Feeding in Arctic darkness: mid-winter diet of the pelagic amphipods Themisto abyssorum and T. libellula

The pelagic amphipods Themisto abyssorum and Themisto libellula represent important links between the herbivore zooplankton community and higher trophic levels of the Arctic marine food webs. Large double structured eyes of both of these hyperiid species are assumed to be used for visual prey detection. However, no information is available on the feeding strategies of these visually searching predators for the period of the polar night, a time of year with no or very low levels of daylight. Here, we report on the stomach and gut content of both Themisto species collected during a January expedition around Svalbard (78° to 81°N). Results indicate that T. abyssorum and T. libellula feed actively during the Arctic winter. The major food source of both amphipods consisted of calanoid copepods, most frequently Calanus finmarchicus.     

Ontogenic variations in fatty acid and alcohol composition of the pelagic amphipod <Emphasis Type="Italic">Themisto libellula</Emphasis> in Kongsfjorden (Svalbard)

The fatty acid and alcohol composition of the pelagic amphipod, Themisto libellula, was monitored during the 5 first months of its life cycle (4–20 mm length) in an Arctic fjord, Kongsfjorden, Svalbard. Fatty acids of the three major lipid classes, polar lipids (PL), triacylglycerol (TAG), and wax esters (WE), were analyzed to highlight ontogenic changes in their diet and metabolism. The PL composition of T. libellula did not show any strong variations along their growth except during the first month where an important increase of 20:5(n-3) (EPA) and 22:6(n-3) (DHA) was observed. The TAG composition revealed a clear gradient corresponding to a diet shift from omnivorous juveniles toward carnivorous sub-adults and adults. Indeed, fatty acid trophic markers of diatoms were dominant in the juveniles, whereas 20:1(n-9) and 22:1(n-11), the Calanus sp. trophic markers, overwhelmed in the older stages. The WE composition highlighted the same general trend, however, differences were found with the TAG and are discussed as a result of differences in turnover rates and assimilation pathways between the two lipid classes.     

Feeding four species of pelagic copepods under experimental conditions

Different qualitative and quantitative aspects of feeding with varied compositions of diets have been studied in 4 species of pelagic copepods: Calanus helgolandicus, Centropages typicus, Temora stylifera and Acartia clausi. By feeding copepods different algal concentrations, it was shown that when food concentration increases grazing rate decreases; the ingestion rate remains fairly constant in the lower range of concentrations, but then increases, reaching a plateau at higher algal concentrations. There is a significant correlation between daily food intake and fecal pellet production. On a pluri-algal diet, selective grazing is observed: larger phytoplankton cells are more efficiently removed than smaller ones. Using Artemia nauplii, it is shown that the copepods studied are also able to eatch and ingest animal prey. Increased daily food intake affects respiration and oviposition. Metabolic requirements, gross growth-efficiency, and food assimilation have been ealculated in Calanus helgolandicus, Centropages typicus and T. stylifera for a large range of algal concentrations.     

Feeding habits of the pebble crab Eriphia smithi (Crustacea,Brachyura, Menippidae)

Eriphia smithi is a very common East African intertidal xanthoid crab. Its dietary input is based mainly in algal grazing (during the day) and predation (particularly at night). Predation appears quite opportunistic and mostly directed at crustaceans and molluscs, among which bivales largely predominate. Echinoderms are actively avoided and, contrary to common belief, predation on Prosobranchia is only accessory.     

Feeding habits of marine cladocerans in the Inland Sea of Japan

Natural food items of five species of marine cladocerans, Evadne nordmanni, E. tergestina, Penilia avirostris, Podon leuckarti and P. polyphemoides, were investigated in the Inland Sea of Japan between April 1986 and May 1987. Gut content examination with SEM (scanning electron microscopy) revealed that feeding was largely limited to centric diatoms and a few exceptions of pennate diatoms and dinoflagellates. No animal remains were detected, and some unidentified materials were also found. Phytoplankton smaller than 35 m in size (cell diameter in centric diatoms and longest dimension in others) was found most frequently in the gut of cladocerans. The role of grazing of marine cladocerans in trophodynamic pathways of the pelagic realm is discussed.     

Feeding by larvae of intertidal invertebrates: assessing their position in pelagic food webs

One of the leading determinants of the structure and dynamics of marine populations is the rate of arrival of new individuals to local sites. While physical transport processes play major roles in delivering larvae to the shore, these processes become most important after larvae have survived the perils of life in the plankton, where they usually suffer great mortality. The lack of information regarding larval feeding makes it difficult to assess the effects of food supply on larval survival, or the role larvae may play in nearshore food webs. Here, we examine the spectrum of food sizes and food types consumed by the larvae of two intertidal barnacle species and of the predatory gastropod Concholepas concholepas. We conducted replicated experiments in which larvae were exposed to the food size spectrum (phytoplankton, microprotozoan and autotrophic picoplankton) found in nearshore waters in central Chile. Results show that barnacle nauplii and gastropod veligers are omnivorous grazers, incorporating significant fractions of heterotrophs in their diets. In accordance with their feeding mechanisms and body size, barnacle nauplii were able to feed on autotrophic picoplankton (<5 microm) and did not consume the largest phytoplankton cells, which made the bulk of phytoplankton biomass in spring-summer blooms. Balanoid nauplii exhibited higher ingestion rates than the smaller-bodied chthamaloid larvae. Newly hatched C. concholepas larvae also consumed picoplankton cells, while competent larvae of this species ingested mostly the largest phytoplankton cells and heterotrophic protozoans. Results suggest that persistent changes in the structure of pelagic food webs can have important effects on the species-specific food availability for invertebrate larvae, which can result in large-scale differences in recruitment rates of a given species, and in the relative recruitment success of the different species that make up benthic communities.     

Feeding habits and food requirements of some amphipods in the Black Sea

Food composition and food requirements of four amphipod species of the Black Sea — Dexamine spinosa (Mont.), Amphithoe vaillanti Lucas, Gammarellus carinatus (Rathke), Gammarus locusta L. — were studied and an attempt made to assess the quantitative composition of the food, daily rhythms of feeding, and daily food rations. The composition of the food of the above-named amphipods is similar and consists primarily of seaweeds. Daily feeding intensity is characterized by a distinct increase during the dark period (D. spinosa, A. vaillanti); however, in G. locusta feeding intensity increases in the morning. The daily food rations of amphipods vary greatly (from 1.7 to 360% of the specimens' body weight), depending upon factors such as the kind of food, age, sex, the physiology of specimens, and the water temperature. The total quantity of food eaten by the populations of these amphipods in the coastal zone of the Black Sea amounts to 4000 g organic matter per square metre within 1 year.     

Feeding habits of the spotted flounder Citharus linguatula off the eastern coast of Spain

The composition of the diet of Citharus linguatula (L.) off the coast of the Gulf of Valencia, Spain, was determined between October 1989 and October 1990. The percentage of empty stomachs remained constant throughout the year, except for the period August to September, when a maximum was recorded coinciding with the reproductive period. Crustaceans (Mysidacea and Decapoda) and teleosts constituted the main prey. The composition of the prey ingested varied with predator size; small specimens contained a greater number of mysids in their stomachs, while decapods and fishes were more abundant in the stomachs of larger specimens. Diets varied seasonally: mysids were more important during autumn, whereas decapods were more important during winter and spring. Fishes were numerically most important in the diet in summer, but the frequency of occurrence was constant throughout the year.     

Reproductive biology of a deposit-feeding amphipod, Casco bigelowi, with extended parental care

The reproductive traits of a deposit-feeding amphipod that engages in extended parental care were examined. At the study site in Lowes Cove, Maine, USA, Casco bigelowi (Blake, 1929) occurred in highest densities in soft sediments just below mean low water (MLW). During most months, the sex ratio was ≃1. Many females hosted males in their burrows throughout the summer, but after fertilization of females in September, all adult males disappeared from the study area. In October almost 80% of the females were ovigerous, and in November about the same percentage was parental, i.e. caring for juveniles in their burrows. The females produced only one brood each in late fall which they accommodated in their burrows for 2 mo or longer. The average number of juveniles per female was ∼20 in November, and continuously decreased until January. Juveniles reached sizes >10 mm length in the maternal burrows. In early December the first juveniles were found in their own burrows, but major recruitment took place in late December and January. It is concluded that for C. bigelowi, the delay of recruitment into the winter months with low predation pressure and the large offspring size at this time are major advantages gained by extended parental care. C. bigelowi is host to the peritrich ciliate Cothurnia sp. on its gills, and during the summer months >70% of all amphipods had ciliates on their gills. Juveniles still living in their mother's burrows showed infestation rates similar to that of the parent; those of highly infested mothers were more heavily infested than those of “clean” mothers. Facilitated epibiont transmission during intimate and long-lasting (2␣mo) parent–offspring associations may be a consequence of extended parental care. Received: 25 November 1997 / Accepted: 14 April 1998     

Feeding habits of pasiphaeid shrimps close to the bottom on the Western Mediterranean slope

The composition of the diet and daily cicle of predatory activity of pasiphaeid shrimps in the Northwestern Mediterranean were established; special attention was focussed on nocturnal feeding habits close to the bottom. Daily activity in both species was studied in two continuous 24-h sampling periods. Samples were obtained using bottom trawls between 1988 and 1990. Both species fed on benthic prey items at night. The nocturnal diet of Pasiphaea multidentata consisted of gammarid amphipods, isopods (Cirolana borealis) and macruran decapods (Calocaris macandreae). Nocturnal feeding activity was carried out only by large specimens (cephalothorax length >28 mm), which stayed close to the bottom during the nighttime. In contrast, the specimens collected during the daytime exhibited highly digested remains of pelagic prey (hyperiids, fishes, euphausiids, chaetognaths) ingested as a result of predatory higher activity in the water column the night before. The feeding strategy of P. sivado was parallel to that of P. multidentata. At night large specimens were located near the bottom and fed on suprabenthic gammarid amphipods. This nocturnal feeding activity by both these mesopelagic pasiphaeid species furnishes evidence of energy transfer from the benthos to the planktonic system in bathyal communities. Changes were observed in the diet of P. multidentata with depth. Crustaceans made up a larger share of the diet of P. multidentata on the lower slope than on the upper middle slope, probably because of changes undergone by bathyal communities with increasing depth. The feeding rate was higher in the submarine canyons, where the diet is also more specialised. Dietary overlap between the two pasiphaeids was very low, due to the different size range of prey exploited.     

Feeding habits of the dungeness crab Cancer magister as determined by the index of relative importance

Specimens of the Dungeness crab Cancer magister, were collected subtidally and intertidally from an estuary in Washington State, USA in June and September 1980, and January, April, May and July, 1981. Gut contens of freshly collected crabs were analyzed by the Index of Relative Importance; for each prey taxon, this method measured frequency of occurrence, percentage of total biomass, and percentage of total numbers consumed. The most important higher taxon eaten was fish; however, the most important prey genus was the shrimp Crangon spp. There was greater predation on Crangon spp. at night at the intertidal site, and during winter and spring when the shrimp were most abundant there. Feeding activity, as indicated by a weight-specific gut-fullness index, showed no consistent diel pattern. There were significant ontogenetic changes in feeding patterns: first-year crabs preyed primarily on very small bivalves or small crustaceans including their conspecifics; second-year crabs preferred Crangon spp. and fish, and third-year crabs preyed less on Crangon spp. and more on fish. Such changes in feeding habits with ege could be purely due to mechanics of food handling, but might also reduce competition among age groups of crabs, possibly partitioning resources within the estuary. Findings are discussed in terms of optimal foraging and compared to other similar studies.Contribution No. 599, School of Fisheries, University of Washington, Seattle, Washington 98195, USA     

Reproductive biology of a filter-feeding amphipod, Leptocheirus pinguis , with extended parental care

Leptocheirus pinguis (Stimpson, 1853) is a widely distributed, abundant, endobenthic amphipod that engages in extended parental care, i.e. females host their juveniles in their burrows for extended time periods. I examined reproduction and population biology of L. pinguis at mean low water (MLW) in muddy sediment in Lowes Cove, Maine, USA. Cores around individual burrows were taken monthly in 1994 and 1995, and four seasonal samples were taken at different tidal heights. During the major reproductive periods in spring/early summer and in the fall, females produced several consecutive broods and hosted growing offspring in their burrows. Juveniles remained in their mothers' burrows until they reached a length of 5 mm (approximately one-third adult size) or more. At the study site, the majority of amphipods in individual burrows were adult females. Following the main reproductive periods, subadult individuals were found in their own burrows, but densities did not increase following the reproductive period in spring/early summer 1994, probably because large numbers of L. pinguis emigrated via the water column between June and December. L. pinguis is an annual species. Many members of the cohort born in spring/early summer start reproducing in the fall, and survive until the following spring when they produce several broods. Members of the cohort born in the fall start reproducing the following spring and also produce several consecutive broods. Both the spring/early summer and fall cohorts die off after the major reproductive period in the following spring/early summer. High standing stocks of microphytobenthos occur in soft-bottoms at MLW, and I conclude that L. pinguis can engage in extended parental care there because its food is abundant year-round. The limited expandability and low stability of burrows in soft-bottoms at MLW do not permit long persistence of parent–offspring groups in L.␣pinguis. Received: 27 March 1997 / Accepted: 30 July 1997     

Intertidal distribution,population dynamics and production of the amphipod Uhlorchestia spartinophila in a Georgia,USA, salt marsh

The talitrid amphipod Uhlorchestia spartinophila Bousfield and Heard occurs in close association with the smooth cordgrass Spartina alterniflora Loisel in salt marshes along the U.S. Atlantic coast. In order to assess its potential as a prey resource for secondary consumers, we followed the population and production dynamics of this amphipod from 3 November 1990 to 2 October 1991 in a salt marsh on Sapelo Island, Georgia. Semimonthly samples were taken along an intertidal transect, which extended from a vegetated creekbank levee landward 140 m to a site near the upland edge of the marsh. Amphipod densities ranged from 9 to 826 indm^-2 and were greatest in the levee and high marsh habitats at opposite ends of the transect. The highest densities occurred in March to May and the lowest in November and December. Specific growth rates, which ranged from 0.001 to 0.024 mm mm^-1 d^-1 length and 0.003 to 0.068 mg mg^-1 d^-1 AFDW (ash-free dry weight), were least in winter and decreased with increasing amphipod size. Although some reproduction occurred year-round, most of the population's reproductive output was from January to May, when adults had the greatest size-specific mass. Sex ratio usually favored females, which were larger than males. Minimum adult female size was 5.4 mm total length (TL). Broods included from 1 to 28 eggs or young and increased with increasing body size, averaging 7.4 young for an average-size female of 7.6 mm TL. Annual production, which ranged from 0.769 to 1.444 g AFDW m^-2, was least in the low marsh and greatest in the levee habitat. Production:biomass ratios were 15.4 to 17.3 in different habitats. Such high turnover rates, together with the population's broad intertidal distribution, suggest a greater contribution to trophic dynamics than is implied by the relatively low standing stock biomass of U. spartinophila in this marsh system.     

On detritus as a food source for pelagic filter-feeders

Data on the amount of organic detritus within the particle size fraction 1 to 150 are presented for the Western Kiel Bight. Grouped into a mixed surface layer and a stratified lower layer in accordance with the main hydrographic features of the Western Baltic Sea, the rounded-off values show a seasonal variation between 100 and 600 mg m^-3 expressed as dry weight of organic matter. The overall average for both layers is about 200 mg m^-3. Organic detritus thus comprises more than 40% of total organic matter in the above size class, which is the class most easily accessible to the relatively small filter-feeders in this area. Attempting to trace the origin of organic detritus, a positive correlation to phytoplankton standing stock was found in some cases, suggesting the predominance of autochthonous detritus. Proceeding from the assumption that pelagic filter-feeders select their food mainly by size and not by taste, it is concluded that organic detritus plays an important role as a supplementary food source, being ingested together with phytoplankton and small nonmotile heterotrophs. The nutritive value of detritus is increased by the adsorption of dissolved organic matter and above all through the subsequent colonization by bacteria, which utilize the dissolved substances. Detritus particles serving as a substratum for bacteria thus form a means whereby dissolved organic substances reenter the food chain. The ingestion of detritus by filter-feeders is, therefore, thought to be instrumental in increasing the effectivity of energy transfer from the primary to the secondary food-chain level.     

Feeding ecology of Spinocalanus antarcticus, a mesopelagic copepod with a looped gut

Spinocalanus antarcticus, an abundant mesopelagic copepod in polar seas, has a greatly elongated and looped midgut, contrary to most other copepod species. The total gut length is 1.77, 1.86 and 1.90 times the total body length in adult females, CV and CIV, respectively. Gross morphology of the midgut is similar in all copepodite stages and adults. It is described here from specimens collected in the Arctic Ocean. In stratified samples from the deep Amundsen and Makarov Basins S. antarcticus showed a clear preference for the depth layer between 100 and 500 m. Generally, the guts were packed with material, but most of it was impossible to identify. In most specimens the digestive tract was filled with undefined detritus particles ("detritus balls"). They were almost spherical, heterogeneous organic aggregates of 40-100 µm diameter, with small clay-sized mineral flakes imbedded. Mineral particles in the size range of 1-10 µm were found in large quantities in the guts of many specimens. Cysts of Chrysophycea and dinoflagellates and fragments of dinoflagellates, diatoms, tintinnids and radiolarians, as well as skeletons of silicoflagellates, were rather rare; some animal remnants were also found. A high carbon/nitrogen ratio (8.9) and very high lipid content (54% of dry weight) indicated a very good nutritional state. The adaptive significance and possible feeding strategy of this deep-water copepod is discussed.     

Discreteness of pelagic faunal regions

A variety of statistical techniques were used to examine the discreteness of mesopelagic fish species assemblages in the western North Atlantic. Although faunal groups could be isolated, the results were more consistent with a gradation between faunas rather than abrupt change. Few species were regularly associated, thus suggesting that, while individual species may respond to environmental gradients, recurrent groups of species do not.     

Significance of pelagic bacteria as a trophic resource in a coral reef lagoon,One Tree Island,Great Barrier Reef

Bacterial numbers and frequency of dividing cells (FDC) were estimated in surface waters at seven stations along a transeet across One Tree lagoon and reefs at 2 h intervals over one tidal cycle in May 1984 and once a day on 8 d in December 1984/January 1985. Cell numbers ranged between 1.24 and 13.10x10⁵ ml^-1, with an overall mean value of 4.81x10⁵. There was a progressive depletion in standing stock from windward to leeward stations across the lagoon, also reflected in the FDC, which generally showed similar dynamic patterns to cell numbers throughout the study. Predator-free seawater (filtered through 2 m pore-size filters) and mucus-enriched incubations were also used to establish the relationship between the growth rate () and FDC for coral-reef populations. Average growth rates predicted from FDC values ranged from 0.062 to 0.174 h^-1, which is equivalent to doubling times of 5.74 to 16.00 h, with an overall mean value of 11.5 h. These fast doubling times suggest that bacteria respond rapidly to pulses of enrichment as they float over the lagoon and reefs, characteristically achieving about one doubling per tidal cycle. This probably ensures that the bulk of labile organic matter and dissolved nutrients is conserved within the immediate coral-reef environment. Estimates of bacterial production also suggest that they may contribute up to 40% of total picoplankton production and about 25% of total microplankton production. Pelagic bacteria are therefore potentially a major food resource for benthic filter-feeders, especially for those mainly dependent on the smaller (pico) components of the plankton.     

Local adaptation of immunity against a trematode parasite in marine amphipod populations

Resources allocated to defence against parasites are not available for investment in other functions such as growth or reproduction, resulting in trade-offs between different components of an organism’s fitness. In balancing the cost of infection and the cost of immunity, selection should only favour individuals that allocate more energy to resistance and immune responses in populations regularly exposed to debilitating parasites. Here, we compare the ability of amphipods, Paracalliope novizealandiae, to (1) avoid becoming infected and (2) to respond to infection by encapsulating and melanizing parasites, between two natural populations exposed to different risk of parasitism. One population faces high levels of infection by the debilitating trematode parasite Maritrema novaezealandensis, whereas the other population is not parasitised by this trematode nor by any other parasite. Under controlled experimental conditions, with exposure to a standardized dose of parasites, amphipods from the parasite-free population acquired significantly more parasites than those from the population regularly experiencing infection. Furthermore, a lower frequency of amphipods from the parasite-free population succeeded at melanizing (and thus killing) parasites, and they melanized a lower percentage of parasites on average, than amphipods from the parasitised population. These differences persist when individual factors, such as amphipod sex or body length, are taken into account as potential confounding variables. These results support the existence of local adaptation against parasites: an amphipod population that never experiences trematode infections is less capable of resisting infection, both in terms of its first line of defence (avoiding infection) and a later line of defence (fighting parasites following infection), than a population regularly exposed to infection.     

Terrestrial, benthic, and pelagic resource use in lakes: results from a three-isotope Bayesian mixing model

Fluxes of organic matter across habitat boundaries are common in food webs. These fluxes may strongly influence community dynamics, depending on the extent to which they are used by consumers. Yet understanding of basal resource use by consumers is limited, because describing trophic pathways in complex food webs is difficult. We quantified resource use for zooplankton, zoobenthos, and fishes in four low-productivity lakes, using a Bayesian mixing model and measurements of hydrogen, carbon, and nitrogen stable isotope ratios. Multiple sources of uncertainty were explicitly incorporated into the model. As a result, posterior estimates of resource use were often broad distributions; nevertheless, clear patterns were evident. Zooplankton relied on terrestrial and pelagic primary production, while zoobenthos and fishes relied on terrestrial and benthic primary production. Across all consumer groups terrestrial reliance tended to be higher, and benthic reliance lower, in lakes where light penetration was low due to inputs of terrestrial dissolved organic carbon. These results support and refine an emerging consensus that terrestrial and benthic support of lake food webs can be substantial, and they imply that changes in the relative availability of basal resources drive the strength of cross-habitat trophic connections.     

The vertical structure of a pelagic community in the tropical ocean

The vertical structure of the ecosystem in the euphotic zone of the western Equatorial Pacific has been studied. The studies were based on a continuous sounding of the bioluminescence field, with simultaneous, vertically aimed sampling made with the aid of a 5l water bottle and plankton nets. The bioluminescence field has a two-maxima structure with a more pronounced and permanent lower maximum found in the oligotrophic regions as deep as 60 to 100 m. In the narrow (10 m) layer of maximum bioluminescence, the concentration of zooplankton, as well as the concentration, activity and production of bacteria and phytoplankton, are several times higher than those in immediately adjacent waters at greater or lesser depth. At the same time, the concentration of nutrient salts in that layer diminishes sharply and approaches zero in the overlying water. Analysis of the data obtained shows that the production processes above the lower maximum layer are dependent not only on the turbulent influx of nutrient salts from the underlying layers, but also on the horizontal supply from the zone of upwelling. A pelagic community, in its different time aspects from the moment of water ascending until its sinking in the convergence zone, is suggested as a common system for modelling.