Growth and production of Sterechinus neumayeri (Echinoidea: Echinodermata) in McMurdo Sound,Antarctica

Population dynamics of Sterechinus neumayeri were investigated at four sites in McMurdo Sound: Cape Evans, McMurdo Station, East Cape Armitage and New Harbor. The annual formation of natural growth bands in the jaws of the Aristotle's lantern was verified by a tagging-recapture experiment. Growth functions based on natural growth bands indicated differences among stations but showed S. neumayeri to be a slow growing species, reaching its maximum diameter of 70 mm at an age of about 40 yr. Annual production ranged between 2.4 (C. Evans) and 0.65 g ash free dry mass m^-2 (New Harbor) and was related to differences in food conditions. More than 95% of total production was invested in reproduction, and less than 5% was invested in somatic growth. Consumption estimates for S. neumayeri showed this species to play a significant role in the benthic trophic web of McMurdo Sound.     

Food web of an Antarctic midwater ecosystem

The diets of 93 species of plankton and micronekton taken in the upper 1000 m of Croker Passage (Gerlache Strait) in the austral fall, 1983, were examined and the principal features of the food web were characterized. Most species were small particle feeders, with phytoplankton and debris (of phytoplankton and krill) being the principal diet components. Krill remains were found in the guts of the majority of species examined, with the krill playing a greater role in the form of molts and debris than as living prey. Carcinores fed mostly on copepods, coelenterates and salps. Some of the larger species fed on live krill. No-hierarchical cluster analysis of diet information supported the concept of resource partitioning and determined the arrangement of the species into 21 feeding groups. Cluster analysis groupings tended to be along genetic lines with closely related and morphologically similar species having similar diets. These analyses were based on collections made in the austral fall (March–April, 1983) when phytoplankton standing crop was low, most zooplankton species had descended into the mesopelagic zone, and some of the more abundant species, such as Calanoides acutus, had ceased feeding. Because the trophodynamics of Antarctic ecosystems is strongly pulse-induced, it is essential to examine the food web at different periods in the seasonal cycle.     

Using fatty acid analysis to elucidate the feeding habits of Southern Ocean mesopelagic fish

Fatty acid biomarkers were used to investigate the feeding ecology of 17 mesopelagic fish species occurring in the Southern Ocean. Fatty acid signatures of species where little or no dietary information exists were compared to fatty acid signatures of species of known diets in order to elucidate their trophic position. Principal component analysis grouped species of known diets into two clusters with amphipods and copepods comprising the main prey species, respectively. Although the majority of species of unknown diet were grouped with either of these feeding guilds, a third cluster comprising only Gymnoscopelus bolini was identified suggesting a significantly different diet for this species. Electrona antarctica also exhibited significant changes in fatty acid signatures with size. Furthermore, discriminant analysis of the four most abundant species classified species with a 90% success rate thus validating the usefulness of fatty acid signatures when trying to resolve the trophic position of species where no or little dietary information exists.     

Microbial uptake of dissolved organic matter in Mcmurdo Sound,Antarctica

The distribution and activity of bacterioplankton, and the turnover of dissolved organic matter (DOM) were examined in McMurdo Sound, Antarctica. On the eastern side of the Sound, bacteria averaged 6.5×10⁸ l^-1, and turnover rates of dissolved adenosine triphosphate, D-glucose and l-leucine averaged 16, 116 and 124 h, respecitvely. These molecules as well as thymidine were taken up maximally from 0° to 5°C and near-maximally from -1.5° to 0°C, indicating bacterial adaptation to rapid turnover of dissolved organic matter at the ambient temperature. On the west side of the Sound, bacteria averaged only 0.65×10⁸ l^-1, and turnover times for adenosine triphosphate, D-glucose and lleucine averaged 59, 20454, and 3070 h, respectively. Total microbial adenosine triphosphate (an indicator of total microbial biomass) and chlorophyll a were also much lower at the western than at the eastern side stations. Moreover, no primary production could be detected at one western side station (New Harbor). Thus, in McMurdo Sound, the western side is highly oligotrophic, but the eastern side has an abundant active bacterioplankton, comparable to that of temperate coastal waters.Communicated by N. Holland, La Jolla     

Paternity analysis shows experience,not age,enhances mating success in an aquatically mating pinniped,the Weddell seal (<Emphasis Type="Italic">Leptonychotes weddellii</Emphasis>)

For polygynous mammals with no paternal care, the number of offspring sired is often the sole measure of male reproductive success. The potential for polygyny is highest when resources or other environmental factors such as restricted breeding sites force females to aggregate. In these circumstances, males compete intensely for females and mating success may vary greatly among males, further intensifying selection for those traits that confer an advantage in reproduction. Hence, determinants of male success in competition for females are likely to be under strong sexual selection. Paternity analysis was used in conjunction with measures of age, site fidelity, and behavior during the breeding season to assess variance in male breeding success in Weddell seals (Leptonychotes weddellii) breeding at Turtle Rock, McMurdo Sound (77.727S, 166.85E) between 1997 and 2000. Paternity could be assigned to 177 pups at relaxed or 80% confidence level or 111 pups at strict or 95% confidence levels. Weddell seals at Turtle Rock show a modest degree of polygyny with the greatest number of pups sired by any individual male in a single season equalling 5 or ∼10% of the pups born. Over four consecutive years, most (89.2%) males sired at least one pup. In a generalized linear model (GLM), age and the age first seen at the study site as an adult were unrelated to mating success, but adult experience, either site-specific or elsewhere in McMurdo Sound, over the reproductive life span of males explained nearly 40% of variance in total mating success with 80% confidence and 24% of variance at 95% confidence. While learning where females are likely to be may enhance male reproductive success, aquatic mating reduces the ability of males to monopolize females, and thereby increases equity in mating success.     

Trophic ecology of the stomiid (Pisces: Stomiidae) fish assemblage of the eastern Gulf of Mexico: Strategies,selectivity and impact of a top mesopelagic predator group

The trophic ecology of the stomiid assemblage (Pisces, Stomiiformes, Stomiidae) in the eastern Gulf of Mexico, a region with physical and biological characteristics typical of oligotrophic low-latitude regimes, was investigated. Over 1400 specimens representing 69 species and 17 genera were examined. Four patterns of feeding were evident among the abundant stomiids: (1) myctophid predation; (2) zooplankton/small micronekton predation; (3) penaeidean shrimp predation; and (4) copepod/micronekton predation. One rare species preyed on cephalopods. Il was concluded that stomiids exhibited a high level of prey-selectivity, particularly considering the broad range of prey types available in the eastern Gulf of Mexico. The absence of numerically dominant potential prey (e.g.Cyclothone spp., sternoptychids) in the diets of piscivorous stomiids is possibly a function of feeding periodicity coupled with stomiid vertical migration. Stomiids may feed at night in the upper 200 m on vertically migrating myctophids while disregarding co-occurring nonmigrating prey during the daytime. Integration of stomiid abundance and diet data suggests that: (1) stomiids are the dominant upper trophic-level predators of the Gulf of Mexico mesopelagial, (2) stomiids inflict the highest predation impact on myctophids in low-latitude midwater ecosystems, and (3) the historic use of predation-avoidance arguments to explain certain mesopelagic phenomena (e.g. vertical migration, ventral photophores) appears to be substantiated by estimates of stomiid predation-impact. The stomiids may serve as key trophic mediators in the transfer of energy from the mesopelagial to the bathyand benthopelagial.     

Sexual segregation in distribution,diet and trophic level of seabirds: insights from stable isotope analysis

Considerable attention has focused on inter- and intraspecific variation in trophic niches of marine predators. Although this has revealed evidence for sexual segregation in distribution in some species, few studies have been able to address sex-related dietary specialisation. Stable isotope analysis of blood cells collected from albatrosses and petrels at South Georgia during chick-rearing indicated a difference in δ¹³C, suggesting that females fed to the north of males, only in two species with male-biased sexual size dimorphism; in no species did sexes differ in trophic level (δ¹⁵N). Based on a wider review, significant differences between sexes in isotope signatures were much more common in seabirds during the pre-laying or breeding than the nonbreeding period, presumably reflecting greater between-sex partitioning of resources when foraging ranges are more constrained and competition is greater. Sex differences, or their absence, were usually consistent across successive stages during the pre-laying and breeding periods, but not necessarily year-round nor between populations. Significant differences in isotope signatures between males and females were extremely rare in monomorphic species, suggesting a link between sexual size dimorphism and segregation in diet or distribution. Among the Southern Ocean albatrosses, sex differences in δ¹³C suggested the underlying mechanism was related to habitat specialisation, whereas in other size-dimorphic taxa (both male- and female-biased), sex differences were more common in δ¹⁵N than δ¹³C and therefore more consistent with size-mediated competitive exclusion or dietary specialisation.     

Trophic segregation between sexes in the Black Skimmer revealed through the analysis of stable isotopes

The degree of individual or gender variation when exploiting food resources is an important aspect in the study of foraging ecology within a population. Previous information on non-breeding skimmers obtained through conventional methodologies suggested sex-related differences in prey species. In this study, stable isotope techniques were used to investigate the intraspecific segregation in diet and foraging habits of the Black Skimmer (Rynchops niger intercedens) at Mar Chiquita Coastal Lagoon (37°40′S, 57°22′W), Argentina. These results were compared with contemporary data on the trophic composition obtained by conventional methodologies. Blood samples were taken from birds captured with mist-nets during their non-breeding season. The isotopic signatures of skimmers showed a diet mainly composed of marine prey with some degree of estuarine fish intake. When comparing diet between sexes, males showed enrichment in ¹⁵N compared to females, while no differences were observed in ¹³C. The use of mixing models revealed differences in the relative composition of prey in the diet of male and female skimmers. This study highlights stable isotope analysis as a valuable tool to test inter-individual differences and sexual segregation in trophic ecology of Black Skimmers as compared to conventional methodologies. The results show a trophic segregation in the Black Skimmer during the non-breeding season that can be explained by differences in prey species and larger prey sizes of male skimmers. Our findings have significant implications for conservation since any environmental change occurring at wintering areas might have profound effects on several avian life-history traits, and could be different for males and females due to trophic segregation.     

Interactions between light and temperature on the physiological ecology ofGracilaria tikvahiae (Gigartinales: Rhodophyta)

The biology and ecology of the Antarctic scallop Adamussium colbecki has been investigated on the west side of McMurdo Sound, Antarctica, principally at Explorers Cove, during the austral summer of 1981–1982. This conspicuous benthic invertebrate exhibits highest densities, 85 individuals m^-2, in shallow (4 to 6 m) water. Densities decrease to 20 m^-2 at 30 m. Biomass levels are highest in shallow water, 1 600 g wet wt m^-2. Provisional growth information suggests that 8 cm individuals are about 12 yr old. Mortality is apparently caused chiefly by a hyposaline lens of seawater, which forms under the sea ice during the summer melt; predators do not appear to be important. High biomass levels and a short generation time suggest that A. colbecki is an important species in a very productive community on the west side of McMurdo Sound. The shallow benthos of Explorers Cove is an important exception to the generalization that Antarctic ecosystems are dominated by indigestible filter-feeders.     

Sinking rates and dissolution of midwater fish fecal matter

The sinking rates of fecal matter from 7 southern California midwater fish species were investigated. Feces were obtained from 162 specimens of Stenobrachius leucopsarus, Triphoturus mexicanus, Leuroglossus stilbius, Lampanyctus ritteri, Argyropelecus affinis and Parvilux ingens, which were collected in the Santa Barbara and San Clemente Basins between 1977 and 1979. In addition, feces obtained from 6 laboratory-maintained specimens of the midwater zoarcid Melanostigma pammelas were used for repeated sinking-rate measurements. The mean of the measured sinking rates for all species was 1.19 cm s^-1 (1 028 m d^-1), which is much higher than the known descent rates of euphausiid and copepod fecal pellets and of most other particulate organic detritus. Dissolution characteristics were also investigated for fecal matter from 4 species collected by the same series of net hauls: S. leucopsarus, T. mexicanus, A. affinis, and Sternoptyx obscura. The release of dissolved organic compounds from this material is low and does not represent a significant output during the relatively short time required to sink through the water column. These findings suggest that midwater fish fecal matter may represent a major source of organic transfer between the pelagic community and the benthos.     

Topological analysis of the ecological importance of elasmobranch fishes: A food web study on the Gulf of Tortugas, Colombia

We built a trophic network based on a matrix of interspecific trophic relationships to assess the role of elasmobranch fishes in shaping community structure of the Gulf of Tortugas in the Colombian Pacific Ocean. We analyzed diet similarities to define trophic components (nodes) - rather than taxonomical groups - in the network. We evaluated the ecological function of species or trophic entities through topological analysis of their structural importance in trophic networks by applying one local and several mesoscale network indices. We found that top predatory elasmobranchs play an important ecological role in top-down control and in propagating indirect effects through the system owing to high values of the node degree, centrality and topological importance indices. However, invertebrates and teleost fishes had higher connectivity and topological importance than other elasmobranchs in the network before and after removal of top predators from the system. Results from our study thus suggest that elasmobranchs at intermediate trophic levels - commonly referred to as “mesopredators” - are not so important in all complex coastal ecosystems as previously reported.     

Dietary study of the midwater polychaete Poeobius meseres in Monterey Bay,California

This study presents the first quantification of the diet of a gelatinous midwater organism on a temporal basis. Using the Monterey Bay Aquarium Research Institute's remotely operated vehicle Ventana, regular collections of the polychaete Poeobius meseres (Heath, 1930) over a 1 yr period (October 1990 to November 1991) in Monterey Bay yielded intact organisms for the study of feeding behavior and quantitative analysis of stomach contents. In situ observations showed P. meseres feeding in two different ways: (1) by deployment of a mucus web in the water column that passively collects particles for consumption; and/or (2) by grasping detrital material in the water column with its ciliated tentacles. Stomach-content analyses showed that P. meseres is primarily coprophagic, its diet being dominated by fecal pellets from euphausiids and copepods. These fecal pellets appear to provide P. meseres with essentially all its carbon. Although fecal pellets were the most important food item volumetrically, P. meseres also consumed large numbers of diatoms and small numbers of dinoflagellates, chrysophytes, radiolarians, foraminiferans and eggs. The diet of P. meseres appears to reflect primary productivity in the surface waters, with different food items predominant in the diet at different times of the year. Pennate diatoms were most abundant in the diet during the fall, centric diatoms were most abundant during the sumnier, and fecal pellets during the winter. The composition of P. meseres diet suggests that this and other midwater gelatinous organisms have a significant role in the remineralization of particles as they sink from the surface to the deep sea.     

Chemical defense in the eggs and embryos of antarctic sea stars (Echinodermata)

Fifteen species of reproductively mature echinoderms (11 sea stars, 3 sea urchins, 1 sea cucumber) were collected from McMurdo Sound, Antarctica, during austral spring and summer of 1985 and 1986; eggs and embryos were obtained, and were tested for ichthyonoxicity using the common marine killifishFundulus grandis as a model predator. Chemical deterrents occurred in the large, yolky eggs of the pelagic lecithotrophic sea starPerknaster fuscus and the planktotrophic sea starPorania antarctica. Brooded embryos of the sea starsDiplasterias brucei andNotasterias armata were also noxious. Significant ichthyonoxicity was not detected in the remaining 7 species of sea stars, 3 sea urchins, and 1 sea cucumber. Chemical deterrents were generally effective at concentrations below a single egg or embryo per agar test-pellet. Although chemicals found in these eggs and embryos are noxious to an allopatric fish, they may not be noxious to sympatric fish.     

Population distribution and structure of the free-living nematodes of Long Island Sound

The distribution and structure of nematode populations in 4 sedimentary environments (muds, muddy sands, fine sands and medium-coarse sands) in Long Island Sound were studied. Mean population densities were highest in muds and muddy sands. Cluster analysis suggested the presence of two basic faunistic units; a mud unit characterized by high species dominance, low species diversity and low species endemism, and a sand unit characterized by low species dominance, high species diversity and high species endemism. Species diversity in all habitats was a direct function of both species richness and equitability. Limited niche separation among deposit feeders, usually the dominant nematode trophic type in muddy sediments, is proposed as the cause for the high species dominance so often characteristic of shallow marine muds. The study afforded the opportunity to examine the quantitative and qualitative aspects of population structure in relation to environmental impact (as indicated by large differences in the heavy metal and organic carbon concentrations within each of the 4 sedimentary regimes). Within each sediment type no differences in population densities, species composition or species diversity of nematodes existed between heavily impacted and apparently non-impacted sediments, or between Long Island Sound and similar coastal regions. These findings cast doubt on (1) the use of heavy metal and organic carbon concentrations as indicators of environmental stress for marine nematodes; (2) the use of diversity indices alone as indicators of environmental deterioration; and (3) the usefulness of field monitoring studies alone for the assessment of pollution impact on marine nematodes.Contribution no. 100 from the Institute of Marine and Atmospheric Sciences, City College of New YorkCommunicated by M.R. Tripp, Newark     

Incorporation of plant carbon into the soil animal food web of an arable system

We used stable isotopes to examine the incorporation of plant carbon into the belowground food web of an agricultural system. Plots were established and planted with maize (Zea mays) in a rye field (Secale cereale) near G?ttingen (northern Germany) in May 1999. In October 1999, April 2000, and October 2000, meso- and macrofauna and maize and rye litter were collected in each plot and analyzed for 13C and 15N content. 15N signatures suggested that the soil animal species analyzed span three trophic levels with the trophic position of species varying little in time. The species investigated formed a continuum from primary to secondary decomposers to predators. On average, predator species differed from primary and secondary decomposers by 3.9 sigma15N suggesting that they fed on a mixed diet of both decomposer groups. The combined analysis of 13C and 15N signatures allowed us to identify links between prey and consumer species. In October 1999, shortly after maize residues had been incorporated into the plots, maize-born carbon was present in each of the animal species investigated, including top predators. The incorporation of maize carbon into the belowground food web increased during the following 12 months but the concentration of maize-born carbon never exceeded 50% in any of the species. Furthermore, the ranks of the incorporation of maize-born carbon of the species changed little. The results suggest that the belowground food web relies heavily on carbon originating from plant residues from before the recent two growing seasons. In most species the amount of maize-born carbon increased continuously; however, in some species it decreased during winter, suggesting that these species switched to a diet based more on C3 plants during winter, or predominantly metabolized carbon incorporated during the last growing season. The study documents that the combined analysis of 13C and 15N signatures in soil invertebrate species, after replacement of C3 by C4 plants, is a powerful tool to better understand the structure of the belowground food web and the flux of carbon through it.     

An inverse model for the analysis of the Venice lagoon food web

A steady-state model of the Venice lagoon food web was constructed, based on a comprehensive set of data, which were collected in the years 2001-2005. Energy flows were estimated by means of an inverse methodology of constrained optimization based on the Minimum Norm criterion, i.e. on the minimization of both the sum of squares of the residuals and of the sum of squares of energy flows. The solution was constrained by a set inequalities, which were derived from general eco-physiological knowledge and site specific data on energy flows. The trophic network was represented by thirty-two nodes, including single-species compartments for the species of high economical or ecological relevance. Mass balance equations were weighted, in order to obtain meaningful results in presence of large differences, up to 5 orders of magnitude, among biomasses. A perturbation technique was applied, with the purpose of reducing the risk of finding solutions heavily affected by the set of constraints and of obtaining a more robust representation of the energy flows. The main patterns of energy flow are consistent with those obtained in previous attempts at modelling the Venice lagoon food web. Micro- and macro-phytobenthos account for the largest fraction of the primary production. Energy is then transferred towards higher trophic levels by means of two main pathways: the recycling of dead biomass through the detritus compartment and the direct consumption by grazers. The first pathway is the most important and accounts for approximately two/thirds of the energy transferred to the second trophic level.     

Population structure and energetics of the shallow-water antarctic sea star Odontaster validus in contrasting habitats

Individuals and populations of Odontaster validus Koehler differed markedly among different habitats, as revealed in a study from October 1984 through January 1986 in McMurdo Sound, Antarctica. At McMurdo Station, individual sizes (wet weight) and population biomass (g wet wt m^-2 and kJ m^-2) decreased significantly with increasing depth. Individuals from shallow (10 to 20 m) habitats were in superior nutritional condition to those from deeper water (30 and 165 m), as shown by higher gonad and pyloric cecum indexes, and by higher lipid and energetic levels in the pyloric ceca. Moreover, gonadal output (reproductive output) was higher in shallow-water individuals. Higher levels of chlorophyll in the pyloric ceca and richer yellow to red coloration of the body wall in the shallow-water individuals indicate that they utilize the higher levels of primary production at shallow depths. At East Cape Armitage, where nearly permanent, thick, snow-covered ice most of the year resulted in very low levels of benthic primary production, the lowdensity sea stars were all very small and nutritionally similar to the deep-water individuals at McMurdo Station. At Cape Evans, where the generally snow-free sea-ice that broke up in mid-summer resulted in a luxurient benthic cover of diatoms and macroalgae, the sea stars were smaller than at McMurdo Station at comparable depths, but population densities were higher, resulting in 4 to 9 times greater biomass. Growth rates of sea stars fed in the laboratory were very low, especially compared to laboratory-reared temperate and tropical species; well-fed individuals need about 9 yr to reach 30 g wet weight, near the mean size of shallowwater individuals at McMurdo Station. No growth was detected in individuals caged at McMurdo Station for one year, suggesting even lower growth rates in the field. The stable size-frequency distributions at the different sites and depths throughout the year-long study suggest highly stable populations with low temporal variability in recruitment, migration and mortality. These data indicate that individuals and populations of O. validus quantitatively and qualitatively reflect the general level of productivity of a habitat. Differences noted in size, coloration, nutrition, and reproductive effort may be the result of long-term integration of local levels of primary production. These ubiquitous sea stars may serve as a biotic indicator of productivity in localized habitats around the continental shelf of Antarctica.     

Use of isotopic analysis of vertebrae in reconstructing ontogenetic feeding ecology in white sharks

We conducted stable 13C and 15N analysis on white shark vertebrae and demonstrated that incremental analysis of isotopes along the radius of a vertebral centrum produces a chronological record of dietary information, allowing for reconstruction of an individual's trophic history. Isotopic data showed significant enrichments in 15N with increasing sampling distance from the centrum center, indicating a correlation between body size and trophic level. Additionally, isotopic values verified two distinct ontogenetic trophic shifts in the white shark: one following parturition, marking a dietary switch from yolk to fish; and one at a total length of >341 cm, representing a known diet shift from fish to marine mammals. Retrospective trophic-level reconstruction using vertebral tissue will have broad applications in future studies on the ecology of threatened, endangered, or extinct species to determine life-long feeding patterns, which would be impossible through other methods.     

The trophic ecology of two abundant mesopredators in south-east coastal waters of Tasmania, Australia

Investigating predator–prey relationships is an important component for identifying and understanding the factors that influence the structure and function of ecosystems. Mesopredators, defined as mid-level predators, have a profound effect on ecosystem structure by contributing an important link between apex predators and lower trophic levels. The diet of two elasmobranch mesopredators, Squalus acanthias and Mustelus antarcticus, was investigated in three locations in south-east Tasmania. Squalus acanthias consumed predominantly pelagic teleosts and cephalopods, while M. antarcticus predominantly consumed benthic crustaceans. As a result, there was low dietary and niche overlap between the two species. There was however evidence of intra-specific dietary variations between locations for both the species. This study has contributed to a better understanding of the top-down dynamics of the food web in coastal Tasmania, by providing important dietary information of two abundant mesopredators. In addition, the similar dietary patterns for S. acanthias and other Mustelus species over much of their global range suggest they may be consistent in their trophic roles across systems, with limited competition between these two sympatric mesopredators to be expected.     

Independent gradients of producer, consumer, and microbial diversity in lake plankton

Interactions between trophic levels during food web assembly can drive positive correlations in diversity between producers, consumers, and decomposers. However, the contribution of trophic interactions relative to local environmental factors in promoting species diversity is poorly understood, with many studies only considering two trophic levels. Here we examine correlations in diversity among zooplankton, phytoplankton, and bacteria in the pelagic zone of 31 lakes in British Columbia, Canada. We sampled species diversity of zooplankton and phytoplankton through morphological identification, and bacterial genetic diversity was estimated by denaturing gradient gel electrophoresis (DGGE) of 16S rDNA polymorphisms. We looked for correlations in diversity that were independent of the abiotic environment by statistically controlling for 18 limnological variables. No significant correlations were found between the diversity of zooplankton, phytoplankton, and bacteria. In addition, the physical factors that were associated with species composition in one trophic level were independent of those that were important for another. Our results provide no support for the importance of direct feedbacks between producers, consumers, and decomposers in maintaining diversity. Zooplankton, phytoplankton, and bacterial diversity and composition are regulated independently from one another and respond to different environmental variables. These results suggest that species of lake plankton show loose trophic associations with one another due to broad diets in consumers and decomposers.