Description: Genetic variability of marine fish species is much higher than in most other vertebrates. Nevertheless, some species with large population sizes including flatfish such as European plaice Pleuronectes platessa show signs of population collapse and inbreeding. Taking plaice as a flagship example for fisheries-induced genetic changes also affecting the Wadden Sea, we determined the amount of genetic variability at antigen-presenting genes of the Major Histocompatibility Complex (MHC) and its potential interaction with the microbiota associated to gill tissue using a next-generation parallel tag sequencing approach. Genetic variation at MHC class IIB genes was extremely large, with 97 alleles found in 40 fish from different age cohorts. Although a strong signal of positive selection was present (dN/dS = 4.01) and we found significantly higher allelic diversity in 0+ fish than in older age classes, the amount of genetic variation maintained within the population may not have exceeded neutral expectations derived from mitochondrial markers. Associated microbes revealed significant spatiotemporal structure with 0+ fish displaying the highest microbial diversity as well as the highest diversity of potentially pathogenic genera. Overall the correlation between MHC genotypes and bacterial abundance was weak, and only few alleles significantly correlated with certain bacterial genera. These associations all conferred susceptibility (i.e. presence of an allele correlated to higher number of bacteria), either suggesting age-dependent selection on common alleles or weak selection on resistance against these bacterial genera. Taken together, our data suggest that selection coefficients of balancing selection maintaining immunogenetic diversity may be relatively small in large marine populations. However, if population sizes are further reduced by overharvesting, the response to increasing balancing selection coefficients will be largely unpredictable and may also negatively influence the adaptive potential of populations. Highlights ► Diversity of MHC class IIB immune genes of European plaice was very high. ► Positive selection ultimately formed the high diversity found in European plaice. ► 0+ fish displayed higher allelic diversity than 1+ fish. ► Gill microbial communities varied between trawl sites.

Description: Ecological network analysis (ENA) was used to study the effects of Pomatoschistus microps on energy transport through the food web, its impact on other compartments and its possible role as a keystone species in the trophic webs of an Arenicola tidal flat ecosystem and a sparse Zostera noltii bed ecosystem. Three ENA models were constructed: (a) model 1 contains data of the original food web from prior research in the investigated area by Baird et al. (2007), (b) an updated model 2 which included biomass and diet data of P. microps from recent sampling, and (c) model 3 simulating a food web without P. microps. A comparison of energy transport between the different models revealed that more energy is transported from lower trophic levels up the food chain, in the presence of P. microps (models 1 and 2) than in its absence (model 3). Calculations of the keystone index (KSi) revealed the high overall impact (measured as eps_i) of this fish species on food webs. In model 1, P. microps was assigned a low KSi in the Arenicola flat and in the sparse Z. noltii bed. Calculations in model 2 ranked P. microps first for keystoneness and eps_i in both communities, the Arenicola flat and the sparse Z. noltii bed. Taken together, our results give insight into the role of P. microps when considering a whole food web and reveal direct and indirect trophic interactions of this small-sized fish species. These results might illustrate the impact and importance of abundant, widespread species in food webs and facilitate further investigations.

Description: This study was conducted in the Sylt-Rømø bight (SRB) (54°52' N - 55°10' N, 8°20' E - 8°40' E), which is situated between the islands of Sylt (Germany) and Rømø (Denmark). Fish, meio- and macrofauna were sampled employing block nets and tube cores in a S. muticum forest in 2010. Additionally the meio- and macrofauna attached on thalli of Sargassum muticum were collected. Sampling data including Biomass, length weight relationships, and gut content analyses, together with literature data were used to calculate necessary parameters (Biomass, Respiration, Consumption, Production, Egestion) for each component of the food web. Carbon was used as currency to measure the flow of energy between the single components of the food web. To calculate the flows of energy, ash free dry weight (AFDW) to carbon (C) ratios were employed. Flows of energy inside the food web were calculated employing the routines of the program Netwrk 4.2 (Ulanowicz and Kay, 1991; Ulanowicz et al., 2005).