Carotenoids indicate differences in diet of the hydrothermal vent crabBythograea thermydron (Brachyura)

Carotenoid pigments were used as markers to investigate the sources of energy to two deep-sea hydrothermal vent communities. Specimens of the hydrothermal-vent brachyuran crabBythograea thermydron were collected at 2 500 m depth from the Rose Garden vent site in the Galápagos Rift Valley in February, November and December 1979, and 2 600 m depth on the East Pacific Rise at 21°N in May 1982. Four carotenoids (astaxanthin, canthaxanthin, echinenone and beta-carotene) have been identified as the pigments responsible for the red color of the eggs of the crabs from the Galápagos Rift site. Consistent with the fact that animals are unable to synthesize carotenoidsde novo, precursors were not present in the crabs' tissues, affirming that these pigments are of dietary origin. The number of ovigerous female crabs and the concentrations of carotenoids in the eggs suggest a readily available source of these pigments in the Galápagos vent environment. In contrast, the developing eggs ofB. thermydron from the 21°N vent site were cream-colored, with only trace quantites of carotenoids and fewer types of carotenoids. Analysis of carotenoid distribution in both females and males in these two populations indicates a very low level of carotenoids in the diet of the 21°N vent crabs, and reflects differences in trophic interactions and primary production at the two vent sites. The few types and low concentration of carotenoids inB. thermydron indicate a diet that is different from non-vent, deep-sea crustaceans. We hypothesize that the source of carotenoids are bacteria within the vent community, and not ultimately from photosynthetic production.     

Trophic relationships among invertebrates at the Kairei hydrothermal vent field (Central Indian Ridge)

Exploration of hydrothermal vent systems in locations remote from well-studied sites allows ecologists to determine the degree of site-specific variation in trophic relationships among communities. A preliminary outline of the trophic structure of the Kairei hydrothermal vent community on the Central Indian Ridge (25°19.23′S; 70°02.42′E) is provided here, based on analysis of collections from an April 2001 expedition. Invertebrate biomass at Kairei is dominated by organic carbon with a δ¹³C isotopic value of about –13‰, due to the abundance of primary consumers (shrimp: Rimicaris aff. exoculata) and secondary consumers (anemones: Marianactis n. sp.) with this isotopic composition. Filamentous thiotrophic episymbionts on shrimp have been interpreted to be the major diet items of the shrimp and hence are the dominant primary producers within the community. Free-living autotrophic microorganisms are implicated as the dietary base for other invertebrate species. Four trophic groups are identified within the Kairei invertebrates based on carbon- and nitrogen-isotope ratios, but these groups do not always define discrete trophic levels. Ontogenetic shifts in diet are documented for R. aff. exoculata and brachyuran crabs (Austinograea n. sp.). Diets of symbiont-bearing mussels (Bathymodiolus aff. brevior) and two species of gastropods are isotopically constant throughout the range of sizes analyzed. There is a consistent but unexplained pattern of increasing nitrogen isotopic composition with increasing carbon isotopic composition in vent communities from geographically disjunct oceanic regions. Given the assumptions associated with interpretations of isotopic data, there remains a missing pool of carbon (presumably unsampled bacterial biomass) that contributes to the maintenance of the ¹³C- and ¹⁵N-enriched primary consumers in these ecosystems.     

Digestive enzymes in marine invertebrates from hydrothermal vents and other reducing environments

The present study demonstrates the potential hydrolytic activities in the symbiont-containing tissues of the vent invertebrates Riftia pachyptila, Bathymodiolus thermophilus (collected in 1991 at the East Pacific Rise) and the shallow-water bivalve Lucinoma aequizonata (collected in 1991 from the Santa Barbara Basin). Activities of phosphatases, esterases, -glucuronidase and leucineaminopeptidase were comparable to those of digestive tract tissues of other marine invertebrates. A lack in most glycosidases as well as in trypsin and chymotrypsin was observed. Activities of lysozyme and chitobiase were rather high. In all vent invertebrates with symbionts and in L. aequizonata, the symbiont-containing tissues and the symbiont-free tissues had similar levels of enzymatic activities, indicating that polymeric nutrients could be hydrolysed after release from the symbionts and cellular uptake. The high activities of -fucosidase in all vent invertebrates as well as in the shallow-water bivalve L. aequizonata could point to the existence of a yet undescribed substrate available to hydrolysation. The ectosymbionts-carrying polychaete Alvinella pompejana (collected in 1991 at the East Pacific Rise, EPR) shows high lysozyme activities in its gut, consistent with the proposed food source of bacteria. For the vent crab Bythogrea thermydron (also collected in 1991 at the EPR) hydrolytic activities were highest in the gut, dominated by esterase and peptidase activities which support their proposed carnivorous food source. A snail and a limpet collected from R. pachyptila tubes showed high levels of chitobiase suggesting a food source of grazed bacteria or ingested R. pachyptila tube.     

Nematode community composition in hydrothermal vent and adjacent non-vent fields around Myojin Knoll,a seamount on the Izu-Ogasawara Arc in the western North Pacific Ocean

In contrast to specific large benthic invertebrates in chemosynthetic ecosystems such as hydrothermal vents, meiofaunal communities in such habitats have been reported to have strong taxonomic overlap with meiofauna in the adjacent “normal” environments. However, meiofauna have only recently been included in studies of those environments and detailed information on these communities is still rare. This is especially true in the Northwest Pacific Ocean, even though there are many seamounts with active vents in the calderas of the region. Nematode community composition at the genus level in sediments from a hydrothermal vent field in the caldera of Myojin Knoll (32°06′N, 139°52′E, depth 1,300 m), a seamount on the Izu-Ogasawara Arc, Japan, was investigated for the first time and was compared with adjacent non-vent areas inside and outside the caldera. Multivariate analyses showed that the composition of nematodes in the hydrothermal field was significantly different from that in the non-hydrothermal fields around the caldera. However, the common genera, such as Oxystomina, Pareudesmoscolex, Desmoscolex, and Microlaimus were found in two, or all three vent fields while their rank contributions differed among the three fields. When the data from Myojin Knoll were compared with those from other deep-sea vent environments in different regions (e.g., North Fiji Basin, East Pacific Rise, Mid-Atlantic Ridge), the nematode composition in the vent field of the Myojin caldera was more similar to that of the non-vent fields around the caldera than the composition in vent fields of other regions. These data from the Northwest Pacific Ocean also suggest the absence of long-range transport systems and local adaptations for meiofauna in hydrothermal vent fields.     

POM in macro-/meiofaunal food webs associated with three flow regimes at deep-sea hydrothermal vents on Axial Volcano,Juan de Fuca Ridge

Deep-sea hydrothermal vent ecosystems host both symbiotic and non-symbiotic invertebrates. The non-symbiotic vent fauna is generally assumed to rely on free-living chemoautotrophic bacteria as their main food source but other sources such as detritus have recently been suggested to be a part of the invertebrate diets. Little is known about how food availability influences the distribution of vent organisms on a small scale. In addition, the feeding ecology and role of small, often numerically dominant invertebrates, the meiofauna is poorly understood at vents. In this study, we used stable carbon and nitrogen isotopic analysis to investigate the role of particulate detritus in the diets of macro- and meiobenthic invertebrates within three vent assemblages at Axial Volcano, Juan de Fuca Ridge, and Northeast Pacific. Particulate organic matter of a detrital origin became more important in the diet of invertebrates in assemblages typically associated with low-hydrothermal flow intensities. Meiobenthic species occupied several different feeding guilds and trophic levels in the assemblages investigated. We conclude that small-scale spatial variability in food sources is an important feature of vent food webs and that spatial patterns observed here and elsewhere are shaped by variations in hydrothermal discharge.     

Extensive gene flow among mytilid (Bathymodiolus thermophilus) populations from hydrothermal vents of the eastern Pacific

Prior studies of the hydrothermal vent mussel Bathymodiolus thermophilus (Bivalvia: Mytilidae), provided conflicting predictions about the dispersal ability and population structure of this highly specialized species. Analyses of morphological features associated with its larval shells revealed a feeding larval stage that might facilitate dispersal between ephemeral vent habitats. In contrast, an allozyme study revealed substantial genetic differentiation between samples taken from populations 2370 km apart on Galápagos Rift (Latitude 0°N) and the East Pacific Rise (13°N). To resolve the discrepancy between these studies, we examined allozyme and mitochondrial (mt) DNA variation in new samples from the same localities plus more recently discovered sites (9° and 11°N) along the East Pacific Rise. Although analysis of 26 enzyme-determining loci revealed relatively low levels of genetic variation within the five populations, no evidence existed for significant barriers to dispersal among populations. We estimated an average effective rate of gege flow (Nm) of 8 migrants per population per generation. Two common mtDNA variants predominated at relatively even frequencies in each population, and similarly provided no evidence for barriers to gene flow or isolation-by-distance across this species' known range. Larvae of this species appear to be capable of dispersing hundreds of kilometers along a continuous ridge system and across gaps separating non-contiguous spreading centers.     

Reproductive biology and population structure of the deep-sea hydrothermal vent worm Paralvinella grasslei (Polychaeta: Alvinellidae) at 13°N on the East Pacific Rise

Paralvinella grasslei is a polychaetous annelid living in the harsh, unstable and heterogeneous environmental conditions found at deep-sea hydrothermal vent sites in the eastern Pacific. The aim of this work was to examine the possible influence of the reproductive biology of P. grasslei on the structure of its populations. Maximum observed oocyte size inside the oviduct is 275 m, and fecundity is relatively low. Examination of gametes and young specimens suggested a direct benthic development for this species. The population structure of P. grasslei at 13°N/EPR (EPR=East Pacific Rise) revealed a discontinuous recruitment which seems to be synchronized within vent sites and fields. The data also suggested the occurrence of discrete breeding periods. P. grasslei probably reproduces several times a years, with an apparent periodicity. Tidal signals could be a possible cue for the coordination of the reproductive cycle. The life-history of P. grasslei is discussed in light of the reproductive biology of other terebellomorph polychaetes, and seems to be well adapted for colonizing the unstable environment of hot vents. Two main hypotheses can explain the dissemination processes of this species along axial oceanic ridges. The influence of nearbottom currents occurring along the central graben of the East Pacific Rise can be cosidered to account for part of the transport of larvae and juveniles, but the observations of polychaete erpochaetes on the test of hydrothermal bythograeid crabs and evidence that crab migrations occur between vents also support the possibility of zoochory for the dissemination of alvinellid polychaetes.     

Na+-K+-adenosine triphosphatase activities in gills of marine teleost fishes: Changes with depth,size and locomotory activity level

Activities of the primary enzyme responsible for monovalent ion regulation, Na⁺-K⁺-adenosine triphosphatase (Na⁺-K⁺-ATPase), were measured in gills of marine teleost fishes with different depths of occurrence (0 to 4800 m), body weights (a range of five orders of magnitude), and locomotory capacities. Specimens were collected off the coasts of California and Oregon in 1983–1989, and at the Galápagos Spreading Center and 13°N East Pacific Rise hydrothermal vent sites in 1987 and 1988, respectively. Except for two hydrothermal vent fishes, deep-sea species had much lower Na⁺-K⁺-ATPase activities g^–1 gill filament than shallow-living species, indicating that osmoregulatory costs, like total metabolic rate, are greatly reduced in most deep-living fishes. Within a species, the total branchial Na⁺-K⁺-ATPase activity per individual was dependent on size; the average allometric scaling exponent was 0.83. Using published values for oxygen consumption rates, and the total branchial Na⁺-K⁺-ATPase activities as an index of osmoregulatory costs, we estimated the maximal cost (as percent of ATP turnover) for osmoregulation in ten teleosts. Osmoregulatory costs averaged about 10% of total ATP turnover among these species, and maximal costs were no greater than about 20%. The percent costs of osmoregulation did not differ between shallow- and deep-living fishes. The reduced total ATP expenditure for osmoregulation in deep-living fishes is proposed to result from the sluggish locomotory habits of these fishes, not from selection for reduced osmotic coastper se. Thus, the reduced swimming abilities of these fishes lead to lower rates of water flow over the gills and less blood flow through the gills due to reduced demands for oxygen. Consequently, passive flux of water and ions through the gills is much lower than in more active fishes, and osmotic costs are thereby minimized. The relatively high activities of Na⁺-K⁺-ATPase in gills of the two hydrothermal vent fishes suggest that these fishes may be more active and have higher metabolic rates than other deep-sea fishes.     

Food web structure of a restored macroalgal bed in the eastern Korean peninsula determined by C and N stable isotope analyses

Loss of macroalgae habitats has been widespread on rocky marine coastlines of the eastern Korean peninsula, and efforts for restoration and creation of macroalgal beds have increasingly been made to mitigate these habitat losses. Deploying artificial reefs of concrete pyramids with kelps attached has been commonly used and applied in this study. As a part of an effort to evaluate structural and functional recovery of created and restored habitat, the macroalgal community and food web structure were studied about a year after the establishment of the artificial macroalgal bed, making comparisons with nearby natural counterparts and barren ground communities. Dominant species, total abundance, and community structure of macroalgal assemblage at the restored macroalgal bed recovered to the neighboring natural bed levels during the study period. The main primary producers (phytoplankton and macroalgae) were isotopically well separated. δ¹³C and δ¹⁵N values of consumers were very similar between restored and natural beds but varied greatly among functional feeding groups. The range of consumer δ¹³C was as wide as that of primary producers, indicating the trophic importance of both producers. There was a stepwise trophic enrichment in δ¹⁵N with increasing trophic level. A comparison of isotope signatures between primary producers and consumers showed that, while suspension feeders are highly dependent on pelagic sources, invertebrates of other feeding guilds and fishes mainly use macroalgae-derived organic matter as their ultimate nutritional sources in both macroalgal beds, emphasizing the high equivalency of trophic structure between both beds. Isotopic signatures of a few mollusks and sea urchins showed that they use different dietary items in macroalgal-barren grounds compared with macroalgal beds, probably reflecting their feeding plasticity according to the low macroalgal biomass. However, isotopic signatures of most of the consumers at the barren ground were consistent with those at the macroalgal beds, supporting the important trophic role of drifting algae. Our results revealed the recoveries of the macroalgal community and trophic structure at the restored habitat. Further studies on colonization of early settlers and the following succession progress are needed to better understand the process and recovery rate in the developing benthic community.     

A new bathymodioline mussel symbiosis at the Juan de Fuca hydrothermal vents

Until recently, the only major hydrothermal vent biogeographic province not known to include bathymodioline mussels was the spreading centers of the northeast Pacific, but deep-sea dives using DSV Alvin on the Endeavor segment of the Juan de Fuca Ridge (47°56N 129°06W; ∼2,200 m depth) in August 1999 yielded the only recorded bathymodioline mytilids from these northeastern Pacific vents. One specimen in good condition was evaluated for its relatedness to other deep-sea bathymodioline mussels and for the occurrence of chemoautotrophic and/or methanotrophic symbionts in the gills. Phylogenetic analyses of the host cytochrome oxidase I gene show this mussel shares evolutionary alliances with hydrothermal vent and cold seep mussels from the genus Bathymodiolus, and is distinct from other known species of deep-sea bathymodiolines, suggesting this mussel is a newly discovered species. Ultrastructural analyses of gill tissue revealed the presence of coccoid bacteria that lacked the intracellular membranes observed in methanotrophic symbionts. The bacteria may be extracellular but poor condition of the fixed tissue complicated conclusions regarding symbiont location. A single gamma-proteobacterial 16S rRNA sequence was amplified from gill tissue and directly sequenced from gill tissue. This sequence clusters with other mussel chemoautotrophic symbiont 16S rRNA sequences, which suggests a chemoautotrophic, rather than methanotrophic, symbiosis in this mussel. Stable carbon (δ¹³C = −26.6%) and nitrogen (δ¹⁵N = +5.19%) isotope ratios were also consistent with those reported for other chemoautotroph-mussel symbioses. Despite the apparent rarity of these mussels at the Juan de Fuca vent sites, this finding extends the range of the bathymodioline mussels to all hydrothermal vent biogeographic provinces studied to date.     

A new method for high-resolution bivalve growth rate studies in hydrothermal environments

The age and shell growth rate of deep-sea hydrothermal bivalves were investigated for the first time using in situ chemical staining combined with high-resolution micro-increment analysis. A staining chamber developed for this purpose was applied to a patch of Bathymodiolus thermophilus mussels at 2,500 m depth at the 9°47′N vent field on the East Pacific Rise (EPR) in May 2010. This approach minimizes disturbance of the mussels in their habitat. Bathymodiolus thermophilus grows according to a circalunidian rhythm, with one increment formed each day, and displays tide-related growth rate variability. Based on the von Bertalanffy growth rate model, the largest shell collected (SL = 20.5 cm) would be 10.0 year old, with a growth rate of 4.2–1.1 cm year^?1 as the shell ages. This fast growth rate is consistent with the instability of the environment in this section of the EPR and observed recolonization rates and could reflect a specific adaptation of this species.     

Seasonal variations in the stable carbon and nitrogen isotope ratios (13C/12C and 15N/14N) of primary producers and consumers in a western Mediterranean coastal lagoon

Stable carbon and nitrogen isotope ratios (¹³C/¹²C and ¹⁵N/¹⁴N) of primary producers and consumers were investigated seasonally throughout 1999, in order to describe the food web in a western Mediterranean coastal lagoon (Lake of Sabaudia, central Italy). Particulate organic matter and algal material (seagrass epiphytes and macroalgae) seem to constitute the main food sources for primary consumers (zooplankton and small benthic invertebrates, respectively) throughout the sampling year, while the seagrass Cymodocea nodosa appears to play a negligible trophic role. As regards the ichthyofauna, carbon stable isotopes differentiated between planktivore and benthivore fish species. However, a benthic-pelagic coupling seems to occur, with some fish of higher trophic levels feeding both on benthic and pelagic materials. Analysis of variance showed that the interaction between the three main factors (species2size2season) significantly affects the isotopic composition of fish, suggesting the presence of intra- and inter-specific resource partitioning. Wide seasonal variations in the isotopic composition were observed in organic matter sources, invertebrates and fish, with a general trend towards depleted values in winter and enriched values in summer. The winter depletion of organic matter sources may be due to several environmental factors and seems to be mirrored in the upper trophic levels. Primary producers and invertebrates are known to have shorter time-integrated isotopic signatures than vertebrates, yet fish also exhibited seasonal isotopic differences. We concluded that the examined fish species can assume a new muscle isotopic signature relatively quickly in response to changes in the isotopic composition of their diet and/or diet shifts.     

Identification of archaeogastropod larvae from a hydrothermal vent community

Dispersal is essential in order that endemic species living in ephemeral, patchy vent environments may persist over evolutionary time. Quantitative field studies of larval dispersal, however, require specieslevel identification of the larval forms because each individual must be distinguished from related vent species, and from non-vent species living in the surrounding deep-sea environment. Methods for culturing these larvae to an identifiable stage have not yet been developed. To solve the larval identification problem for the archaeogastropod molluscs (a prominent component of vent communities), we used a scanning electron microscope (SEM) to image shells of larvae collected in the water column near vents along the East Pacific Rise (9°40′ to 9°50′N; 104°W). Larval shell size, shape and ornamentation were compared to protoconchs retained in juvenile or adult shells of identified species, and used to assign five larval groups unequivocally to species (Cyathermia naticoides Warén and Bouchet, 1989; Neomphalus fretterae McLean, 1981; Clypeosectus delectus McLean, 1989; Rhynchopelta concentrica McLean, 1989; and Lirapex granularis Warén and Bouchet, 1989) and seven groups tentatively to species or genus [Lepetodrilus spp. (three groups); Gorgoleptis sp; Peltospira ?operculata McLean, 1989; and ?Melanodrymia sp. (two groups)].     

Activities of sulfur-oxidizing bacteria at the 21°N East Pacific Rise vent site

The incorporation of inorganic and organic carbon into cell material, as well as the activities of carboxylating enzymes (ribulose bisphosphate carboxylase and phosphoenol pyruvate carboxylase), were measured in waters emitted from warm vents at the 21°N ocean spreading site (depth 2 600 m) of the East Pacific Rise. Both obligately and facultatively chemoautotrophic bacteria were present and comprised a significant but variable portion of the total microbial population as indicated by comparisons of microscopic cell counts with liquid enrichments and colony counts on media containing reduced sulfur compounds. The proportion of chemoautotrophic, sulfur-oxidizing bacteria maximally reached 79% of the total microbial population based on ribulose bisphosphate carboxylase activity. Variability of chemoautotrophic activity occurred between vents at different locations, but was also observed in one individual vent. Maximum rates of CO₂ incorporation in warm vent water were similar to levels measured previously in the O₂/H₂S interface of the Black Sea. In shipboard experiments, these rates were virtually unaffected by in-situ pressures (ca 260 atm). Rate measurements at various temperatures, as well as the observed mesophilic characteristics of all isolates obtained, suggest that the microbial, chemoautotrophic activity decreases rapidly as freshly emitted vent water is diluted with cold, ambient, deep-sea water.Contribution No. 6071 of the Woods Hole Oceanographic InstitutionContribution No. 1708 of the Center for Environmental and Estuarine Studies of the University of Maryland     

Carbon and nitrogen isotopic composition of the fauna from the Broken Spur hydrothermal vent field

 The carbon and nitrogen isotopic composition of 18 faunistic groups collected during the 39th cruise of the R.V. “Akademik Mstislav Keldysh” in September 1996 at the Broken Spur vent field, MAR, was studied. The trophic structure of the Broken Spur vent community is considered. All age stages of the shrimp Rimicaris exoculata living 5 m below the main aggregations at black smokers show higher δ¹⁵N and more depleted δ¹³C values than the same stages inhabiting the black smokers themselves. The shrimps R. exoculata and Chorocaris chacei demonstrate ontogenetic changes in δ¹³C (the former also in δ¹⁵N), with smaller individuals showing higher δ¹⁵N and more depleted δ¹³C values than larger shrimps. Benthopelagic and benthic components of the vent community differ significantly in δ¹³C and δ¹⁵N, the benthic fauna being less dependent upon chemosynthetic production. Received: 30 March 1999 / Accepted: 28 September 1999     

Long-term trends in the trophic structure of the North Sea fish community: evidence from stable-isotope analysis, size-spectra and community metrics

Fishing has wide-ranging impacts on marine ecosystems. One of the most pervasive signs of intensive fishing is "fishing down the food web", with landings increasingly dominated by smaller species from lower trophic levels. Decreases in the trophic level of landings are assumed to reflect those in fish communities, because size-selective mortality causes decreases in the relative abundance of larger species and in mean body size within species. However, existing analyses of fishing impacts on the trophic level of fish communities have focused on the role of changes in species composition rather than size composition. This will provide a biased assessment of the magnitude of fishing impacts, because fishes feed at different trophic levels as they grow. Here, we combine body size versus trophic level relationships for North Sea fishes (trophic level assessed using nitrogen stable-isotope analysis) with species-size-abundance data from two time-series of trawl-survey data (whole North Sea 1982-2000, central and northern North Sea 1925-1996) to predict long-term trends in the trophic structure of the North Sea fish community. Analyses of the 1982-2000 time-series showed that there was a slow but progressive decline in the trophic level of the demersal community, while there was no trend in the trophic level of the combined pelagic and demersal community. Analyses of the longer time-series suggested that there was no trend in the trophic level of the demersal community. We related temporal changes in trophic level to temporal changes in the slopes of normalised biomass size-spectra (which theoretically represent the trophic structure of the community), mean log₂ body mass and mean log₂ maximum body mass. While the size-based metrics of community structure showed long-term trends that were consistent with the effects of increased fishery exploitation, these trends were only correlated with trophic level for the demersal community. Our analysis suggests that the effects of fishing on the trophic structure of fish communities can be much more complex than previously assumed. This is a consequence of sampled communities not reflecting all the pathways of energy transfer in a marine ecosystem and of the absence of historical data on temporal and spatial changes in the trophic level of individuals. For the North Sea fish community, changes in size structure due to the differential effects of fishing on species and populations with different life histories are a stronger and more universal indicator of fishing effects than changes in mean trophic level.     

Long-term changes in the Georges Bank food web: trends in stable isotopic compositions of fish scales

Fish scales from seven species of demersal fish in an archival collection were analyzed for stable isotopic compositions of carbon and nitrogen to study long-term changes in trophic structure of the Georges Bank food web. Nitrogen isotopic compositions are often used to infer trophic level. In the case of haddock, Melanogrammus aeglefinus (Linnaeus), there was a trend towards feeding at 2/3 of one trophic level (2.45% in ¹⁵N) lower in 1987 than in 1929. Values of °¹³C, frequently employed to identify sources of organic carbon to consumers, declined by 1.5 from 1929 to 1960, and then increased again toward the present, suggesting changes in the food web at the level of the primary producers. Superimposed on long-term isotopic trends were short-term variations (1 to 10 yr).To identify potential causes for these isotopic trends, canonical correlation analysis was performed between isotopic data and a suite of environmental and population factors including sea surface temperature, the Greenland Regional Pressure Anomaly (GRPA), the North Atlantic Oscillation (NAO), and the following haddock stock parameters; stock size, fishing mortality, recruitment, and weight-at-age-2 (a measure of growth rate). Isotopic variation was significantly correlated with a combination of environmental and population variables: GRPA, NAO, weight-at-age-2, stock size, and fishing mortality. On the basis of published gut content analyses, the seven fish species were predicted to vary in trophic level (TL) from American plaice, Hippoglossoides platessoides (Fabricius), (TL 2.9) to summer flounder, Paralichthys dentatus (Linnaeus), (TL 4.5), whereas measured °¹⁵N values suggested smaller differences in the trophic levels of these species (less than one TL). Four species showed good agreement between gut-predicted and measured ¹⁵N values, while three species did not. Inclusion of information on ontogenetic dietary shifts in our predictions improved the agreement in some cases but not in other. Differences between stable isotope analysis and gut content analysis in terms of what they measure, i.e., integrated assimilated diet vs short-term ingested diet, respectively, may account for some of the differences in results. Based on our analyses and previous studies, feeding habits of these fish may undergo considerable year-to-year and geographic variation, some of which may have been missed in gut content analyses. To the extent that these fish are representative members of the food web, trophic variation in these fish may indicate more general changes in the food web.     

Stable carbon isotope variability in marine macrophytes and its implications for food web studies

Stable carbon isotope ratios (¹³C) of eelgrass Zostera marina and of kelp Laminaria longicruris showed considerable variation in time and space. The isotopic composition of Z. marina varied seasonally from a mean of-6 for leaves formed in June to a mean of-10 for leaves formed in February. The maximum range for individual leaves was from-5 to-11.4. Once a leaf was fully formed, its isotopic composition appeared not to change. In L. longicruris there was no clear seasonal pattern of variation, but in any given blade there was a spatial pattern of variation, with the thickened central band tending to be least negative and the margins most negative. In one blade the range was from-12 to-20. Since this range overlaps values that are found in various other macrophytes and in seston, the value of the stable carbon isotope ratio as a tracer in food webs involving macroalgae is questioned. For Z. marina, in which the values are seasonally predictable, the technique may be useful if interpreted with care. The most probable explanation of variation in the ratios is differential storage of biochemical components of different isotopic compostion.     

Effect of ambient oxygen concentration on activities of enzymatic antioxidant defences and aerobic metabolism in the hydrothermal vent worm, Paralvinella grasslei

The alvinellid Paralvinella grasslei is a common endemic polychaete from the deep-sea hydrothermal vent communities located on the East Pacific Rise (EPR). These organisms colonise a large range of microhabitats around active sites where physico-chemical conditions are thought to generate reactive oxygen species (ROS). Furthermore, in this aerobic organism, ROS could also be generated by the activity of the mitochondrial respiratory chain. In this paper, we investigated the effect of ambient oxygen concentration on the activities of three essential antioxidant enzymes (superoxide dismutase, SOD; catalase, CAT; glutathione peroxidase, GPX) and their relationships with the activity of enzymes involved in aerobic metabolism (cytochrome c oxidase, COX; citrate synthase, CS). Results of incubation of P. grasslei in a high-pressure vessel with circulating seawater at different oxygen partial pressures indicate that this worm regulates COX and CS activities differently in gills and body wall. CAT and GPX activities increase in these tissues when animals are maintained in filtered surface seawater. Moreover, levels of malondialdehyde increase in gills, testifying that oxidative damage occurs under these conditions. CAT and GPX activities are positively related to COX and CS activities, but no correlation was detected between SOD and the metabolic enzyme activities. In comparison with littoral annelids, SOD activities are very high whereas CAT activities are very low or absent in P. grasslei. The possible reasons for the occurrence of such differences are discussed.     

Trophic relationships of hydrothermal vent and non-vent communities in the upper sublittoral and upper bathyal zones off Kueishan Island,Taiwan: a combined morphological,gut content analysis and stable isotope approach

This study used morphological, gut content analysis and carbon- and nitrogen-stable isotope analysis to investigate the trophic structure of upper sublittoral (15–30 m deep) and upper bathyal (200–300 m deep) hydrothermal vents and the adjacent non-vent upper bathyal environment off Kueishan Island. The sublittoral vents host no chemosynthetic fauna, but green and red algae, epibiotic biofilm on crustacean surfaces, and zooplankton form the base of the trophic system. Suspension-feeding sea anemones and the generalist omnivorous vent crab Xenograpsus testudinatus occupy higher trophic levels. The upper bathyal hydrothermal vent is a chemoautotrophic-based system. The vent mussel Bathymodiolus taiwanensis forms a chemosynthetic component of this trophic system. Bacterial biofilm, surface plankton, and algae form the other dietary fractions of the upper bathyal fauna. The vent hermit crab Paragiopagurus ventilatus and the vent crab X. testudinatus are generalist omnivores. The vent-endemic tonguefish Symphurus multimaculatus occupies the top level of the trophic system. The adjacent non-vent upper bathyal region contains decapod crustaceans, which function as either predators or scavengers. The assemblages of X. testudinatus from sublittoral and upper bathyal vents exhibited distinct stable isotope values, suggesting that they feed on different food sources. The upper bathyal Xenograpsus assemblages displayed large variations in their stable isotope values and exhibited an ontogenetic shift in their δ¹³C and δ¹⁵N stable isotope signatures. Some individuals of Xenograpsus exhibited δ¹⁵N values close to those of non-vent species, suggesting that the highly mobile Xenograpsus may transfer energy between the upper bathyal hydrothermal vents and the adjacent non-vent upper bathyal environment.