Size-selective predation by fourhorn sculpin,Myoxocephalus quadricornis (L.) (Pisces) on Mesidotea entomon (L.) (Crustacea,Isopoda)

During three different seasons Mesidotea entomon specimens from fourhorn sculpin stomachs were analyzed, and compared with the M. entomon population in the field. The field samplings were carried out in the northern Bothnian Sea. Feeding experiments revealed fourhorn sculpins to be highly selective when feeding on M. entomon.The fourhorn sculpin most preferably selected large M. entomon in the field as well as in the laboratory experiments. The preference for large M. entomon remained after correction for availability of differently sized M. entomon. In summer the actual sizes of M. entomon eaten by fourhorn sculpin were smaller than in autumn and winter. During all three seasons the oldest M. entomon were the ones most preferred. Ultimate effects of predation by fourhorn sculpin on the life-history of M. entomon are discussed.     

Effects of large Saduria entomon (Isopoda) on spatial distribution of their small S. entomon and Monoporeia affinis (Amphipoda) prey

We performed laboratory experiments to investigate the effects of predator avoidance and numerical effects of predation on spatial distribution of small Saduria entomon (Isopoda) and Monoporeia affinis (Amphipoda), with large S. entomon as predators. The horizontal distribution and mortality of the prey species, separately and together, were studied in aquaria with a spatial horizontal refuge. We also estimated effects of refuge on mortality of small S. entomon and M. affinis by experiments without the refuge net. In addition, we investigated whether predation risk from large S. entomon influenced the swimming activity of M. affinis, to clarify the mechanisms behind the spatial distribution. Both small S. entomon and M. affinis avoided large S. entomon. The avoidance behaviour of M. fffinis contributed about 10 times more to the high proportion in the refuge than numerical effects of predation. Due to the low mortality of small S. entomon the avoidance behaviour of this species was even more important for the spatial distribution. The combined effect of avoidance behaviour and predation in both species was aggregation, producting a positive correlation between the species in density. M. affinis showed two types of avoidance behaviour. In the activity experiments they reduced activity by 36% and buried themselves in the sediment. In the refuge experiments we also observed avoidance behaviour with the emigration rate from the predator compartment being twice the immigration rate. The refuge did not lower predation mortality in M. affinis, probably due to the small scale of the experimental units in relation to the mobility of the species. Predation mortality in small S. entomon was higher in absence of a refuge and especially high in absence of M. affinis.     

Direct and indirect effects of predation by Saduria entomon (Isopoda) on the size-structure of Monoporeia affinis (Amphipoda)

We performed a 6-month laboratory experiment to investigate the direct and indirect effects of predation by the benthic invertebrate predator Saduria entomon on the growth and survival of Monopreia affinis prey individuals in different age-cohorts at low and high prey densities. The experimental results were compared with changes of growth and abundance in corresponding age-cohorts of M. affinis at sites with different S. entomon and M. affinis densities in the deep sublittoral zone of the Bothnian Sea during the same year. In the experiment, the presence of S. entomon reduced growth rate of M. affinis in the 1-year and 2-year age-cohorts at low amphipod density. Increased refuge use by M. affinis, expressed as a decrease in swimming activity in the presence of S. entomon, is suggested to have reduced feeding rate and therefore growth of the amphipods. The recruitment of M. affinis offspring was reduced in presence of S. entomon. In the field, the growth rate of amphipods in the 1-year cohort increased with increasing S. entomon density at low amphipod density. We found no corresponding increase of M. affinis growth in the 2-year cohort. The positive effect on 1-year amphipod growth indicated that predation reduced intra-cohort competition of M. affinis and increased growth of surviving prey at high predator density. In both the experiment and the field data we found indications of size-selective predation on smaller M. affinis specimens. This was because of the changed ratio between number of individuals in the juvenile age-cohorts and lower recruitment of amphipod offspring connected to S. entomon density. The experimental results and field data suggest that predation by S. entomon may have both direct and indirect effects on the size-structure of M. affinis populations. Received: 22 June 1998 / Accepted: 15 March 1999     

Predation by the isopod Saduria entomon on the amphipods Monoporeia affinis and Pontoporeia femorata: experiments on prey vulnerability

Summary Predation by Saduria entomon on the depositfeeding amphipods Monoporeia (=Pontoporeia) affinis and Pontoporeia femorata was studied in laboratory experiments. Prey vulnerabilities were compared in singleprey treatments and in mixed-prey treatments, where the proportions of the two species were varied at one total density. In a pilot experiment, P. femorata was the more vulnerable prey, both in single-prey and mixed-prey treatments. In later experiments, the amphipod species were equally vulnerable in single-prey treatments, while in mixed-prey treatments Saduria preyed preferentially on M. affinis, even when this prey was rare, i.e. it did not switch. We suggest that the different result in the later experiments may have been due to a change in the search mode of the isopods.     

Why do juvenile fourhorn sculpin, Triglopsis quadricornis, appear in the pelagic habitat at night?

Synopsis Challenging the generally accepted view of the fourhorn sculpin Triglopsis quadricornis as a cold-stenothermal and true benthic species, novel findings of juveniles in the pelagic habitat of Lake Vättern, Sweden, have been examined and critically evaluated. Young-of-the-year and small-sized, sexually mature individuals, ranging 27–45 and 82–110 mm in total length, were recorded in nocturnal trawl catches from pelagic midwater in August. The sculpin had fed extensively on both benthic and semi-pelagic crustaceans, including large cyclopoid copepods, Mysis relicta, Pallasea quadrispinosa, Monoporeia affinis, Gammaracanthus lacustris, and Saduria entomon. Based on an extensive literature review a series of ecological and evolutionary reasons for a pelagic behavior of this profundal coldwater fish is suggested. Nocturnal, semi-pelagic feeding, and consecutive, thermotactic, vertical migrations up in midwater as shown by Wurtsbaugh & Neverman (1988) in order to maximize energy intake after feeding, are possible adaptations of these juvenile sculpin for minimizing size-dependent mortality due to intra- and interspecific predation and competition from other deepwater fishes.     

Chemical composition of antarctic zooplankton during austral fall and winter

Water level, ash content, proximate (protein, lipid, carbohydrate and chitin) and elemental (carbon and nitrogen) composition were analyzed in twentythree species of Antarctic Zooplankton collected during the austral fall (1986) and winter (1988) from the Scotia/Weddell Sea region. Extremes in water level, ash content and organic components were typified by copepods and gelatinous forms. Ostracods and polychaetes were generally similar in composition to copepods, being only slightly higher in water level and ash content. Chaetognaths exhibited a composition intermediate in character with some components similar in value to that shown by crustaceans (i.e. protein) while other components were more in the range of values seen in gelatinous forms (i.e. water level and ash content). Protein was the major proximate component and measured values (as % Afdw) were fairly uniform among non-gelatinous species (x=33.9±6.9). Lipid levels were variable, with high values (>30% AFDW) only found for the copepods Calanoides acutus, Calanus propinquus and Euchaeta antarctica. Carbohydrate values were low in all species examined. Chitin was measured in crustacean species only. With the exception of C. acutus (x=2.5% AFDW chitin), values were similar among species with mean values being slightly higher in fall (x=11.8±2.5) than in winter (x=6.7±1.8). Among non-gelatinous species, the ratio of carbon to nitrogen was positively correlated with the lipid to protein ratio, underscoring the compositional association between elemental and proximate components in these groups. In gelatinous species, the relationship between carbon:nitrogen and lipid:protein was inconsistent and less pronounced. Caloric content was estimated from recovered organic matter for nongelatinous species. As a function of wet weight and dry weight, values reflected differences in water level and ash content among individual species. As a function of ashfree dry weight, values were similar among all species (x=3.6±0.9 kcal/g).Seasonal comparisons were possible for 12 of the 23 species. Among crustaceans, changes in water level and organic components were variable reflecting dissimilar trophic, reproductive or ecological habits among different species. Essentially no change in composition between fall and winter was observed for diapause species (e.g. Calanoides acutus and Rhincalanus gigas) as well as for omnivorous/ carnivorous species (e.g. Gaetanus tenuispinus). Conversely, large compositional changes were evident for Calanus propinquus, a small-particle grazer that relies heavily on lipid reserves. Chaetognaths and some gelatinous species exhibited a considerable decrease in ash content from fall to winter which, for most cases, was mirrored by some degree of increase in lipid level. At present, however, scant data are available to help explain the observed patterns of compositional change within non-crustacean species.     

Ontogenetic diet shifts of bull-rout, Myoxocephalus scorpius (L.), in the south-western Baltic Sea

The stomach contents of 533 bull‐rout, Myoxocephalus scorpius (L.), were collected during February–March and October–November 1997 in the south‐western Baltic Sea in order to study the feeding ecology and investigate diet changes between seasons and with the increasing size of the fish. The isopod Mesidotea entomon, and herring Clupea harengus, were the dominant species in the diet, accounting for 99% in weight of all bull‐rout sampled in spring and autumn. Mysis mixta was frequent but less important in weight; analysis of the ontogenetic diet shift showed the presence of M. mixta essentially in the smaller size‐classes. Herring was eaten almost exclusively by individuals larger than 21 cm total length; its importance increased with increasing size of bull‐rout. The isopod M. entomon was abundant in all size‐classes examined. Prey‐size and the size of the principal prey, M. entomon, showed a significant increase with increasing bull‐rout size. The diet shift from mysids to fish was followed by a change in the weight‐length relationship. Cluster analysis confirmed two distinct size groups of >26 cm and <24 cm. In conclusion, the data indicate that the diet of bull‐rout partially overlaps with that of herring in the first trophic stage, and with that of cod in the second part of the bull‐rout life cycle.     

Divergence in host use ability of a marine herbivore from two habitat types

We studied ecological divergence of host use ability in a generalist marine herbivore living in two distinct host plant assemblages. We collected Idotea balthica isopods from three populations dominated by the brown alga Fucus vesiculosus and three dominated by the seagrass Zostera marina. In two reciprocal common garden feeding experiments for adult and laboratory‐born juvenile isopods, we found that isopods from both assemblages performed better with their sympatric dominant host species than did isopods allopatric to this host. This indicates parallel divergence of populations according to the sympatric host plant assemblage. Furthermore, initial body size and body size‐dependent mortality differed between populations from the two assemblages. In nature, this may result in lower fitness of immigrants compared with that of residents and consequently reinforce divergence of the populations. Finally, we discuss how phenotypic plasticity and maternal and random effects may associate with the results.     

Comparative studies on membranes isolated from Rhodopseudomonas viridis grown in the presence and in the absence of yeast extract

Rhodopseudomonas viridis was grown in the presence and in the absence of yeast extract. The cells grown under this latter condition present a ten fold diminished bacteriochlorophyll (bchl) content. This decrease was paralleled by a similar increase in the ratio lipid phosphorous/bchl, whereas the ornithine lipid/bchl ratio remains constant. Some quantitative differences in the fatty acid composition are also reported.The protein composition of both membranes was also studied, only indicating quantitative differences.An active reaction center preparation was obtained from both types of cells. When isolated from cells grown in the presence of yeast extract, this reaction center preparation shows the presence of proteins a, b, c and d. Further treatment of this active reaction center results, in cells grown under either condition, in the isolation of green (oxidized behl) and brown (inactive reaction center) bchl containing fractions.The protein composition and absorption spectrum of the inactive reaction centers obtained from both types of cells were identical (proteins a, c and d).On the other hand the green complexes differ in their protein composition as well as in their absorption spectrum.     

Metabolic adaptations for isopod specialization in three species of Dysdera spiders from the Canary Islands

The spider genus Dysdera is considered to comprise specialist isopod feeders, although the degree of specialization varies between species, depending on morphological (shape of chelicerae), behavioural (attack tactics) and metabolic (food quality of prey) adaptations. Dysdera has radiated extensively in the Canary Islands (currently 47 endemic species are described) and codistributed species have different cheliceral shapes and body sizes indicating different feeding niches. In the present study, we investigate the existence of metabolic adaptations to feeding on isopods by three endemic species (Dysdera insulana Simon, Dysdera macra Simon and Dysdera verneaui Simon) from Tenerife. We hypothesize that there is enhanced extraction efficiency of fundamental macronutrients from isopods compared with control prey in species with special morphological and behavioural adaptations for this prey type. We measure quantitatively spider growth, dry mass consumption, lipid and nitrogen consumption, and calculate growth efficiency and efficiency of utilization of dry mass, lipid and nitrogen. The results show that all three species are able to utilize both prey types, indicating that none of them are strict isopod specialist. Dysdera insulana shows enhanced growth efficiency and D. macra shows enhanced nitrogen extraction efficiency compared with D. verneaui when feeding on Porcellio rather than on Musca. Both traits indicate likely adaptations for the utilization of isopods. Spider species, sex and prey type all affect lipid and nitrogen extraction efficiencies, indicating that spiders do not simply extract nutrients in the proportions available. The results support the hypothesis that adaptations for enhanced digestion of focal prey evolve in species that already have adaptations for enhanced capture success.     

Effects of intrinsic and extrinsic factors on the diet of Bathyraja macloviana,a benthophagous skate

The effects of intrinsic (sex, maturity stage and body size) and extrinsic (depth and region) factors on the diet of Bathyraja macloviana, in the south‐west Atlantic Ocean, were evaluated using a multiple‐hypothesis modelling approach. Bathyraja macloviana fed mainly on polychaetes followed by amphipods, isopods and decapods. Effects of intrinsic and extrinsic factors on diet composition of this species were found. The consumption of polychaetes had a humped relationship with total length (L_T), and isopods and decapods increased with increasing L_T. Immature individuals preyed on amphipods more heavily than mature individuals. Furthermore, region and depth had an important effect on the consumption of isopods, decapods and amphipods. Such ontogenetic changes and spatial patterns may provide insights into understanding the regulatory mechanisms of marine communities.     

Behaviour,survival and glycogen utilisation in the baltic isopod Saduria entomon exposed to long‐term oxygen depletion

The benthic isopod Saduria entomon was exposed to different oxygen saturations (92, 6, and < 1%, T = 8°C). The behaviour of S. entomon under hypoxia and anoxia differs from that observed under normoxia. During the first few hours under low oxygen conditions animals became more active. After that time they lay motionless on the sediment surface. A few of them were partially buried. When oxygenation decreased, the percentage of individuals that emerged from the sediment rapidly increased. It was four and seven times higher under hypoxia and anoxia, respectively. S. entomon is able to survive long periods (41 days) under severe hypoxia, whereas the LT₅₀ under anoxia varied between 12 and 18 days according to animal size. The glycogen content, which is the essential substrate for anaerobic metabolism of this species, decreased after 41 days under anoxia down to 48% of the normoxic level.     

Occurrence, diversity and community structures of peracarid crustaceans (Crustacea, Malacostraca) along the southern shelf of Greenland

The species composition of peracarids (Crustacea: Malacostraca) of the Greenland shelf between 60°N and 65°N was investigated by means of 10 qualitative epibenthic samples in relation to environmental factors. In total, 59,234 specimens were collected belonging to 219 species. The relative abundance was much higher on the western shelf (total of 41,594 specimens) than on the eastern shelf (total of 17,640 specimens with same effort). Three species were new to science, while five records were new for the investigated area. The species composition was dominated by amphipods (58%), while the relative abundances of isopods (25%), cumaceans (11%) and tanaidaceans (6%) were much lower. Diversity and evenness were similar in the eastern and the western areas. Multivariate analyses of the species relative abundances divided the peracarids into a southeastern and southwestern Greenland fauna. Based on a correlation analysis between faunal data and five environmental variables, the separation between the two areas was mainly based on sediment type. Species contributing most to the separation between eastern and western fauna included the amphipods Hardametopa nasuta, Photis reinhardi and Phoxocephalus holboelli, the isopods Pleurogonium spinosissimum, Iolella laciniata and Nannoniscus oblongus and the cumaceans Leucon cf. nasicoides and Campylaspis horrida. Species distribution patterns are discussed in the light of habitat and feeding preferences.     

Temporal variation of energy reserves in mayfly nymphs (Hexagenia spp.) from Lake St Clair and western Lake Erie

1. We analysed changes in energy reserves (lipid and glycogen) and length–weight relationships of burrowing mayflies (Hexagenia spp.) in 1997–99 to compare an established population in Lake St Clair with a recovering population in western Lake Erie of the Laurentian Great Lakes. In addition, we measured changes in water temperature and potential food in both water columns and sediments. 2. Although overall mean values of lipid and glycogen levels of Hexagenia nymphs from Lake St Clair and western Lake Erie were not significantly different, there were differences in seasonal patterns between the two lakes. In Lake St Clair, levels were highest in early spring, declined throughout the year, and reached their lowest levels in fall during all 3 years of study. In contrast, levels in western Lake Erie were lower in spring, increased to a maximum in summer, then declined in fall. Seasonal patterns in length–weight relationships were similar to those for lipid and glycogen. 3. Total lipid as a percentage of dry weight did not increase with developmental stage of nymphs until just prior to metamorphosis and emergence from water. However, the major reserve lipid, triacylglycerols, increased systematically with development stage. In the final stage of development, triacylglycerols declined, probably as a result of energy consumption and its conversion to other biochemical components for metamorphosis and reproduction. 4. Indicators of potential food (algal fluorescence in the water column and chlorophyll a and chlorophyll a/phaeophytin ratio in sediments) suggest that Hexagenia in Lake St Clair have a food source that is benthic based, especially in early spring, whereas in western Lake Erie nymphs have a food source that is water column based and settles to the lake bottom during late spring and summer.     

A laboratory-based comparison of osmoregulatory ability at different water temperatures and salinities in the stenothermic isopod Saduria entomon (Linnaeus, 1758) and the eurythermic amphipod Corophium volutator (Pallas, 1766) from the Baltic Sea

The aim of the investigation was a comparison of osmoregulatory ability at different water temperatures and salinities by the stenothermic isopod Saduria entomon (Linnaeus, 1758) and the eurythermic amphipod Corophium volutator (Pallas, 1766) from the Baltic Sea. The experiments were performed under laboratory conditions at different water temperatures (5, 10 and 15?°C) and salinities (3, 6.6, 15 and 25 PSU for S. entomon and 3, 6.3, 15 and 25 PSU for C. volutator). The osmotic concentrations of the crustaceans’ haemolymph were determined using the melting point method. Temperature had a statistically significant effect on the osmoregulation in S. entomon, but had generally no statistical influence on the osmoregulation in C. volutator. This physiological information regarding adaptation provides a basis for predicting the distribution of these species in changing environments. The implications of the results for the comparative adaptability of the two species are considered.     

Dietary effects on growth, reproduction, body composition and stress resistance in the terrestrial isopods Oniscus asellus and Porcellio scaber

The effect of differences in food composition on growth, body composition and cold tolerance of the isopods Porcellio scaber (Latreille) and Oniscus asellus (L.) has been studied. The effect on reproduction of P. scaber was included. Total lipid and protein content remained the same in O. asellus, given food with a nitrogen content of 2.4, 2.5, 2.8 or 3.1% for 8 weeks. Those fed on 3.1% N gained significantly more weight than those fed on 2.4, 2.5 or 2.8% N. Cold tolerance after a period of low temperature acclimation could not be tested, due to high mortality at the subzero treatment. Haemolymph osmolality increased after the winter acclimation for all food treatments. Juvenile P. scaber, given food of 1.8, 2.5 or 3.1% N, showed no difference in growth over 18 weeks, nor in body composition over 14 weeks. Diet did not influence cold tolerance ability and haemolymph osmolality remained the same with changing cold tolerance of the animals. Gravid females of P. scaber were separated to examine the effect of diet on reproduction. Diet did not influence the number of juveniles, nor their weight. In all food treatments, a significant correlation was found between the weight of the female and the number and weight of her offspring.     

Differential palatability of leaf litter to four sympatric isopods in a Hong Kong forest

Summary The feeding behaviour of four sympatric isopods from a Hong Kong forest has been investigated. The study included two armadillids (Formosillo raffaelei and Orodillo maculatus) and two philosciids (Burmoniscus ocellatus and Burmoniscus sp.). When given a choice of eight types of litter, all isopod species showed significant dietary selection, and food preferences were similar. Berchemia racemosa (Rhamnaceae) was most readily consumed, followed by Celtis sinensis (Ulmaceac), while feeding rates on Cinnamomum camphora (Lauraceae) were low. The armadillids exhibited a narrower dietary spectrum than the philosciids. All isopods showed significant differences in consumption rates when each litter type was presented separately to them. Cinnamomum camphora was eaten the slowest, while Celtis sinensis and Berchemia racemosa were the top-ranked species. Although patterns of litter ranking based on percentage eaten or feeding rates were similar, all isopods ate more food when given a mixture of leaves than when presented with a single litter type. Assimilation rates were, in general, positively related to feeding rates. On this basis, it appears that assimilation may influence food preference. There were no clear relationships between food preference or feeding rates and ash, calcium, copper, soluble tannin or energy content of the litter. This finding may indicate the benefits of isopods maintaining a mixed diet, consuming certain litter species to meet their calcium or copper requirements and then switching to others so as to meet daily energy needs and to avoid excessive injection of tannins or plant allelochemicals.     

Response of macrozoobenthos of lagoon Lake Izmenchivoye (Sakhalin Island) to the discontinuance of water exchange with the sea

Hydrobiological surveys in lagoonal Izmenchivoye Lake (Sakhalin Island) were carried out in 2004, 2005, and 2007 in order to examine the main changes in the composition and structure of macrozoobenthos after the cessation of its communication with the sea. Changes in the hydrological regime of the lake led to a significant reduction in the species list, a decline in the diversity of benthic communities, as well as a twofold increase in the population density of constituent species. The characteristic communities of the seawater cold spreading layer and tidal littoral communities disappeared after the water exchange with the sea ended. The latter communities were replaced with a lacustrine exposed littoral community dominated by amphipods and isopods. The previously shallow-water community dominated by the bivalve Musculista senhousia became the main benthic community. The number of warm-water elements increased markedly. Water temperature proved to be the main factor affecting the composition and structure of benthos of Sakhalin lagoons at the initial stages of evolution of macrobenthos.     

Chemotaxonomic characterisation of the thaumarchaeal lipidome

Thaumarchaeota are globally distributed and abundant microorganisms occurring in diverse habitats and thus represent a major source of archaeal lipids. The scope of lipids as taxonomic markers in microbial ecological studies is limited by the scarcity of comparative data on the membrane lipid composition of cultivated representatives, including the phylum Thaumarchaeota. Here, we comprehensively describe the core and intact polar lipid (IPL) inventory of ten ammonia‐oxidising thaumarchaeal cultures representing all four characterized phylogenetic clades. IPLs of these thaumarchaeal strains are generally similar and consist of membrane‐spanning, glycerol dibiphytanyl glycerol tetraethers with monoglycosyl, diglycosyl, phosphohexose and hexose‐phosphohexose headgroups. However, the relative abundances of these IPLs and their core lipid compositions differ systematically between the phylogenetic subgroups, indicating high potential for chemotaxonomic distinction of thaumarchaeal clades. Comparative lipidomic analyses of 19 euryarchaeal and crenarchaeal strains suggested that the lipid methoxy archaeol is synthesized exclusively by Thaumarchaeota and may thus represent a diagnostic lipid biomarker for this phylum. The unprecedented diversity of the thaumarchaeal lipidome with 118 different lipids suggests that membrane lipid composition and adaptation mechanisms in Thaumarchaeota are more complex than previously thought and include unique lipids with as yet unresolved properties.     

Wolbachia infection in crustaceans: novel hosts and potential routes for horizontal transmission

Thirty‐five percent of isopods are estimated to be infected by Wolbachia, an intracellular maternally inherited α‐Proteobacterium. Previous studies have indicated that horizontal transfer of Wolbachia strains may occur, although the mechanisms are unclear. The wsp gene was sequenced from 17 Wolbachia strains harboured by crustacean host species and three from their associated predators and parasites. Two major clades of Wolbachia were found in crustacean, with relatives also found in insects, the other restricted to crustaceans. Highly divergent Wolbachia strains were found in a woodlouse‐eating spider and its prey, suggesting no intertaxon bacterial exchange via the predator–prey route. The phylogenetic proximity of Wolbachia from parasitoid flies or phoretic mites to those from isopods suggests that horizontal symbiont transmission may have occurred between those taxa. Two distant Wolbachia strains were detected in two intertidal amphipods; these strains were closely related to different coastal isopod symbionts, suggesting Wolbachia transmission may occur between distantly related crustacean hosts living under the same ecological conditions.