stomach content index, was generally higher during spring and summer than autumn and winter, but showed incidentally high values during win- ter, suggesting sporadic but intense feeding opportunities. Fish condition fell during winter and then recovered rapidly during May and June. Recovery coincided with superabundance of N. cristatus during blooming, so that this species is essential for the recovery of pollock from wintering and spawning.

Growth of larval and juvenile Diaphus theta (Pisces: Myctophidae) in the transitional waters of the western North Pacific

Diaphus theta is one of the most common myctophid fish species in the subarctic and transitional waters of the North Pacific. The growth of larval and juvenile D. theta was investigated using sagittal otolith increment analysis of specimens caught in transitional waters of the western North Pacific. Samples taken over a 24-h period demonstrated that otoliths exhibited daily growth cycles, allowing accurate determination of age. Calcification of the incremental zone of otoliths took place only at night, suggesting that the formation cycle of the increment of juvenile D. theta was different from that of shallow-water fishes and would be related to their diel vertical migration. The relationships between standard length (SL) and daily growth increment (D) were expressed as linear equations: SL = 2.65 + 0.141D (r 2 = 0.942) for larvae of 5.1–9.6 mm SL and SL = 3.54 + 0.129D (r 2 = 0.933) for juveniles of 13.7–27.6 mm SL. The growth rates were 0.14 mm d−1 in larvae and 0.13 mm d−1 in juveniles; this is slow compared with tropical or subtropical mycto-phid species, in which growth occurs at about twice these rates. The larval period, including the metamorphic stage, was long compared with species at lower latitudes and was estimated to be 71 days. The slow growth rate and long period of larval stage of D. theta would be the life history pattern of high-latitudinal species adapted to a low-temperature habitat.

Wax ester biosynthesis in the liver of myctophid fishes.

The biosynthetic properties of wax esters in the liver were compared between two types of myctophid fishes having different body lipid composition, i.e., three triglyceride-rich species (Lampanyctus jordani, Diaphus theta, and Symbolophorus californiensis) and three wax ester-rich species (L. regalis, Stenobracius nannochir, and Stenobracius leucopsarus). n-Heptadecanol (17:0-ALC) and/or 10-cis-heptadecenoic acid (17:1-ACID) was incubated with liver homogenate of the six myctophid fishes and with co-factors such as NADPH and ATP for 2 to 5 h. Considerable amounts of wax esters with odd-numbered fatty acids and/or alcohols were produced in the liver homogenate of the wax ester-rich species. Stenobracius nannochir and L. regalis, which exclusively contained wax esters as neutral lipids, showed the highest activity of wax ester synthesis, followed by S. leucopsarus, which contained triglyceride as the minor constituent. Only trace amounts at most of odd-numbered fatty acids and alcohols were incorporated into the wax esters after incubation with the liver homogenates of the triglyceride-rich fishes. Active interchange between the fatty acids and the alcohols occurred during wax ester biosynthesis in the wax ester-rich fishes. The chain elongation and shortening of acyl moieties were also observed during incubation. These results suggested that the deposition of lipids in myctophid fishes is mainly due to their biosynthetic activities.

Effects of the Parasitic Copepod Cardiodectes medusaeus on the Lanternfishes Diaphus theta and Tarletonbeania crenularis off Central California

In two distinct survey areas, offshore from the Farallon Islands and in the vicinity of Pioneer Canyon off central California, Diaphus theta was significantly more prone to infection by the parasitic copepod Cardiodectes medusaeus than Tarletonbeania crenularis. In addition, D. theta was more heavily parasitized in the Pioneer Canyon area than in the Farallon Islands area. A linear relationship was shown between loge-transformed otolith size and loge-transformed SL and between loge-transformed dry weight and loge-transformed SL for D. theta and T. crenularis in both survey areas. A test of slopes showed differences between the two survey areas for both species. Lower dry weights were observed for both D. theta and T. crenularis, in parasitized versus non-parasitized specimens in the Pioneer Canyon area. In the Farallon Islands area, however, no differences were observed. Larger otolith sizes were observed in parasitized D. theta versus non-parasitized specimens in the Pioneer Canyon area, while no differences were observed in the Farallon Islands area. In contrast, T. crenularis had larger otolith sizes in parasitized versus non-parasitized specimens in the Farallon Islands area, while no significant differences were observed in the Pioneer Canyon area. These findings provided evidence of geographic and interspecific variability in both the prevalence and the physiological effects of parasitism.

Diel vertical migration of myctophid fishes (Family Myctophidae) in the transitional waters of the western North Pacific

The diel vertical migration patterns of adult myctophid fishes were determined in the transitional waters of the western North Pacific off Japan, using day–night sampling from 20 to 700 m depths with a commercial otter trawl in the summer of 1995. A total of 12 species belonging to 9 genera were collected. Four patterns were recognized in the diel vertical migration of 11 of the 12 species. (1) Migrants showing clear day–night habitat separation with peak abundance above 200 m at night: Symbolophorus californiensis, Tarletonbeania taylori, Notoscopelus japonicus, Diaphus theta, Ceratoscopelus warmingi, and Diaphus gigas. (2) Semi‐migrants, in which part of the population often remains in the daytime habitat at night. The distribution depths of migratory and nonmigratory individuals do not overlap: Stenobrachius leucopsarus. (3) Passive‐migrants, in which there is no separation of day–night habitats, but the upper limit of daytime distribution depth shifts to a shallower layer at night, probably as the fish follow migratory prey: Lampanyctus jordani. (4) Nonmigrants: Stenobrachius nannochir, Lampanyctus regalis (&gt; 140 mm SL), and Protomyctophum thompsoni. The day–night habitat temperature ranges are also given for the 11 species. No remarkable east–west differences were seen in the vertical migration patterns compared with previous knowledge of eight of these species in the eastern Pacific. The diel migration patterns are newly described for three other species endemic to the western Pacific. The standing stock of myctophids in the study area was conservatively estimated at 18.5 ± 4.7 g m−2 (avg. ± SD).

Occurrence of inflated swimbladders in five species of lanternfishes (family Myctophidae) from waters off southern California

The swimbladders of three species of myctophid fishes (Symbolophorus californiensis, Tarletonbeania crenularis and Diaphus theta, collected in the Southern California Bight between 1963 and 1978), previously observed to have both inflated and noninflated swimbladders as adults, were examined to determine if the swimbladders either vary in their inflation on a diel basis or cease to be inflated and possibly become nonfunctional in some individuals. The swimbladders of two lanternfishes reported to lack inflated swimbladders as adults (Lampanyctus regalis and L. ritteri) were included in the study for comparison. Only the swimbladder of L. regalis did not increase in size with increasing fish size. In the remaining four species, both the lengths and volumes of noninflated swimbladders were positively correlated with fish standard length (SL). Although the swimbladder continued to increase in size, inflated swimbladders were not found in L. ritteri>2 mm SL. Inflated and noninflated swimbladders occurred in overlapping size ranges of S. californiensis, T. crenularis, and D. theta. Only 11% of the D. theta swimbladders were inflated, and the occurrence of inflated swimbladders dit not differ significantly with day vs night capture or fish size. Although the frequency of occurrence of inflated swimbladders decreased significantly in larger S. californiensis, both S. californiensis and T. crenularis caught at the surface at night with a neuston net had significantly higher proportions of inflated swimbladders than did those collected below the surface with either daytime or night-time trawls. The swimbladder does not appear to become nonfunctional in either species. Rather than maintaining either constant volumes or constant masses of gas in their swimbladders during their vertical migrations, both species may inflate their swimbladders only while in the surface waters at night, and have noninflated swimbladders while at other depths of their vertical ranges.

Adaptations in mesopelagic fishes

Mesopelagic fishes were collected from depths of 400 to 900 m off Oregon (USA) in September 1983, October 1985 and June 1989. Species of mesopelagic fish without diel vertical migrations (non-migrators) are known to have high body-and muscle-water contents relative to epipelagic and vertically migrating mesopelagic species. This characteristic is reported to correlate with low food availability. Through histology, we show that four species of swimbladderless non-migrators (Bathylagus pacificus, B. milleri, Tactostoma macropus and Chauliodus macouni) have large deposits of gelatinous material which stain positively for acidic glycosaminoglycans (GAGs) and negatively for lipid and protein. GAGs are highly hygroscopic and are thus responsible in part for the high body-water content of these species. The material is located between myotomal (swimming) muscle cells, along the dorsal midline and surrounding the spine in all four species, and the two Bathylagus species have an additional subcutaneous layer. The subcutaneous layer of B. pacificus has very high (96%) water, low protein (3%) and low ion contents and is positively buoyant, unlike myotomal muscle (89% water, 6.7% protein, negatively buoyant). In contrast, four species of vertical migrators (B. ochotensis, Stenobrachius leucopsarus, Tarletonbeania crenularis and Diaphus theta) have no such deposits. The role of this gelatinous material as a possible buoyancy mechanism in an energy-poor habitat is discussed.

The biological and acoustical structure of sound scattering layers off oregon

Four discrete acoustical frequencies were used to determine the depths of migratory and non-migratory scattering layers. Layers were insonified with a broad-band, low frequency (1–30 kHz) acoustic source and sampled with a 100 m 2 rope trawl with five opening and closing codends. The predominant fishes with gas-filled swimbladders caught in deep (188–250 m) day time layers were Protomyctophum crockeri and P. thompsoni, whereas Protomyctophum spp. and Diaphus theta were the most common species in deep (125–225 m) night-time layers. The common fishes with gas-filled swimbladders in shallow (0–80 m) night-time layers were D. theta, Stenobrachius leucopsarus (<35 mm), Tarletonbeania crenularis, Symbolophorus californiensis and Engraulis mordax. Arcer estimates of distributions of bubble radii were similar for deep day and deep night scattering layers, but a wider range of bubble sizes existed in shallow night-time layers than in deep day and deep night layers. A model which considered species-specific parameters of morphology, physiology and behavior provided estimations of the distributions of swimbladder radii that most closely matched the acoustically estimated distributions of bubble radii. Comparisons of acoustic data and net catch data indicate that myctophids with gas-filled swimbladders may maintain swimbladder volume at a level below that required for neutral buoyancy. Furthermore, there is some evidence suggesting that large myctophids maintain a constant swimbladder volume during vertical migrations, but at volumes that result in negative bouyancy; smaller myctophids may maintain a constant swimbladder mass during vertical migration.

Spatial and temporal variations in the population size structure of three lanternfishes (Myctophidae) off Oregon, USA

Size-frequency distributions were determined for 3 common lantern-fishes (Stenobrachius leucopsarus, Diaphus theta, and Tarletonbeania crenularis) off Oregon in the summer. The fishes were caught mainly in sound-scattering layers by a large pelagic trawl with 5 opening-closing nets. Changes in depth distribution and diel vertical migration with growth were evident for all 3 species. The size of S. leucopsarus increased markedly with depth both at 0 to 90 m at night and 250 to 500 m during the day. Larger D. theta were also found deeper during the day (between 250 and 450 m), but neither D. theta nor T. crenularis demonstrated size segregation in the upper 90 m at night. Large D. theta and small T. crenularis did not appear to migrate into surface waters at night. Age-Group O (15 to 20 mm) S. leucopsarus were most abundant in deep water (400 to 480 m) in the daytime and did not migrate into near-surface waters at night. Age-Group I (30 to 40 mm) S. leucopsarus were common at about 300 m by day and within the upper 30 m at night. Age-Group II–III (50 to 60 mm) apparently followed the evening ascent of Age-Group I fish and most resided at 75 to 90 m at night, beneath Age-Group I fish. Age-Group III+fish (70 to 80 mm) were associated with Age-Group O at 400 to 480 m by day and usually did not migrate above 200 m at night. The size structure of S. leucopsarus differed among the nets of a single tow at one depth, or between two tows that fished the same depths on successive nights, indicating horizontal patchiness in age structure. D. theta demonstrated low within-tow variability in size composition which indicated a spatially more uniform age structure on a scale of kilometers. The size structures of these 3 lanternfishes were different in the same area and the same season during two different years, suggesting variable survival of year classes or horizontal patchiness of age composition in the area sampled.

The feeding habits of three species of lanternfishes (family myctophidae) off oregon, USA

The common myctophids Stenobrachius leucopsarus, Diaphus theta and Tarletonbeania crenularis were found to feed primarily on the euphausiid Euphausia pacifica, the copepods Metridia lucens and Calanus spp., and the amphipod Parathemisto pacifica. The diets of these species of fishes were diverse and overlapped broadly, suggesting that they are feeding generalists. Most stomachs contained either all copepods or all euphausiids. Euphausiids were the most important prey on the basis of biomass. They comprised over one-half the weight of the stomach contents in over 40% of individuals of each of the three fishes. Stomach fullness and state of digestion of stomach contents differed over the diel period, but not enough to indicate feeding only during the nighttime. Average stomach fullness was greatest during the morning and night and well-digested material, which predominated in most stomachs, was most prevalent in the morning and afternoon. Either some feeding occurs throughout the day or digestion rates are slow.

Swimbladder Morphology and Specific Gravity of Myctophids off Oregon

Three general types of swimbladders were found in the eight species of myctophids studied: gas-filled, fat-invested, and atrophied or reduced. Small specimens of all species had thin-walled, gas-filled swimbladders. Large specimens of Stenobrachius leucopsarus had fat-invested swim-bladders and large Diaphus theta had either gas-filled or atrophied swimbladders, as found by other workers. Large Tarletonbeania crenularis had either gas-filled or reduced swimbladders, large Lampanyctus ritteri and L. regalis had reduced swimbladders, and large Stenobrachius nannochir had fat-invested swimbladders. Protomyctophum thompsoni and P. crockeri retained gas-filled swimbladders.High body lipid content was found in S. leucopsarus, S. nannochir, L. ritteri and D. theta, and low lipid content was found in the other four species. Myctophids with high lipid content had specific gravities close to that of sea water (1.026–1.030). Tarletonbeania crenularis with a reduced swimbladder had a specific gravity of 1.088. Lampanyctus regalis had a lower specific gravity (1.040) due to high water content of the tissue.The swimbladder to body volumes in S. leucopsarus and D. theta were inversely related to body size and lipid content, indicating that lipids assume the primary buoyancy function as the gas-filled swimbladder regresses with age. This change may eliminate the physiological constraints imposed by a gas-filled swimbladder and permit the more extensive diel vertical migrations of adults.

The Distribution of Animals in the Eastern North Pacific and its Relationship to Physical and Chemical Conditions

Sampling with the Isaacs-Kidd midwater trawl yields good quantities of large and easily-identified animals that can be used as "indicator organisms" to identify water masses in the northeastern Pacific Ocean. Particularly useful are the myctophid fishes Lampanyctus leucopsarus, Diaphus theta, Tarletonbeania crenularis, Electrona arctica, E. crockeri, Ceratoscopelus townsendi and Lampanyctus ritteri, the pteropods Limacina helicina, Cavolinia globulosa and Euclio pyramidata, and the heteropod genus Atlanta. In going from Subarctic water to warmer Intermediate water in the open ocean, numbers of animals declined and the species changed markedly. By the time quantities of Central or Tropical water are encountered, catches were about [Formula: see text] of those in Subarctic water. Inshore catches off southern British Columbia and Washington in 1958 were only about [Formula: see text] as great as in comparable hauls in 1957 — a difference which coincides with a greater intrusion of Intermediate water in 1958.