Trophic importance of the hyperiid amphipod, Themisto gaudichaudi, in the Prince Edward Archipelago (Southern Ocean) ecosystem

The feeding dynamics and predation impact of the hyperiid amphipod, Themisto gaudichaudi, on the zooplankton community in the waters surrounding the Prince Edward Archipelago were investigated at 30 stations in late austral summer (April/May) 1998. Ingestion rates of T. gaudichaudi were estimated using two approaches, the gut fullness index and in vitro incubations. Throughout the investigation mesozooplankton, comprising copepods, pteropods and chaetognaths, numerically and by biomass dominated Bongo samples. Zooplankton abundances and biomass ranged from 8 to 271 ind. m⁻³ and between 1.01 and 7.47 mg dwt m⁻³, respectively. Densities of T. gaudichaudi during the study were low, never exceeding 0.4 ind. m⁻³. Gut content analysis (n=61) indicates that T. gaudichaudi is a non-selective, opportunistic carnivore generally feeding on the most abundant copepod and chaetognath species. Peaks in feeding activity were recorded at sunrise and sunset, corresponding to their diel vertical migration patterns. Daily rations estimated from in vitro incubations and gut fullness index were equivalent to 1.2–8.7% and between 11.5 and 19.8% of body dry weight, respectively. The predation impact of T. gaudichaudi averaged over the upper 300 m of the water column was low, accounting for <0.4% of the mesozooplankton biomass or <3% of the mesozooplankton secondary production. Indeed, the predation impact is likely to be lower as the contribution of the smaller copepods (e.g. Oithona spp.) to total zooplankton was underestimated due to the sampling gear employed. The low predation impact recorded during this study can be related to low abundances of T. gaudichaudi. It is likely that the importance of T. gaudichaudi as a secondary production consumer in the waters surrounding the Prince Edward Archipelago demonstrates a high degree of spatio-temporal variability. Accepted: 6 December 1999     

Distribution and feeding of chaetognaths in the epipelagic zone of the Lazarev Sea (Antarctica) during austral summer

This study seeks to determine the effects of local hydrography on the distribution, abundance and feeding of chaetognaths in the Lazarev Sea, an area strongly controlled by physical processes which has been held responsible for initiating the Weddell Polynya. Zooplankton samples were taken at 39 stations on four transects located between 6°W and 3°E and from 60°S to 70°S between surface and 350 m. The dominant species, Eukrohnia hamata, accounted for 86.5% of all chaetognaths, followed by Sagitta gazellae (8.1%) and Sagitta marri (5.4%). These three species showed distinct vertical and horizontal distribution patterns. While E. hamata and S. marri had maximum abundances below 250 m depth, S. gazellae showed a narrow distribution band in the upper 150 m depth. The distribution pattern was strongly modified at the Greenwich meridian with an upward transport of a high abundance of deep dwelling organisms (S. marri and E. hamata) and a displacement of S. gazellae to the surface, likely coupled with the rise of the warm, saline halo around the Maud Rise. Small copepods were the main prey of all three chaetognath species. Feeding rates (FR) varied among species and depth. Sagitta marri showed the highest FR with 0.38 prey d⁻¹, followed by S. gazellae and E. hamata (0.22 and 0.07 prey d⁻¹). Feeding rates were usually highest in the 25–80-m stratum. Size distribution and maturity of E. hamata revealed a dominance of small and immature organisms along all depths and stations, suggesting that this area might be acting as an important source of recently spawned organisms to the surface.     

Vertical distribution and ecological significance of chaetognaths in the Arctic environment of Baffin Bay

Summary The vertical distributions of species of chaetognaths are described to a depth of 1043 m in Baffin Bay during the summer. The dominant species in the southern region of the Bay were Eukrohnia hamata and Sagitta maxima and, in the northern region, E. hamata and S. elegans. The younger stages of all chaetognath species were found at depths above the older more mature stages. There was no evidence of any diurnal vertical migration by any species. The population structure of E. hamata was bimodal suggesting two sub-populations and a two year life cycle. Sagitta maxima showed a multimodal population suggesting a life cycle of four or more years. The population structure of E. elegans was unimodal. Gut contents showed that E. hamata fed at a higher rate than did S. maxima. E. hamata fed at a higher rate during the night but there was no significant difference between day/night feeding rates for S. maxima. It was estimated that E. hamata consumed up to 1.8 prey items day^-1 and S. maxima 0.5 prey items day^-1. The daily ration was influenced by the water temperature. Calculations showed chaetognaths are major predators on copepods consuming between 1.2 and 1.3% of the standing crop of copepods per day. Generally the maximum concentration of chaetognaths was found at a depth below the maximum concentration of copepods.     

Feeding dynamics and respiration of the bottom-dwelling caridean shrimp Nauticaris marionis Bate, 1888 (Crustacea: Decapoda) in the vicinity of Marion Island (Southern Ocean)

The feeding dynamics and oxygen uptake of the bottom-dwelling caridean shrimp Nauticaris marionis were studied during the April/May 1984, 1996 and 1997 cruises to Marion Island (Prince Edward Islands, Southern Ocean). N. marionis is thought to have an opportunistic feeding mode. Prey composition varied considerably between the years and sites investigated. Overall, benthic (mainly hydrozoans and bottom-dwelling polychaetes) and, at times, pelagic (largely euphausiids and copepods) prey items dominated in the stomachs of N. marionis both by occurrence and by volume. Generally, pelagic prey contributed more to the diets of smaller shrimps, while benthic prey was a more important component in the guts of larger specimens. Wet, dry and ash-free dry weight were determined for specimens used in respiration experiments. The respiration rates of N. marionis females with carapace length 6.6–11.1 mm ranged from 80 to 250 μl O₂ individual⁻¹ · h⁻¹, or from 0.588 to 2.756 μl O₂ · mg⁻¹ dry weight h⁻¹. Regression analyses showed highly significant correlations between oxygen consumption and carapace length for N. marionis. Daily ingestion rates estimated using an in situ gut content analysis technique (4.4% of body dry weight) and an energy budget approach (average 4.7% of body dry weight, range 2.0–7.5%) showed good agreement with each other. Accepted: 29 July 1998     

Composition and spatial variability of macroplankton and micronekton within the Antarctic Polar Frontal Zone of the Indian Ocean during austral autumn 1997

Net sampling and continuous acoustic measurements within the Antarctic Polar Frontal Zone (APFZ) and in the vicinity of the Prince Edward Islands were conducted during austral autumn (April/May) 1997 to describe the composition and distribution of macrozooplankton and micronekton, and to investigate their relations to the prevailing oceanographic regime in the area. Two major circulation patterns associated with the Subantarctic (SAF) and Antarctic Polar (APF) Fronts existed in the oceanic environment surrounding the Prince Edward Islands, promoting high cross-frontal mixing both upstream and downstream of the islands. Average abundance and biomass of macroplankton/micronekton in the top 300-m layer were 21 ind. 1000 m⁻³ and 467 mg DW 1000 m⁻³, respectively. Pelagic crustaceans (euphausiids and amphipods), fish, chaetognaths and gelatinous zooplankton dominated numerically and by biomass. Continuous acoustic measurements displayed elevated pelagic biomass at the SAF and APF. Although four groupings of stations were identified using cluster analysis, a single macroplankton/micronekton community was recognized in the top 300-m layer throughout the offshore area of the APFZ. A modification of the APFZ community was observed within the inter-island region. Subantarctic species dominated zooplankton samples throughout the APFZ, although subtropical species were also well represented at stations occupied in the northern region of the APFZ. A biological response reflected in macroplankton community composition, resulting from an extensive cross-frontal mixing, was observed within the APFZ around the Prince Edward Islands. Accepted: 27 November 1999     

Mesozooplankton assemblages in the shallow Arctic Laptev Sea in summer 1993 and autumn 1995

Mesozooplankton distribution and composition in the very shallow part of the Siberian Laptev Sea shelf were studied during the German-Russian expeditions “Transdrift I” (August/September 1993) and “Transdrift III” (October 1995). Maximum abundances were found close to the outflow of the Lena River (7,965 ind. m⁻³) and in the Yana river mouth (38,163 ind. m⁻³). Lowest abundances occurred in the northeast and west of the Laptev Sea (64–95 ind. m⁻³). Highest biomass values (104–146 mg DM m⁻³) were determined in the northern and northeastern part of the shallow Laptev Sea, as well as close to the river outflows, with a record biomass maximum in the Yana river mouth (270 mg DM m⁻³). Biomass minima were situated north of the Lena Delta and in the western part of the shallow Laptev Sea (0.3–1.0 mg DM m⁻³). Copepods dominated in terms of abundance and biomass. Cluster analyses separated four mesozooplankton assemblages: the assemblage “Lena/Yana” in the southern part, “Eastern-central” in the centre, “Kotelnyy” in the eastern part and “Taimyr” in the western part of the shallow Laptev Sea. The small-sized neritic and brackish-water copepods Drepanopus bungei, Limnocalanus grimaldii and Pseudocalanus major occurred in enormous numbers and made up the bulk of zooplankton abundance and biomass in the very shallow part of the Laptev Sea close to the rivers Lena and Yana. In the more northern and northeastern areas, Calanus glacialis, P. minutus and P. major were dominant copepod species, whereas Oithona similis and Acartia sp. became important in the western Laptev Sea. Appendicularians, as well as hydromedusae and the chaetognath Sagitta sp., contributed significantly to abundance and biomass, respectively, but not over the entire area studied. One can identify taxon-specific distribution patterns (e.g. Sagitta predominated the biomass in a zone between the area heavily influenced by Lena/Yana and the offshore area to the north), which differ from the patterns revealed by cluster analysis. Hydrographic features, especially the enormous freshwater inflow, apparently determine the occurrence and formation of zooplankton aggregations. Extremely high numbers of small-sized neritic and brackish-water copepods occurred locally, which were probably also supported by excellent feeding conditions.     

Vertical distribution and abundance of juvenile chaetognaths in the Weddell Sea (Antarctica)

The composition, abundance and vertical distribution of chaetognaths were analysed along a transect in the Weddell Sea during late spring. Three species were identified: Eukrohnia hamata (90.8%), Sagitta marri (6.4%) and S. gazellae (2.8%). Only juvenile stages were collected in the samples, a result related both to the type of sampling gear employed (mesh size: 100 μm) and the species' life-cycles. The vertical distributions showed that the juvenile stages of these species tended to aggregate at considerable depth (1000–500 m). It is postulated that this pattern may be related to the life-cycles of these species in association with seasonal Antarctic conditions, similar to the pattern postulated for krill and other polar crustaceans. Accepted: 10 July 2000     

Meso- and bathypelagic distribution and abundance of chaetognaths in the Atlantic sector of the Southern Ocean

We conducted multinet sampling during winter and summer in the Southern Ocean (Atlantic sector) to investigate the effect of water mass, season and water depth on abundance and species composition of meso- and bathypelagic chaetognaths. Eukrohnia hamata (mean 115 ind. 1,000 m⁻³) and Sagitta marri (mean 51 ind. 1,000 m⁻³) were dominant, complemented by E. bathypelagica (mean 19 ind. 1,000 m⁻³) and E. bathyantarctica (mean 19 ind. 1,000 m⁻³) below 1,000 m. A further six species were identified, among them the rare bathypelagic species Heterokrohnia fragilis and the subtropical Eukrohnia macroneura that is new to the Antarctic. Water depth and season were the principal determinants of abundance and species composition patterns, indicating vertical seasonal migration and vertical segregation of species. The life cycles of E. hamata and S. marri were studied additionally. Their maturity stages were vertically segregated and prolonged reproductive periods are suggested for both species.     

Interannual variability in a predator-prey interaction: climate, chaetognaths and copepods in the southeastern Bering Sea

The interaction of the chaetognath Sagitta elegans with thecopepod community of the southeast Bering Sea middle shelf wasexamined in relation to environmental conditions during 1995–1999.Predation impact was estimated for 2 years, 1995 and 1997, usinggut content analysis, experimentally derived digestion time(DT) and abundances of chaetognaths and prey. Pseudocalanusconcentrations correlated with water temperature and Calanusmarshallae with sea ice extent. Sagitta elegans were less abundantbut individuals were larger in 1995, when C. marshallae predominated,compared to 1997, when Pseudocalanus and Acartia were the primaryprey. Predation by S. elegans removed <1% standing stockday^–1 of Pseudocalanus or C. marshallae in 1995 and 1.7to 2.3% of Pseudocalanus in 1997. The percent of the copepodcommunity biomass required by chaetognaths was estimated tobe <1% in 1995 compared with 8–12% in 1997. Calanusmarshallae may be more vulnerable than Pseudocalanus to cumulativepredation effects because of its reproductive strategy. Theeffect of chaetognath predation on the copepod community dependson which copepod species is predominant and its susceptibilityto cumulative predation effects, as well as on daily predationimpact, both of which varied between years with different climaticconditions.     

Distribution and trophic ecology of chaetognaths in the western Mediterranean in relation to an inshore-offshore gradient

This study examines the distribution patterns and feeding ecologyof chaetognaths in the Catalan Sea in relation to mesoscalefeatures along an inshore–offshore gradient. The studywas conducted during two different periods of the year: latespring of 1995 and late summer of 1996. The two periods differedin hydrographic conditions and mesoscale processes, which affectedthe distribution patterns of the different species of chaetognathsfound. The diet of the chaetognaths was mainly composed of copepodsand differed between species. Prey size was not always stronglyrelated to chaetognath size and for certain species, there wasan overlap in prey size spectrum. Trophic niche breadth (ona ratio scale) appeared to be constant with growth. Ingestionrates and predation pressure by chaetognaths did not followa clear trend related to the mesoscale features in the area,such as the presence of a density front. The impact of chaetognathson copepod standing stock appeared to be extremely low (<1%),but it became more relevant when the species and prey size specificityof the chaetognaths was taken into account.     

Effects of chaetognath predation and nutrient enrichment on enclosed estuarine copepod communities

Summary The effects of predation by the chaetognath Sagitta hispida and nutrient enrichment on estuarine copepod community structure were studied by experimental manipulations in large enclosures. Chaetognath abundance and nutrient addition rates were manipulated in a factorial design. Predation by chaetognaths resulted in a significant decline in the relative abundance of Acartia tonsa and an increase in the relative abundance of Oithona colcarva. However, these effects were evident only at chaetognath densities far higher than observed in natural populations. Nutrient enrichment resulted in a decline in relative abundance of Paracalanus crassirostris, and, in the absence of chaetognath predation, an apparent increase in the relative abundance of A. tonsa. The effects of chaetognath predation were independent of enrichment level, apparently because of the absence of effects of enrichment on total copepod and chaetognath densities.     

Chaetognaths of the United States South Atlantic Bight: Distribution, abundance and potential interactions with newly spawned larval fish

Chaetognaths are one of the most numerous organisms in the zooplankton community off the coast of North Carolina. During two years of offshore sampling in the late winter to early spring, sixteen chaetognath species were identified, four of which had not previously been reported in the waters of the United States South Atlantic Bight. Offshore samples were dominated by Sagitta enflata Grassi, 1881, one of the larger species, which contributed > 61% of total chaetognath abundance while dominant coastal species were S. tenuis Conant, 1896 and S. hispida Conant, 1895. Abundances, body sizes and spatial distributions were determined for the most abundant chaetognath species along with the overall abundance of three common co-occurring larval fish species (spot, Atlantic croaker and Atlantic menhaden). In addition, laboratory feeding experiments were conducted using S. tenuis and S. hispida to estimate the potential impact of chaetognath predation on representative North Carolina larval fish which spawn offshore and subsequently migrate into local estuaries. Feeding rates (no. prey items day^− 1) varied with prey type and the condition (starved/fed) of the chaetognath. Weight specific daily rations (SDR) were found to vary inversely with chaetognath size, decreasing exponentially with an increase in chaetognath length. The observed abundance and distribution data indicate that wintertime chaetognath populations in offshore waters of Onslow Bay, North Carolina have the potential to interact with recently spawned larval fish and may simultaneously act as competitors, predators, or prey. Furthermore, calculations using published values of chaetognath abundance, zooplankton standing crops, and our SDR estimates indicate that chaetognaths in a representative North Carolina estuary would require a minimum of 5.96 cal m^− 3 day^− 1 to sustain their biomass. Allowing for the reported spatial and temporal variability in zooplankton abundance in these systems, chaetognaths should consume 4.4%-20.9% of the estimated total zooplankton production day^− 1. This further emphasizes the role of chaetognaths not only as predators, but also as competitors with larval fish for zooplankton food stocks in southeastern United Sates estuaries.     

The importance of protozooplankton as prey for copepods in the coastal areas of the central Irish Sea

This study evaluates food supply for copepods, highlighting the trophic relationship between copepods and protozooplankton. To test the hypotheses that protozooplankton prey are capable of sustaining the copepod standing stock in the western Irish Sea, the taxonomic and size composition of these two groups and the size-specific predation of copepods on protozooplankton were investigated. Protozooplankton and copepod samples were collected off the southwest coast of the Isle of Man using 1.7 l Niskin water bottles and two nets (64 and 280 μm meshes), respectively. Copepod predation on protozooplankton was calculated using weight-specific clearance rates from the literature, considering the availability of prey that was accessible to a given size of copepod. Low protozooplankton biomass was dominated by small cells (<60 μm), and high copepod biomass was dominated by small species, which were more efficiently collected by a 64-μm mesh net. However, large copepods were only collected by a 280-μm mesh net, suggesting that the combination of the two nets provided a better estimate of copepod biomass. Predation by the copepod assemblage in the Irish Sea removed 1–47% and 0.5–22% of ciliates and dinoflagellates standing stock, respectively, resulting in 1–40% of the copepod feeding requirement per day. Contrary to our hypothesis, copepods could not meet their feeding requirements by grazing only on the microzooplankton prey (15–200 μm), and other food sources (i.e. nanoplankton) must be important additional dietary components to copepods in the Irish Sea. Handling editor: S. M. Thomaz     

The role of the chaetognath Sagitta gazellae in the vertical carbon flux of the Southern Ocean

Chaetognaths are among the most abundant predators in the Southern Ocean and are potentially important components in the biological carbon pump due to the production of large, fast-sinking fecal pellets. In situ S. gazellae abundance, fecal pellet production, sinking rates, carbon content, and vertical carbon fluxes were measured at the Lazarev Sea between December 2005 and January 2006. Sagitta gazellae produce fecal pellets that sink at speeds of 33–600 m day⁻¹ and have carbon contents of 0.01–0.8 mg C pellet⁻¹. Vertical carbon flux was later compared with the total carbon flux measured at 360 m depth at the study area. Rough estimates using published seasonal abundance of S. gazellae indicate that, at 360 m depth in the Lazarev Sea, this specie may contribute 12 and 5% of the total vertical carbon flux in winter (ice-covered) and summer (ice-free), respectively. Thus, the role of chaetognaths in the downward transport of organic matter may be far more important than previously thought.     

Omnivory and predation impact of the calanoid copepod Boeckella poppei in a maritime Antarctic lake

The copepod Boeckella poppei is a major species in high latitude lakes of the Southern Hemisphere. In such lakes the reduced diversity of metazoans contrasts with a rich microbial assemblage, making these systems amenable to the study of predation controls on the microbial food web. However, the diet of B. poppei is subject to conflicting reports, with little information on feeding rates. We incubated this species in water from Sombre Lake, a much-studied maritime Antarctic Lake on the South Orkney Islands, in order to quantify its feeding rates and potential impact on the microbial assemblage. Overall, clearance rates were similar across 4 experiments spanning November 1999–March 2000, but increased with prey size over the range of 2.7–18 μm equivalent spherical diameter (esd). B. poppei fed omnivorously, although small phototrophic flagellates comprised the bulk of the diet because of their overwhelming dominance in the incubation water. Larger motile preys—heterotrophic ciliates of ~18 μm esd—were cleared fastest (mean 555 ml mg⁻¹ dry mass day⁻¹) and at equivalent rates to those found for freshwater and marine copepods of similar size and at similar temperatures. Estimated predation impact on the microbial food web varied with the abundance of copepods; these were ~30-fold greater in March than in December. In March even the relatively abundant B. poppei (1.7 adults l⁻¹) had a negligible impact on nanoflagellates, due to the low clearance rate on these small cells. However, in March, B. poppei adults were estimated to clear 24% of the lake water of ciliates daily. Given the generation time of ciliates (1.6 days measured in a previous summer study), and the fact that other larval stages of B. poppei were not assessed, this species has the potential to control this part of the microbial assemblage in Sombre Lake.     

Ontogenetic diet shift, feeding rhythm, and daily ration of juvenile yellowfin goby Acanthogobius flavimanus on a tidal mudflat in the Tama River estuary, central Japan

To ascertain the feeding habits of benthic juvenile yellowfin goby Acanthogobius flavimanus, the gut contents of 599 specimens (15–41 mm in standard length, SL), collected on a tidal mudflat in the Tama River estuary throughout the diel cycle, were examined. The major prey items changed from harpacticoid copepods to errant and sedentary polychaetes at ca. 20 mm SL. Prey width increased with fish size. Fish of 26–28 mm SL fed mainly from sunset to morning, with highest feeding intensity during twilight hours and/or high tide. Based on the gut evacuation rate estimated from a forced feeding experiment in the laboratory and data for the diel change of mean gut-content volume in the field, the daily ration of juvenile yellowfin goby (26–28 mm SL) was calculated to be 13.8 mm³ fish⁻¹ day⁻¹. This volume is approximately equivalent to 3.9 individuals of the errant polychaete Ceratonereis erythraeensis (9.7 mm in body length, BL) or 8.1 individuals of the sedentary polychaete Prionospio japonica (14.8 mm BL), both species occurring abundantly on the mudflat during the study.     

Diel vertical migration and feeding of chaetognaths in coastal waters of the eastern Mediterranean

This study investigates the diel vertical distribution and the diet of the most important chaetognath species found in the 0–50 m surface layer of a coastal area in the eastern Mediterranean during a 24-hour period in July 2004. Among the recorded chaetognaths, Sagitta enflata was the most abundant species (41.6%), followed by S. minima (32.5%) and S. serratodentata (20.8%). Those three species exhibited a “twilight migration” pattern, with only small differences among them. Vertical separation was found between S. enflata and S. minima. Both species preyed mainly on cladocerans, although copepods were the most abundant group in the zooplankton assemblage. The chaetognath species followed partially the diel vertical migration of their prey. S. enflata showed high feeding intensity at different times in both day and night, while S. minima fed more intensively at midday (12:00) and at dusk (20:00), and S. serratodentata in the morning (08:00). It seems that in order to coexist in an area of low productivity the chaetognath species follow the basic ecological rules of space, time and food-type separation, in order to reduce the inter- and intra-specific competition. The high preference of S. minima and especially of S. enflata for the cladocerans made them probably the most important predators of cladocerans during summer.     

Structure and grazing impact of the mesozooplankton community during late summer 1994 near South Georgia, Antarctica

Mesozooplankton abundance, community structure and grazing impact were determined during late austral summer (February/March) 1994 at eight oceanic stations near South Georgia using samples collected with a Bongo and WP-2 nets in the upper 200-m and 100-m layer, respectively. The zooplankton abundance was generally dominated by copepodite stages C3–C5 of six copepod species: Rhincalanus gigas, Calanus simillimus, Calanoides acutus, Metridia spp., Clausocalanus laticeps and Ctenocalanus vanus. Most copepods had large lipid sacs. All copepods accounted for 41–98% of total zooplankton abundance. Juvenile euphausiids were the second most important component contributing between 1 and 20% of total abundance. Pteropods, mainly Limacina inflata, were important members of the pelagic community at two sites, accounting for 44 and 53% of total abundance. Average mesozooplankton biomass in the upper 200 m was 8.0 g dry weight m⁻², ranging from 4.3 to 11.5 g dry weight m⁻². With the exception of Calanussimillimus, gut pigment contents and feeding activity of copepod species were low, suggesting that some species, after having stored large lipid reserves, had probably started undergoing developmental arrest. Daily mesozooplankton grazing impact, measured using in situ gut fluorescence techniques and in vitro incubations, varied widely from <1 to 8% (mean 3.5%) of phytoplankton standing stock, and from 5 to 102% (mean 36%) of primary production. The highest grazing impact was found northeast of the island co-incident with the lowest phytoplankton biomass and primary production levels. Received: 30 October 1996 / Accepted: 23 February 1997     

Prey selection by larvae of Prochilodus lineatus (Pisces: Curimatidae): indigenous zooplankton versus veligers of the introduced bivalve Limnoperna fortunei (Bivalvia: Mitilidae)

We studied experimentally the feeding selectivity of larvae of Prochilodus lineatus (Pisces), with particular emphasis on the role of veligers of the exotic bivalve Limnoperna fortunei. Three concentrations of veligers were offered to three developmental stages of P. lineatus. Veliger concentrations were: (1) higher than in the field (“enriched”, 0.09 ind. ml⁻¹), (2) unmodified from field conditions (“normal”, 0.06 ind. ml⁻¹), and (3) lower than in the field (“low”, 0.02 ind. ml⁻¹). Fish developmental stages were protolarvae (approx. 10 days old), mesolarvae (17 days), and metalarvae (25 days). Proportions (in terms of numbers and biomass) and selectivity values were calculated for each prey item evaluated: veligers, small cladocerans + nauplii, medium-sized cladocerans, copepodits, and large cladocerans + copepods. Protolarvae and mesolarvae consumed veligers almost exclusively (88–90%, both in numbers and in biomass) when offered prey enriched in veligers, whereas for metalarvae veligers represented only 16.0% of the food consumed. At lower veliger concentrations, only protolarvae preferred Limnoperna veligers, whereas older fishes switched gradually to crustacean plankton. We conclude that veligers are preferred by the early fish developmental stages, and we speculate that this may be because their slower swimming makes them easier to capture than planktonic crustaceans. However, as fish larvae grow larger, veligers become too small a prey for their energetic needs, and they switch to larger items like cladocerans and copepods. We anticipate that this new and abundant food resource has an important impact on the survival and growth of P. lineatus.     

Feeding of <Emphasis Type="Italic">Themisto libellula</Emphasis> (Amphipoda Crustacea) on natural copepods assemblages in an Arctic fjord (Kongsfjorden,Svalbard)

Ingestion rates and selectivity of the Arctic pelagic amphipod Themisto libellula were studied experimentally in Kongsfjorden (Svalbard, 78°N) during the summer period. Feeding incubations were conducted on naturally occurring copepod communities at different concentrations ranging from 25 to 250 preys L⁻¹. The ingestion rates increased with food availability from 1.3 to 17.7 preys ind⁻¹ day⁻¹, which corresponded to 0.3–11% of body carbon day⁻¹. Despite the high prey concentration used in the experiments the satiation level was not reached. We suggested that T. libellula is able to take the maximum benefit from dense patches of preys, which represent a good adaptation to the high variability in food supply characteristic of polar environment. Copepodids stage III of Calanus spp. appeared to be the preferred preys of T. libellula. Smaller copepods such as Oithona similis and Pseudocalanus spp., were also selected but only when their relative abundance exceeded 25% of the total prey available. The potential predation impact of T. libellula is discussed in relation to the mesozooplankton small-scale patchiness and predator abundance.