Observations on the nocturnal feeding of some mesopelagic decapod crustacea

Feeding in relation to temporal changes in the depth distribution of predator and prey is described for 9 species of mesopelagic decapods from an examination of 268 foreguts. Intensive nighttime feeding appears to be the rule in all species. The smaller decapods Sergestes (Sergestes) atlanticus, Sergestes (Sergestes) sargassi and Sergestes (Sergestes) pectinatus exploit the smaller prey, principally copepods and to a lesser extent ostracods. Larger decapod species Sergestes (Sergestes) henseni, Sergestes (Sergestes) curvatus, Sergestes (Sergia) grandis, Systellaspis debilis, and Acanthephyra purpurea mainly prey on macrozooplankton and micronekton, i.e., chaetognaths, euphausiids, decapods and fish, but copepods also occur in the foreguts. Gennadas valens is exceptional for the high incidence of foraminiferal remains, and a predator-prey relationship seems probable. All 9 decapod species have mixed diets, and pronounced feeding preferences are not evident. However, a high incidence of “secondary” feeding or “dietary contamination” has been deduced from the frequent occurrence of remains of the copepods Pleuromamma spp. and Oncaea spp. in the foreguts of the larger decapod species. Direct feeding cannot have occurred, since the depth distributions of these copepods and decapods are disjunct by day and night. It is concluded that the remains of Pleuromamma probably represent the food of the larger prey such as chaetognaths etc. which are eaten by the decapods. The presence of Oncaea is speculatively attributed to a possible ectoparasitic relationship with the larger prey items, but confirmatory evidence is required. These anomalies suggest that caution must be exercised in deducing predator-prey relationships simply from gut contents without consideration of distributional factors.     

Small-scale horizontal distributions of zooplankton taxa

Small-scale (100 to 2 400 m) horizontal distributions of major taxonomic categories (class and order) of zooplankton were measured at a depth of 90 m with an opening-closing plankton net over a 3 d period in October 1978 in the California Current. Some zooplankton categories showed evidence of diurnal vertical migration, while others had long-period temporal changes in mean abundance. Variance-to-mean ratio for large copepods and euphausiids was higher at night than during the day, while the opposite was true for chaetognaths and pteropods. Within a given category, the variance-to-mean ratio generally increased with a category's abundance. Spatial abundance variations were characterized by trends (i.e., fluctuations larger than length of the net hauls) in some taxonomic categories. No consistent differences in scales of variability were found as a function of animal size or from day to night. Correlation analysis of taxonomic counts implied that significant biological interactions occurred. The proportions of counts of taxonomic groups showed no large changes over the time-space scales sampled. However, the proportions of biomass in taxonomic groups differed from day to night due to the large variability of euphausiids. Comparisons of wet weight biomass to taxonomic counts indicated that biomass was usually less variable than taxonomic counts.     

Patterns and selectivity in the feeding of certain mesopelagic fishes

Stomach contents were analysed from the 7 most numerous species of mesopelagic fish caught in a series of 11 hauls over a 24 h period at 230 to 266 m depth in the eastern North Atlantic Ocean. The numerical abundance of organisms per filled stomach and the frequency of occurrence of empty stomachs were used to indicate feeding periodicity. The ecological significance of the feeding periodicity was considered by examining it in connection with an investigation of the day-night vertical distribution of zooplankton and micronekton to 2000 m at the same station. Additional dietary evidence on the 7 species considered was also obtained from the vertical series. Feeding selectivity was examined by comparing the composition of the zooplankton population, sampled separately but simultaneously with the micronekton, with that from the overall stomach contents of the species examined. Feeding periodicity was demonstrated for 6 species, of which 3 were found to be feeding selectively: Valenciennellus tripunctulatus on calanoid copepods, Argyropelecus aculeatus on ostracods, and Lampanyctus cuprarius on amphipods and possibly euphausiids. The limited data available on the other 3 species suggested that they were either random feeders (A. hemigymnus and Lobianchia dofleini) or perhaps selecting against a particular group (Notolychnus valdiviae). No indication of feeding periodicity or selectivity was found for Chauliodus danae. The overall pattern of results confirmed the supposed close correlation between vertical migration and feeding in mesopelagic fish.     

Inter- and intra-specific diurnal habitat selection of zooplankton during the spring bloom observed by Video Plankton Recorder

Diel vertical migration (DVM) is a common behavior adopted by zooplankton species. DVM is a prominent adaptation for avoiding visual predation during daylight hours and still being able to feed on surface phytoplankton blooms during night. Here, we report on a DVM study using a Video Plankton Recorder (VPR), a tool that allows mapping of vertical zooplankton distributions with a far greater spatial resolution than conventional zooplankton nets. The study took place over a full day–night cycle in Disko Bay, Greenland, during the peak of the phytoplankton spring bloom. The sampling revealed a large abundance of copepods performing DVM (up during night and down during day). Migration behavior was expressed differently among the abundant groups with either a strong DVM (euphausiids), an absence of DVM (i.e., permanently deep; ostracods) or a marked DVM, driven by strong surface avoidance during the day and more variable depth preferences at night (Calanus spp.). The precise individual depth position provided by the VPR allowed us to conclude that the escape from surface waters during daytime reduces feeding opportunities but also lowers the risk of predation (by reducing the light exposure) and thereby is likely to influence both state (hunger, weight and stage) and survival. The results suggest that the copepods select day and night time habitats with similar light levels (~10^?9 μmol photon s^?1 m^?2). Furthermore, Calanus spp. displayed state-dependent behavior, with DVM most apparent for smaller individuals, and a deeper residence depth for the larger individuals.     

Swimming and feeding in Periphylla periphylla (Scyphozoa, Coronatae)

Some western Norwegian fjords host extraordinarily abundant and persistent populations of the mesopelagic, coronate scyphomedusa, Periphylla periphylla. In these environments, from late autumn to spring, the medusae undertake regular diel vertical migrations into surface waters. From unique observations obtained with a remotely operated vehicle (ROV), including observations made without artificial light, we observed that 90% of the medusae swam with their tentacles in aboral position. Stomach content analyses of surface-collected specimens revealed that the medusae ate mainly calanoid copepods, but ostracods and large euphausiids were also prominent components of their diets. The clearance rate potential of P. periphylla, assessed from in situ observations and stomach contents, was comparable to that of similar-sized, epipelagic gelatinous species. Our findings suggest that P. periphylla behave as active predators in surface waters.     

Vertical distribution,diurnal and seasonal migration of copepods in Saronic Bay,Greece

The problem of vertical distribution and the movements of copepods was studied from several plankton samples collected by vertical hauls in the Saronic Bay, Greece during two cruises (8 to 17 August and 21 November to 1 December, 1969). The species, whose vertical movements were analyzed, were divided into 3 groups: (1) Those which perform diurnal vertical migration (generally psychrophilic species found in summer samples in deeper layers and in reduced numbers). In autumn, the number of specimens is generally increased and many individuals reach the surface at night. From surface hauls it is known that these species abound in night surface hauls during the cold period. (2) Those species which execute a seasonal vertical migration. These, too are, in general, psychrophilic and found in summer in the deep water layers. In autumn, the population of the upper layers increases. From surface hauls it is known that these species abound in day and night surface samples. (3) Copepods which remained at the surface layer in both seasons of our cruises. These species are thermophilic and are absent from surface hauls during the cold period.     

Diel trophic interactions between vertically-migrating zooplankton and their fish predators in an eelgrass community

Diel changes in the composition of crustacean zooplankton and the diets of fish predators from an intertidal eelgrass flat were monitored concurrently. The zooplankton is characterized by two major components. The obligate zooplankters (holoplanktonic calanoid copepods and meroplanktonic decapod larvae) appear to exhibit vertical migration, being present in higher densities near the surface of the water column at night. The facultative zooplankton (amphipods and ostracods) are benthic during the day, but move up into the water column at night. Planktivorous midwaterdwelling fish consume calanoid copepods and decapod larvae during the day and cease feeding or switch their diet to amphipods at night. Benthic-dwelling fish consume some amphipods during both day and night. The factors important in prey selection by fish and the functional significance of vertical migration in both components of the zooplankton are discussed in the light of the changing patterns of fish predation.This paper is Publication No. 183 in the Ministry for Conservation of Victoria, Environmental Studies Series.     

Vertical distribution and feeding patterns of midwater fish in the central equatorial Atlantic

Fishes and zooplankton were obtained (March–April 1979 and partly in August 1974) from 45 hauls taken during the day and at night in the central equatorial Atlantic between Latitude 3°N and 2°S from the surface to 1250-m depth, using the RMT 1+8, a combined opening-closing plankton and micronekton trawl. The vertical distribution of 30 myctophid species is described. All species migrate in a diel pattern, Ceratoscopelus warmingii and Lampanyctus photonotus down to at least 1250 m. During daytime most species aggregated at 400-to 700-m depth, therefore only partly occupying the depth of the Deep Scattering Layer (400 to 500 m at 15 kHz). The feeding patterns of seven of the most abundant species were compared, with a total of 1 905 stomach contents being analysed. All seven species are regarded as opportunistic predators, which feed predominantly during the night on calanoid copepods. A total of 66 species of calanoid copepods were identified among the prey items, with smaller species definitely being in the minority. Stomachs of C. warmingii (700 to 1 250 m depth) and Lepidophanes guentheri (500 to 900 m depth) from daytime samples contained copepod species restricted to the upper 150 m of the water column, including Undinula vulgaris, Nannocalanus minor, and Euchaeta marina, thereby confirming an extended vertical migration of predators. Differences in diet and preferences between species in their total food spectrum are described.     

Comparison of the food of Triphoturus mexicanus and T. nigrescens,two lanternfishes of the Pacific Ocean

Prey (chiefly euphausiids and copepods) eaten by two myctophids (lanternfishes) are compared from incidence in fish stomachs and from abundance in the environment. One lanternfish species, Triphoturus mexicanus, lives in the California Current, and the other, T. nigrescens, lives in the central Pacific Ocean. Although these two environments are very different physically and biologically, the feeding habits of the two lanternfishes are surprisingly similar. Prey biomass is 94% euphausiids, 3% copepods, and 3% other organisms for T. mexicanus and 88% euphausiids, 4.5% copepods, and 7.5% other organisms for T. nigrescens; the difference between the fish species is not significant when tested statistically. The two fishes resemble one another in frequency distributions of ingested copepod individuals, copepod species, euphausiid individuals, and euphausiid species. During a single diurnal feeding period, both fishes eat a variety of copepod species but tend to eat only a single species of euphausiid. T. mexicanus grows to twice the length of T. nigrescens and eats proportionally larger euphausiids; however, both fishes eat copepods having the same median size. The frequencies of euphausiid species in the diets of both fishes differ from the frequencies in the environment. The chief differences between the feeding habits of the two lanternfishes are that T. nigrescens, in comparison to its congener, eats a greater variety of organisms during one diurnal feeding period and captures smaller euphausiids. The feeding patterns for each lanternfish species are consistent over distances of hundreds of kilometers and over many years of sampling.     

Trophic strategies of euphausiids in a low-latitude ecosystem

Vertical distribution, diet, and morphology of adults were examined in 27 species of euphausiids occurring in the upper 1000 m in the eastern Gulf of Mexico. Vertical distribution patterns were similar to those found in the central ocean gyres and oceanic equatorial waters of the Atlantic, Indian and Pacific Oceans. Most species migrated vertically from their daytime depths of 300 to 600 m to the upper 300 m at night. Exceptions were the non-migrating species of Stylocheiron, which remained in the epipelagic zone day and night, and Nematobrachion boopis, which remained in the mesopelagic zone. Based on gut-contents analysis, the Gulf euphausiids were largely zooplanktivorous, with cyclopoid and calanoid copepods being the most common items in stomachs. ostracods were especially common in the stomachs of Thysanopoda spp. and phytoplankton in the guts of Euphausia spp. Nearly every species' diet contained a considerable amount of olive-colored debris, which may have been marine snow generated in the epipelagic zone. Cluster analysis grouped the euphausiids into nine diet guilds. Euphausiids with a generalized morphology (i.e., spherical eyes, uniform thoracic appendages) tended to group together and demonstrated little variety in stomach contents among species. Euphausiids with a specialized morphology (i.e., bilobed eyes, elongate thoracic appendages) showed considerable variety in stomach contents among species, and several species had diets that were highly specific. Many of the species that had similar gut contents fed on prey of different sizes, as indicated by the width of the calanoid copepod mandibles found in stomachs. Principal-components analysis of seven morphological characters yielded species groups that were similar, but not identical, to those generated by cluster analysis of stomach contents data. We inferred from this that morphological characters partly determine diet, but that behavior is also important. Using the 20 most abundant species and 3 niche parameters, we attempted to identify the degree of separation among euphausiids based on the level of overlap in vertical distribution and diet composition, and on differences in mean prey size. Overlap of <60% in vertical distribution or diet composition was considered to indicate distinction of that parameter. Of 190 total species pairs, only 4 pairs did not demonstrate niche separation in at least one of these categories. We found that differences in these niche parameters were greatest among species with a specialized morphology and least among species that were morphologically generalized.     

Recherches sur la situation trophique d'un groupe d'organismes pélagiques (Euphausiacea). VI. Conclusions sur le rôle des euphausiacés dans les circuits trophiques de l'océan Pacifique intertropical

The role of euphausiids in the food webs of the Intertropical Pacific Ocean is defined through analysis of their nutrition, vertical distributions and migrations, and their utilization by pelagic predators. It is suggested that the abundance of the group, the extensive vertical migrations of many species and the fact that feeding takes place mainly in subsurface layers, result in a leading role of euphausiids in energy transfer between different bathymetric levels. For night-time feeding predators, they represent a noticeable food source only in the 0 to 300 m water layer, as 97% of the euphausiid biomass concentrates in this layer at night. In the daytime, only the smaller specimens (chiefly genus Stylocheiron), accounting for 10 to 15% of the whole biomass of the group, remain available for epipelagic (0 to 400 m) predators, larger individuals dwelling deeper. Euphausiids account for 8 to 10% of the food ingested by micronektonic fishes, but the species are not the same for different categories of fishes. Migrating fishes caught by pelagic trawls, more or less connected with the deep scattering layer, feed on migrating species in subsurface layers at night as well as in deeper layers during the daytime, and on non-migrating species inhabiting shallower and intermediate layers. On the other hand, fishes which comprise the prey of large long-line tunas, which are not caught by trawls because they are fast swimmers, feed almost solely on species which remain above 400 m in the daytime. These results suggest a certain degree of independance between the trophic webs which concern, on the one hand, epipelagic ichthyofauna (including tuna), and, on the other hand, migrating and deep-living faunas. Migrating populations are able to feed at night upon subsurface organisms, a part of this resource being then transmitted during the day to the deep-living fauna; but the epipelagic ichthyofauna, with a feeding activity restricted to light hours, has few possibilities to benefit from the migrating or deepliving biomass. Therefore, energy transfers seem to be intense only from subsurface (0 to 400 m) to deeper layers. From a more general point of view, these investigations suggest that, in the pelagic system, vertical distributions and migrations, and feeding rhythms, are the main factors determining the structure of the food webs.     

Plankton community dynamics of the central Great Barrier Reef Lagoon: Analysis of data from Ikeda et al.

Plankton data collected by Ikeda et al. (1980) from the central region of the Great Barrier Reef, and spanning two years (1976 through 1978) of zooplankton records, have been analyzed extensively for spatial and temporal patterns. Estimates of net zooplankton (including chaetognaths, copepods, and larvaceans) and microzooplankton (juvenile copepods, encompassing nauplii and copepodites, and ciliates) were assessed at three stations across the 60 km lagoon. Temperature, salinity, and chlorophyll a were also measured. A cross-lagoonal gradient was identified in the plankton, concurring with results of related surveys of benthic taxa, such as scleractinian corals, soft corals, macro-algae, fish, sponges, crinoids, etc. Two associations of net zooplankton were identified. The first was associated primarily with the inner lagoon; the second with the outer lagoon. The inshore association was characterized by higher abundances of almost all net zooplankton taxa, particularly chaetognaths, copepods, polychaetes, decapods, and meroplanktonic larvae as well as higher concentrations of chlorophyll a. This inshore association wove back and forth across the lagoon through time, dominating the lagoon entirely during periods of high river discharge, reaching the mid-shelf platform reefs in this region, and sometimes being entirely absent during dry periods. Both seasonal and annual peaks in plankton abundance were generally linked with degree of runoff. Summer/autumn peaks of abundance were evident in chaetognaths, copepods, and larvaceans while annual variation was detected in the former two as well as in chlorophyll a concentrations. Depth stratification was noted in juvenile copepods and chlorophyll a concentrations at the center of the lagoon, with higher abundances recorded in deeper waters. The central Great Barrier Reef lagoon was found to be typical of other tropical coastal waters where plankton community dynamics are controlled primarily by physical factors. We suggest that any substantial changes in river discharge in this area will affect plankton production.A.I.M.S. Contribution No. 242     

Feeding and predation impact of two chaetognath species, Eukrohnia hamata and Sagitta gazellae, in the vicinity of Marion Island (southern ocean)

The predation impact of the two chaetognaths Eukrohnia hamata and Sagitta gazellae on mesozooplankton standing stock were investigated in three depth layers during two 24 h stations occupied in the vicinity of Marion Island in late austral summer (April/May) 1986. The zooplankton community at both stations was dominated by small copepods (Oithona spp., Microcalanus spp.), which accounted for >95% of total zooplankton abundance. Chaetognaths comprised <2% of total zooplankton abundance. E. hamata constituted >95% of the total chaetognath stock. The general trend in both species was decreasing abundance with increasing depth, which appeared to be correlated to the distribution of copepods (r ² = 0.45; P <0.05). Gut-content analysis showed that copepods (mainly Oithona spp., Calanus spp. and Rhincalanus gigas) and ostracods were the main prey of both species, accounting for 87 and 61% of the total number of prey in E. hamata and S.␣gazellae stomachs, respectively. In the guts of S.␣gazellae, pteropods (Limacina spp.) and chaetognaths were also well represented. The mean number of prey items (NPC) for E. hamata ranged from 0.02 to 0.06 prey individual⁻¹ which corresponds to an individual feeding rate (Fr) of between 0.05 and 0.12 prey d⁻¹. For S.␣gazellae, the NPC values were higher, varying between 0.04␣and 0.20 prey individual⁻¹, or between 0.15 and 0.76 prey d⁻¹. The daily predation impact of the two chaetognaths was estimated at between 0.3 and 1.2% of the copepod standing stock or between 7 and 16% of the daily copepod production. Predation by S. gazellae on chaetognaths accounted for up to 1.6% of the chaetognath standing stock per day. Received: 26 November 1996 / Accepted: 31 October 1997     

The neuston of the subtropical and boreal North-eastern Atlantic Ocean. A review

The present paper compiles results of recent studies on distribution, abundance, migratory pattern and feeding of invertebrates and early stages of fish in the uppermost layer of the subtropical and boreal Atlantic Ocean. Three ecological groups are described: euneuston, living permanently at the surface; facultative neuston, invading the uppermost layer mainly during night; pseudoneuston, comprising the uppermost part of populations which are mainly concentrated in deeper layers. For several species of fish, a shift in behaviour was found: eggs and yolk-sac larvae are mainly missing from the uppermost layers, young post-larvae staying permanently close to the surface, and old larvae and juveniles performing diurnal vertical migrations. During daytime, the uppermost layer serves as a refuge for only a small number of welladapted organisms, while at dusk and at night considerable immigration occurs. During daytime, zooneuston is mainly carnivorous or omnivorous. In boreal and turbid waters, the ecological differences between the uppermost and lower layers are reduced. Neuston is an important element of the ecosystem in these areas.     

Comparison of the feeding habits of migratory and non-migratory Stenobrachius leucopsarus (Myctophidae)

Stenobrachius leucopsarus, the most abundant species of myctophid fishes off Oregon, USA, has a bimodal distribution at night, with a peak of abundance in the upper 100 m composed of diel vertical migrants, and another peak at 300 to 500 m composed of fish that did not migrate the night they were caught. We compared the feeding habits of these two groups of fish in an attempt to learn if deep fish migrated to surface waters. Low similarity of diets, differences in the rank order of common prey, and similar states of stomach fullness and digestion of prey suggest that fish captured in deep water at night probably did not feed exclusively in shallow water on previous nights. They probably fed in deep water. The similarity in food habits between deep and shallow fish is most readily explained by daytime feeding by fish in deep water and by broad vertical distributions of prey.     

Vertical distributions of euphausiids and fish in relation to light intensity in the Northeastern Atlantic

The relationship between the vertical distributions of euphausiids and fish and light intensity has been studied directly by using a photometer in conjunction with an acoustically controlled rectangular midwater trawl. Samples were taken at a position centered on 47°N; 17°W on 15 and 16 May 1978. Five species of euphausiid and six species of fish have been analysed, both groups contained migrant and non-migrant species. The population of each of these species occurred throughout a light regime spanning at least three orders of magnitude of intensity; none of them was restricted to, or followed, and isolume. There were no sexual or size differences in the distributions of the euphausiids, but the population of Argyropelecus hemigymnus was probably stratified during the day, with smaller individuals occurring shallower than large ones. The results are discussed in relation to previous observations and to the theories of photic regulation of distributions and migrations.     

Aspects of the community ecology of deep-sea,benthopelagic plankton,with special attention to gymnopleid copepods

Benthopelagic plankton was collected 10 to 100 m above the bottom of the deep sea (1100 to 3200 m) with an opening-closing net. Five samples were taken from the San Diego Trough, 2 from the eastern tropical Pacific and 2 from the northeast Atlantic. Roughly 80% of the individual organisms collected were copepods; the next most abundant groups were usually isopods, ostracods, and chaetognaths. Within the depth range studied, absolute abundances of most animal groups decreased with depth, but relative abundances (%) did not change. Of over 100 species of adult gymnopleid copepods collected, a few species were abundant, but most were rare. Copepod exoskeletons were more abundant than living copepods. For the gymnopleids, similarities among the 3 geographical regions were strong at the family and genus level but weak at the species level. Species that were widely distributed horizontally tended to occupy different positions (in terms of rank order of abundance) in each community. Gut content analysis indicated that the most abundant gymnopleids were generalized particle feeders. However, size-frequency distributions provided indirect evidence for niche separation. The deep-sea benthopelagic plankton was divisible into a truly planktonic component and a bottom-associated component. Abundances of some copepods changed relative to distance off the bottom. Several copepods and other animals, previously described as epibenthic or bottom-associated, were collected more that 10 m above the bottom and thus appeared to move freely between the bottom and lower water column.     

Vertical community structure and ontogenetic distribution of chaetognaths in upper pelagic waters of the Eastern Mediterranean

The vertical distribution of chaetognaths and their developmental stages were investigated in the pelagic waters (0 to 500 m) of the Eastern Mediterranean Sea. Twenty vertical hauls were taken during autumn 1991 in the Ionian, Cretan, Levantine and Rhodes Seas. The chaetognaths Sagitta bipunctata and S. enflata had the same median depth distributions, which differed from those of S. minima, S. serratodentata, S. lyra, Krohnitta subtilis, and S. decipiens, while S. hexaptera occurred only sporadically. S. bipunctata and S. enflata were mainly restricted to <50 m, and while their distributions co-occurred their stage composition differed. The remaining species took up progressively deeper positions in the water column. Ontogenetic vertical distributions, with the older stages occurring at greater depth, were only observed for the mesopelagic species S. lyra and S. decipiens. Diurnal vertical migration was not detected in any of the species nor in their developmental stages.     

Resource-partitioning and predation impact of a low-latitude myctophid community

This study, based on data collected during summer 1985 in the eastern Gulf of Mexico, examined the degree of overlap in two prime niche parameters, space and food, in 17 of the most abundant myctophid species which inhabit the epipelagic zone at night. Cluster-analyses of vertical distribution information and diet characteristics revealed that while large groups of species overlapped (>60%) in either vertical distributions or diet, when both niche parameters were considered together, little interspecific or intraspecific (size class) overlap occurred. Our data suggest that for myctophids trophic competition is reduced through resource-partitioning, although with considerable overlap at niche boundaries. Niche separation presumably is the result of competition during the evolution of the ecosystem and is maintained presently as diffuse competition: the effect on a species of the combined competition from all other species at that trophic level. We suggest that the large degree of niche overlap enables the packing of over 50 myctophid species in the epipelagic zone at night. Our calculations indicate that myctophid predation nightly removes 2% of the zooplankton biomass. Myctophid predation is selective in that greatest pressure is on certain size classes and types of prey (copepods, ostracods and euphausiids). In the case of copepods, impact is greatest on the larger, more mature stages and hence on the breeding population. It is estimated that myctophids account for at least one-third of the daily production of zooplankton removed from the epipelagic zone by micronekton in the eastern Gulf.