Nematodes and meiofauna in sediments obtained from RRS Charles Darwin cruise CD179, Portuguese continental margin

Samples collected at two different depths (ca. 3200 and ca. 4200 m) in the Setúbal and Cascais canyons off the Portuguese coast, during the HERMES RRS Charles Darwin cruise CD179, were analysed for (1) sediment biogeochemistry (TOC, TN) and (2) composition, and structural and trophic diversity of nematode communities. Multivariate PERMANOVA analysis on the nematode community data revealed differences between sediment layers that were greater than differences between canyons, water depths, and stations. This suggests that biogeochemical gradients along the vertical sediment profile are crucial in determining nematode community structure. The interaction between canyon conditions and the nematode community is illustrated by biogeochemical patterns in the sediment and the prevalence of nematode genera that are able to persist in disturbed sediments. Trophic analysis of the nematode community indicated that non-selective deposit feeders are dominant, presumably because of their non-selective feeding behaviour compared to other feeding types, which gives them a competitive advantage in exploiting lower-quality food resources. This study presents a preliminary conceptual scheme for interactions between canyon conditions and the resident fauna.

Nematodes and meiofauna in sediments obtained from RRS Discovery cruise D297, Portuguese continental margin

Samples collected in the deep Nazaré Canyon and at the adjacent slope, during the HERMES RRS Discovery D297 cruise (2005), were analysed for metazoan meiofauna, nematode structure and diversity and its relation to quality and quantity of sedimentary organic material. The amount and quality of organic matter available for direct consumption was much higher in the canyon compared to the slope and positively correlated with high nematode abundances (795-1171 ind. 10 cm**-2) and biomass (93.2-343.5 µg dry weight 10 cm**-2), thus leading to higher standing stocks. Canyon nematode assemblages also showed particular adaptations (e.g. higher trophic complexity, variability of nematode morphology, and presence of opportunistic genera) to canyon conditions, particularly in the deeper sediment layers. The Nazaré Canyon's nematode diversity was slightly lower than that of the adjacent slope and its assemblages were characterised by a higher dominance of certain genera. Still, the canyon contributes considerably to total Western Iberian Margin diversity due to different assemblages present compared to the slope. Furthermore, the harsh conditions in terms of hydrodynamic disturbance and the high organic matter flux are likely to have a negative impact on the establishment of species rich meiobenthic communities, especially in the canyon axis.

Nematodes and meiofauna in sediments obtained during Belgica cruise BG06/13, north-east Atlantic

The Gollum Channels and Whittard Canyon (NE Atlantic) are two areas that receive high input of organic matter and phytodetritus from euphotic layers, but they are typified by different trophic and hydrodynamic conditions. Sediment biogeochemistry was analysed in conjunction with structure and diversity of the nematode community and differences were tested between study areas, water depths (700 m vs 1000 m), stations, and sediment layers. The Gollum Channels and Whittard Canyon harboured high meiofauna abundances (1054-1426 ind. 10 cm**-2) and high nematode diversity (total of 181 genera). Next to enhanced meiofauna abundance and nematode biomass, there were signs of high levels of organic matter deposition leading to reduced sedimentary conditions, which in turn structured the nematode community. Striking in this respect was the presence of large numbers of 'chemosynthetic' Astomonema nematodes (Astomonema southwardorum, Order Monhysterida, Family Siphonolaimidae). This genus lacks a mouth, buccal cavity and pharynx and possesses a rudimentary gut containing internal, symbiotic prokaryotes which have been recognised as sulphur-oxidising bacteria. Dominance of Astomonema may indicate the presence of reduced environments in the study areas, which is partially confirmed by the local biogeochemical environment. The nematode communities were mostly affected by sediment layer differences and concomitant trophic conditions rather than other spatial gradients related to study area, water depth or station differences, pointing to small-scale heterogeneity as the main source of variation in nematode structure and function. Furthermore, the positive relation between nematode standing stocks, and quantity and quality of the organic matter was stronger when hydrodynamic disturbance was greater. Analogically, this study also suggests that structural diversity can be positively correlated with trophic conditions and that this relation is tighter when hydrodynamic disturbance is greater.

Stomach content and parasite characteristics of M. scorpius from Frobisher Bay, Baffin Island

Shorthorn sculpin (Myoxocephalus scorpius) from Frobisher Bay, Baffin Island, is a slow growing long-lived species. A wide range of diet items were present in the stomachs of the shorthorn sculpins sampled but 2-3 diet items (amphipod species) comprised 99.5 % of total food consumed. These amphipods were present in the stomachs in similar proportions among all age classes of shorthorn sculpin. Several new host records for parasites were reported and mean numbers of parasite species increased with shorthorn sculpin age. The increased diversity of parasite species and higher d15N values in older/larger individuals suggest that their diets were more diverse and the prey items consumed had higher d15N values. By contrast, the value of d13C in dominant diet items masked the d13C values of minor diet items. We conclude that parasites and stable isotope values provide complementary data on feeding patterns of the shorthorn sculpin. The ubiquitous marine acanthocephalan, Echinorhynchus gadi, was found at high prevalences (87-100 %) and mean intensities (28-35), and were localized in the midgut. In contrast to other studies on acanthocephalans, E. gadi did not influence fish condition as measured by condition factor, liver somatic and gonado-somatic indices.

The effect of repeated exposure to parasite genotypes of the eyefluke Diplostomum pseudospathaceum on infection rate and immunization in three-spined sticklebacks

Adaptive immunity in vertebrates can confer increased resistance against invading pathogens upon re-infection. But how specific parasite genotypes affect the transition from innate to adaptive immunity is poorly understood. Here, we investigated the effects of homologous and heterologous exposures of genetically distinct parasite lineages of the eye fluke Diplostomum pseudospathaceum on gene expression patterns of adaptive immunity in sticklebacks (Gasterosteus aculeatus). We showed that observable differences were largely attributable to final exposures and that there is no transcription pattern characteristic for a general response to repeated infections with D. pseudospathaceum. Final exposure did not unify expression patterns of heterologous pre-exposed fish. Interestingly, heterologous final exposures showed similarities between different treatment groups subjected to homologous pre-exposure. The observed pattern was supported by parasite infection rates and suggests that host immunization was optimized towards an adaptive immune response that favored effectiveness against parasite diversity over specificity.

Results of an isotope tracer experiment with arctic deep-sea nematodes

A stable isotope (13C)-labeling experiment was performed to quantify the importance of bacterial carbon as a food source for an Arctic deep-sea nematode community. Bacterial functional groups were isotopically enriched with 13C-glucose, 13C-acetate, 13C- bicarbonate, and 13C-amino acids injected into sediments collected from 1280 m depth at 79uN, 6uE, west of Svalbard. Incorporation of the 13C label into bacterial phospholipid-derived fatty acids (PLFAs) and nematodes in the top 5 cm of the sediment was monitored over a 7-d period. The 13C dynamics of nematodes was fitted with a simple isotope turnover model to derive the importance of the different bacterial functional groups as carbon sources for the nematodes. The different substrates clearly labeled different bacterial groups as evidenced by differential labeling of the PLFA patterns. The deep-sea nematode community incorporated a very limited amount of the label, and the isotope turnover model showed that the dynamics of the isotope transfer could not be attributed to bacterivory. The low enrichment of nematodes suggests a limited passive uptake of injected 13C-labeled substrates. The lack of accumulation suggests that the injected 13C-labeled dissolved organic carbon compounds are not important as carbon sources for deep-sea nematodes. Since earlier studies with isotopically enriched algae also found limited uptake by nematodes, the food sources of deep-sea nematodes remain unclear.