ALBERT

Description: Some of the basic assumptions implied in the theory of limiting similarity of coexisting competitors were investigated experimentally in the case of three congeneric, deposit feeding snails. The species show character displacement with respect to size when coexisting. In experimental containers with a natural substrate, interspecific, exploitative competition between snails of the same size range is as intense as intraspecific competition. Diatoms of a given size range, which constitute the most important food, show a "logistic" growth response to grazing, and individual growth of the snails is linearly related to diatom density. The previous observation that the snails show size dependent selection for ingested particle sizes is extended to show that this mechanism leads to a real resource partitioning between snails of different sizes. Size frequency distribution of diatoms in the sediment is a function of the sizes of grazing snails. Size selection of ingested sand grains may also be significant for resource partitioning since the migration rate of the attached microflora between sand grains seems to be slow relative to its growth on the individual sand grains.

Description: In both terrestrial and aquatic environments introductions of non-indigenous species are continuing and represent one important component of global change. Negative biotic interactions by resident species may prevent successful invaders from becoming pests. Few experimental data are available on the presence and significance of such biotic resistance other than predation or competition. This study addresses the role of habitat structure provided by a native eelgrass (Zostera marina) canopy on growth and survival of the non-indigenous mussel Musculista senhousia, a habitat-modifying gregarious suspension feeder with strong effects on native infauna and eelgrass. In 2 southern California bays, a series of transplantation experiments using tagged mussels revealed that inside an eelgrass canopy, Musculista growth rates were reduced by more than half in 3 of 4 experiments compared to adjacent unvegetated areas. Musculista survival also decreased inside the vegetation in a 4-mo experiment. As one element of habitat structure, we tested the effects of eelgrass patch size, using natural (1 site) and planted (1 site) eelgrass patches of defined sizes. Growth rates of Musculista were highest outside the vegetation and decreased as eelgrass patch size increased. As a potential mechanism for the canopy effects, we suggest that Musculista receives less food inside the vegetation. In the experimental plots, the presence and spatial extent of the macrophyte canopy strongly affected near bottom (10 cm) horizontal water flow assessed with a direct dye tracking method. Reduced mussel growth rates were linearly associated with lower water flow, and presumably, food flux. Over a period of 7 mo, food resources (particulate chlorophyll a) were consistently lower 1 and 5 cm above the sea floor inside eelgrass patches compared to the sand flat. The reduction in food availability matched the growth reduction of Musculista. Also, mussel condition (dry flesh mass/shell mass) was worse in individuals growing in eelgrass than in the sand flat. Previous experiments revealed that dense beds of Musculista impede the rhizome growth and vegetative propagation of eelgrass, yet mussels attain abundances sufficient for interference only if eelgrass beds are patchy. Thus, anthropogenic disturbances on eelgrass beds, which often result in meadow fragmentation, and the proliferation of Musculista may have synergistic negative effects on the persistence of eelgrass beds.

Description: The use of stable isotope analysis in ecological and wildlife studies is rapidly increasing. Studies include evaluating flow of nutrients in ecosystems and studying dietary composition of individual animals. Several mixing models have been developed to evaluate the relative contribution of different foods to the diet of consumers. All these mixing models require that all prey types will be significantly different in bivariate space. This requirement usually poses a problem in analyzing data of stable isotope ratios because sample sizes in most studies are small and seldom normally distributed. We propose a randomization test that we based on the K nearest-neighbor approach. Results from our simulations of power revealed that the K nearest-neighbor test appears to have high power even with small sample sizes and comparatively low displacement. The K nearest-neighbor test described here provides the preliminary statistical analysis necessary for the use of the mixing models, and therefore is a new, powerful tool for analyzing stable isotope data. In evaluating the test performance on data collected from American martens (Martes americana) and their prey on Chichagof Island, Southeast Alaska, we were able to reject our null hypothesis that all samples of prey were drawn from identical populations (P = 0.05). A program written in Pascal or S-Plus is available from the authors to evaluate the K nearest-neighbor statistic for several groups.

Description: In brackish water the variety of marine and freshwater parasite species is considerably reduced. The distribution in brackish water of most marine endoparasites is restricted by the salinity tolerance of their hosts, most of the parasite species are more tolerant than their hosts. The influence of salinity and temperature on nine species has been examined; first stage larvae of Contracaecum aduncum develop in 0-32‰ salinity; Cryptocotyle lingua proved to be infective at salinities down to 4‰. The greatest resistance was found in Anisakis larvae from herring Clupea harengus, which survived for more than half a year. Parasites in the fish intestines appear to be unaffected by changing water salinities, as the osmolarity in the intestines stays nearly constant. Marine ectoparasites (Acanthochondria depressa, Lepeophtheirus pectoralis) survive about three times longer than freshwater species (Piscicola geometra, Argulus foliaceus) when salinity is 16‰. High temperature increases the effects of adverse salinities on parasites. There is evidence that none of these ecto-parasitic species can develop within the range of 7-20‰ salinity.

Description: In situ experiments were run with the seastar Asterias rubens to investigate the influence of epibiosis on predation preferences. Mussels (Mytilus edulis) monospecifically fouled by different epibiont species (the barnacle Balanus improvisus, the red filamentous alga Ceramium strictum, the sponge Halichondria panicea and the hydrozoan Laomedea flexuosa) and macroscopically clean mussels were exposed and seastar predation was monitored by SCUBA. Asterias rubens preferred macroscopical unfouled mussels as prey. Fouling generally reduced predation pressure on the mussel hosts (associational resistance). Barnacles protected mussels less efficiently than hydrozoans or algae. We hypothesize that in top-down controlled communities this influence of epibiosis on predation pressure should affect mussel community patterns. A survey of natural mussel-epibiont distribution in the presence or absence of A. rubens showed that the prevalence of differently fouled mussels differed between predation-exposed and predation-protected habitats. Natural mussel-epibiont associations reflected the preferential predation of the major local predators. Additionally, higher epibiotic diversity and evenness could be observed at locations accessible to benthic predators as compared with habitats protected from predation. As blue mussels and seastars are important structuring and controlling elements in the shallow water community of Kiel Fjord, major consequences of epibiosis on the entire system are discussed.

Description: Newly hatched Baltic cod Gadus morhua larvae are typically found at depths 〉60 m. This is a region of low light and prey availability, hence generating the hypothesis that larvae have to migrate from hatching depth to the surface layer to avoid starvation and improve their nutritional condition. To test this hypothesis, Baltic cod larvae were sampled during the spawning seasons of 1994 and 1995 with depth-resolving multiple opening/closing nets. Each larva was aged by otolith readings and its RNA/DNA ratio was determined as a measure of nutritional condition. The RNA/DNA ratios of these larvae aged 2-25 days (median 10 days) ranged from 0.4 to 6.2, corresponding to levels exhibited by starving and fast-growing larvae in laboratory calibration studies (starvation, protein growth rate, Gpi= -12.2% day−1; fastgrowing larvae, Gpi=14.1%day−1) respectively. Seventy per cent of the field caught larvae had RNA/DNA ratios between the mean values found for starving and fed laboratory larvae. Only larvae aged 8-11 days had higher mean RNA/DNA ratios above 45 m than below (t-test, P〈0.05). However, the instantaneous protein growth rates were significantly higher for all larval age groups in the surface layers (t-test, P〈0.05). Starving larvae were found in all depths sampled (10-85 m), whereas growing larvae (positive Gpi) were restricted to samples taken shallower than 45 m. These superior growth rates above 45 m corroborate the hypothesis and imply that migration to the shallow water layers is a prerequisite for good nutritional condition, growth and survival of Baltic cod larvae. The frequent occurrence of cod larvae older than 8 days in the deep water in poor condition suggests that a proportion of the larvae will die from Starvation in the deep layers of the Baltic Sea.

Description: We propose that ocean conditions of the Near Islands in the western Atleutian Arc mimic those of the shallow continental shelf of the eastern Bering Sea to the extent that the marine community, including assemblages of forage fishes and their avian predators, has disinctly coastal characteristics. In contrast, marine avifauna and their prey at neighbouring Buldir Island are distinctly oceanic. For example, at the Near Islands, the ratio of thick-billed to common murres, Uria lomvia and U. aalge, is low and black.legged kittiwakes, Rissa tridacytla, but not red-legged kittiwakes, R. brevirostris, nest there. Diets of murres and kittiwkaes are dominated by sand lance, Ammodytes hexapterus, an abundant coastal species. At Buldir Island, thick-billed murres greatly outnumber common murres, red-legged kittiwakes and black-legged kittiwakes are both abundant, and diets of the birds consist primarily of oceanic squid and lantern-fish (Myctophidae). This mesoscale difference in food webs is apparently a consequence of the local physiography. A broad escarpment on the Near physiographic block creates a comparatively expansive, shallow, shelf-like habitat around the Near Islands, where a pelagic community typical of coastal regions flourished. Buldir Island is the only emergent feature of the Buldir physiographic block, with little shallow water surrounding it and, apparently, little opportunity for other than oceanic species to exist. Patterns in the distribution of fishes, and thus of sea birds, throughout the Atleutian Islands might be largely explained by the relationship between physical environments and food webs. In the larger context of fisheries oceanography, this model for the Aleutian Islands improves our ability to interpret physical and biological heterogeneity in the ocean and its relationship to regional community dynamics and trends in the abundance and productivity of individual species at higher tropic levels.

Description: The effects of hydrostatic pressure and dissolved oxygen concentration on oxygen consumption rates were examined in the mesopelagic, vertically migrating squid Histioteuthis heteropsis from southern California offshore waters. Elevated hydrostatic pressure (to 136 atm) had no significant effects on oxygen consumption rates at 5°C. Lox oxygen concentrations (〈0.5 ml O2 liter -1) depressed rates of oxygen consumption. These findings suggest that biosis may be necessary to supply the total nergy needs of this species at daytime depths in the oxygen minumum layer off southern California.