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1. We derived a functional response function that accounted for both predator and prey size. Laboratory experiments were then performed to investigate the size relationships of the functional response of the benthic isopod Saduria entomon (L.) feeding on Monoporeia affinis (Lindström) amphipods. 2. The S. entomon predators, in general, showed sigmoid (type III) functional responses. The attack rate of S. entomon was a bent ridge-shaped function of predator and prey size. The maximum attack rates of small predators were found on small prey, and then attack rate declined rapidly as prey size increased. In large predators attack rate increased slowly up to a maximum for large prey. The shape of the attack rate was dependent on the size scaling of predator and prey body mass in the attack rate function. We suggest that the ridge-shaped attack rate function found in our study is likely to be general for predators that feed on prey with a large size range. The handling time of small S. entomon increased rapidly as prey size increased. In contrast, we found only small differences in handling time of large predators. The complete functional response may provide size-refuges for very small and large M. affinis depending on the size-structure of S. entomon. 3. To investigate the realism in our functional response estimates, we incorporated the functional response in a consumption model based on the densities and size-structures of the predator and the prey in the field (Bothnian Sea). We adjusted the attack rate using a correction factor to predict the observed survival of the different age-classes of M. affinis from field samples. The field data based consumption model accounted for 1 year of size-dependent predation by the size-structured population of S. entomon on M. affinis. In addition, using a growth model for S. entomon we checked that the predicted consumption by the predator gave realistic growth curves. We found that the experimentally derived attack rates may have been over-estimated by a factor of 400.

Knowledge about ecological interactions between species is of paramount importance in ecology and ecosystem-based fisheries management. To understand species interactions, studies of feeding habits are required. In the Baltic Sea, there is good knowledge of the diet of cod, but little is known about the diet of flounder, the second most abundant demersal fish in the region. In this study, we investigated the diets of cod and flounder for the first time using stomach content data collected simultaneously in 2015–2017 over a large offshore area of the southern Baltic Sea. The diet of flounder was relatively constant between sizes and seasons and was dominated by benthos, with a high proportion in weight of the benthic isopod Saduria entomon. The diet of cod differed between seasons and showed an ontogenetic shift with a relative decrease of benthic prey and an increase of fish prey with size. Historic diet data of cod were used to explore cod diet changes over time, revealing a shift from a specialized to generalist feeding mode paralleled by a large relative decline in benthic prey, especially S. entomon. Flounder populations have increased in the past 2 decades in the study area, and therefore we hypothesized that flounder have deprived cod of important benthic resources through competition. This competition could be exacerbated by the low benthic prey productivity due to increased hypoxia, which could contribute to explaining the current poor status of the Eastern Baltic cod. The results of this study point to the importance of including flounder in multispecies end ecosystem models.

Intraspecific competition has been shown to favor diet specialization among individuals. However, the question whether the competition takes the form of interference or exploitative in driving diet specialization has never been investigated. We investigated individual diet specialization in the isopod Saduria entomon, in relation to forager and resource biomasses in a system that exhibits predator-prey fluctuations in density. We found that individual diet specialization was only affected by the biomass of their preferred prey (Monoporeia affinis) and not by Saduria biomass; diet specialization was higher when Monoporeia biomass was low compared to when there were high Monoporeia biomass. Population diet breadth increased at low Monoporeia biomass whereas individual diet breadths were marginally affected by Monoporeia biomass. Overall, this led to the increase in diet specialization at low Monoporeia biomass. This study shows that predator-prey dynamics might influence diet specialization in the predator and that resource biomass, not forager biomass might be important for individual diet specialization.

This study addresses the problem of concordance in fluctuating asymmetry (FA) across traits by analyzing the relationship between FAs and the mean values of character measurements in a set of morphological traits. Regression slopes vary in natural populations, thus, revealing discordance in FA across traits among these populations. Hence, commonly accepted techniques for measuring developmental instability with FA result in uncertainties. Here, I relate FA to mean as a two-dimensional complex to demonstrate the uniformly negative slopes of standardized FA vs. mean value regressions for sets of morphological traits from eighteen distinct natural marine and aquatic populations. Comprehensive analysis of the FA–mean complex cannot be recommended for wide use in assessing stress and fitness, but it offers promise to improve FA measuring methodologies and to better understand the nature of developmental instability.

Environmental genotoxicity in the Gulf of Riga was assessed using different bioindicators (fish, clams, and isopods) collected from 14 study stations. Comparison of genotoxicity responses (micronuclei (MN) and nuclear buds (NB)) in blood erythrocytes of herring ( Clupea harengus ), eelpout ( Zoarces viviparous ), and flounder ( Platichthys flesus ) revealed the species- and site-specific differences. For the first time, the analysis of genotoxicity was carried out in gill cells of isopods Saduria entomon . The highest inductions of MN and NB in gill cells of investigated S. entomon and clams ( Macoma balthica ) were evaluated in specimens from station 111A (offshore zone). In fish, the highest incidences of MN were measured in eelpout and in herring collected in the southern part of Gulf of Riga (station GOR3/41S). Moreover, in the southern coastal area, the assessment of genotoxicity risk (according to micronuclei levels) indicated exceptionally high risk for flounder, eelpout, and clams.

The parasite fauna of cod (Gadus morhus) is well described, but the life cycles of Baltic cod parasites are known only in general terms. Invertebrates commonly found in the stomach of cod are recognized as intermediate hosts in the life cycles of nematodes or acanthocephalans. The aim of this study was to determine the source of infection of Baltic cod with parasites found in situ in invertebrates present in the cod stomach. Our results indicate that Saduria entomon is both a source of infection of Baltic cod with parasites and an intermediate host in the life cycle of Hysterothylacium aduncum in the Baltic Sea.

Major climate changes are underway in the Canadian Arctic, but our ability to monitor and predict their impact on faunal community structure is hindered by the lack of baseline diversity data. This study combined megabenthic community data sampled at 78 stations from 2007 to 2011 across the Western and Eastern Canadian Arctic biogeographic units. These large biogeographic units were divided into five geographical regions to provide regional estimates of observed and predicted taxon richness. We did not detect a strong regional difference in benthic community characteristics, observing only a lower richness in the Amundsen Gulf region than in the neighboring Beaufort Sea region. The Amundsen Gulf region had the highest turnover (beta) diversity, coincident with high environmental heterogeneity. The strong and distinctive presence in the Beaufort Sea region of Saduria spp., a euryhaline isopod, demonstrated the particular influence of the Mackenzie River on the community composition ofthat region. Our analysis showed that in various regions, about 34% to 59% of megabenthic taxa in Canadian Arctic waters are still to be documented. This study provides useful baseline data for both national and pan-Arctic evaluations of benthic diversity in the Arctic Ocean. Des changements climatiques majeurs sont en cours dans l'Arctique canadien, mais notre capacité à surveiller et à prévoir leurs impacts sur la structure des communautés est entravée par le manque de données de référence sur la diversité. Cette étude combine des données sur les communautés mégabenthiques échantillonnées à 78 stations de 2007 à 2011 à l'intérieur des unités biogéographiques de l'ouest et de l'est de l'Arctique canadien. Ces grandes unités biogéographiques ont été divisées en cinq régions géographiques afin de fournir des estimations régionales de richesse taxonomique observée et prédite. Nous n'avons pas détecté de fortes différences régionales dans les caractéristiques des communautés, benthiques. Seule la richesse observée est inférieure dans le golfe d'Amundsen par rapport à la région voisine de la mer de Beaufort. La région du golfe d'Amundsen a la diversité bêta la plus élevée, ce qui coïncide avec une grande hétérogénéité de conditions environnementales. La composition taxonomique de la mer de Beaufort est différente de celle des autres régions. La présence distinctive et forte de Saduria spp., un isopode euryhalin, dans la région de la mer de Beaufort illustre l'influence du fleuve Mackenzie sur la composition taxonomique de cette région. Notre analyse démontre que régionalement, environ 34 % à 59 % des taxa mégabenthiques restent à être répertoriés dans les eaux arctiques canadiennes. Cette étude fournit des données de référence utiles pour les évaluations nationales et panarctiques de la diversité benthique de l'océan Arctique.

Benthic fauna in Tsivolki Bay (Novaya Zemlya Archipelago, Kara Sea) has been studied during the voyage of the R/V Professor Shtokman in 2013 and 2014. A peculiar feature of the bay is the presence of the Serp i Molot glacier in its inner part, which determines the extremely high content of suspended particles in the water column. The bay is divided into three parts: the inner part (close to the glacier), the middle basin, and the outer slope. These parts are separated from each other by several rises. Benthic communities changed gradually from the inner part of the bay towards the outer slope. Three communities were described from the data of nine grab stations (26 samples). The apex of the bay is occupied by the depleted community dominated by the isopod Saduria sabini and the bivalve Yoldiella lenticula , which can successfully survive the increased mineral sedimentation. In the middle basin, it is replaced by the transitional community with Ennucula tenuis and Portlandia arctica being the main dominants. Finally, the outer slope is inhabited by the community typical for the open parts of the Kara Sea. It is dominated by Astarte crenata, Ophiacantha bidentata , and Ophiopleura borealis . The main reason for macrobenthic distribution in the studied region is the content of mineral particles in the water column and bottom layers.

Representatives of at least six crustacean taxa managed to establish a terrestrial life style during their evolutionary history and the Oniscidea (Isopoda) are currently held as the most successfully terrestrialized malacostracan crustaceans. The brain architecture of terrestrial isopods is fairly well understood and studies on this field suggest that the evolutionary transition from sea to land in isopods coincided with a considerable size reduction and functional loss of their first pair of antennae and associated brain areas. This finding suggests that terrestrial isopods may have no or poor abilities to detect volatile substances but that their chemosensory ecology is most likely restricted to contact chemoreception. In this study, we explored how the brain of a marine isopod and particularly its olfactory system compares to that of terrestrial relatives. Using histochemical and immunohistochemical labeling, brightfield and confocal laser-scan microscopy, we show that in the marine isopod Saduria entomon aesthetascs on the first pair of antennae provide input to a well defined deutocerebrum (DC). The deutocerebral chemosensory lobes (DCL) are divided into spherical neuropil compartments, the olfactory glomeruli (og). Secondary processing areas in the lateral protocerebrum (lPC) are supplied by a thin but distinct projection neuron tract (PNT) with a contralateral connection. Hence, contrary to terrestrial Isopoda, S. entomon has at least the neuronal substrate to perceive and process olfactory stimuli suggesting the originally marine isopod lineage had olfactory abilities comparable to that of other malacostracan crustaceans.

Background. At the end of 1980s, ecological regime shifts occurred in the Baltic. Since, the Baltic fish assemblage has been dominated by sprats, whereas before cod was the dominant fish species. The majority of previous food studies refer to the period before the shift. This raises the question, how changes in the Baltic ecosystem have affected the diet of cod. The aim of the presently reported study was to identify the differences in the food composition of cod depending on the area, depth, season, and the cod size and compare it with the findings from the 1970s and 1980s. Materials and methods. Food composition and selection of prey size for Baltic cod was evaluated by examining the stomach contents of 556 adult cod, collected in 2006 and 2007. Fish stomachs were sampled from research catches within the Polish Exclusive Economic Zone. Results. The main component of the cod diet were the clupeids, constituting more than 67% of the total weight of the food recovered from the specimens studied. Other fishes in the diet represented Gobiidae, Ammodytidae, as well as young cod. The dominant invertebrate in the diet was the isopod Saduria entomon which accounted for 13% of the diet by weight. The weight proportion of larger fish, such as herring, Clupea harengus (L.), and cod, increased with the cod size, while the proportion of small- and medium-sized fish (sprats, gobies) decreased. The food composition of cod varied seasonally. The weight proportion of European sprat, Sprattus sprattus (L.), was much higher in winter (55%) than in fall (27%), while the proportion of cod in the diet was higher in the fall (10%) than in winter (4%). The proportion of sprat in the diet increased with depth, while that of Ammodytidae and Gobiidae showed a decreasing trend. At depths greater than 40 m, the proportion of invertebrates in cod stomachs decreased. This study demonstrated a significant area effect for sprat; year effect for sprat and Saduria entomon; a seasonal effect for young cod and S. entomon; and a depth effect for sprat, young cod, and S. entomon. The occurrences of herring and small cod increased in the stomach as the predator increased, while no significant relation was found for sprat and S. entomon. The data collected do not indicate that cod select a specific size of sprat or herring, though cod were found to use size selection for S. entomon and juvenile cod. Conclusion. The food composition of cod has undergone certain changes compared to results of Zalachowski (1977, 1985), which covered the 1970s and 1980s. Currently, the proportion of clupeids in the food is approximately 67% (in weight), while during the 1977-1982 it was in the range of 25%-50% depending on cod length. In 2006 and 2007 sprat was the main cod diet component rather than herring.