Absence Of Mussels Research Articles

Active hydrothermal vents in the Woodlark Basin may act as dispersing centres for hydrothermal fauna

Here we report the discovery of a high-temperature hydrothermal vent field on the Woodlark Ridge, using ship-borne multibeam echosounding and Remotely Operated Vehicle (ROV) exploration. La Scala Vent Field comprises two main active areas and several inactive zones dominated by variably altered basaltic rocks, indicating that an active and stable hydrothermal circulation has been maintained over a long period of time. The Pandora Site, at a depth of 3380 m, is mainly composed of diffuse vents. The Corto site, at a depth of 3360 m, is characterized by vigorous black smokers (temperature above 360 °C). The striking features of this new vent field are the profusion of stalked barnacles Vulcanolepas sp. nov., the absence of mussels and the scarcity of the gastropod symbiotic fauna. We suggest that La Scala Vent Field may act as a dispersing centre for hydrothermal fauna towards the nearby North Fiji, Lau and Manus basins.

STATUS OF FRESHWATER MUSSELS (UNIONIDAE) OF THE RÍO CONCHOS BASIN, CHIHUAHUA, MEXICO

Before this study, researchers had not systematically surveyed the Rio Conchos, a tributary of the Rio Grande located in northern Mexico, for freshwater mussels. The Texas hornshell, Popenaias popeii, an endangered species under the United States Endangered Species Act, occurs in the main stem of the Rio Grande near its confluence with the Rio Conchos. The Texas hornshell's proximity to the Rio Conchos prompted us to survey to determine whether P. popeii also occurs within the Conchos basin. Additionally, previous researchers described the Conchos disk, Disconaias conchos, from the Rio Conchos but, to date, no one has reported live individuals of this species. The goal of this study was to determine the status of mussels within the Rio Conchos basin and to provide a baseline for future monitoring efforts. We qualitatively surveyed 11 sites within the Rio Conchos basin in May 2018. In total, we found two live individuals of the paper pondshell, Utterbackia imbecillis, at one site in the Rio San Pedro, a tributary of the Conchos River, and as shells of recently live individuals at two sites in the Rio Conchos. We found D. conchos as relict shells only at three sites, two in the Rio Conchos and one in a tributary of the river at Coyame. The lack of live or fresh-dead material suggests that D. conchos is extinct. We also found live individuals of nonnative Asian clams (Corbicula spp.) at 10 sites. Sand and gravel mining, high organic pollution, and inadequate instream flows were evident throughout the basin and might explain the general absence of mussels.

Fish vs. Aliens: predatory fish regulate populations of Limnoperna fortunei mitigating impacts on native macroinvertebrate communities

Limnoperna fortunei, an invasive mussel altering the structure of benthic communities, is preyed upon by several fish species in South America. To investigate the impact of predatory fish on populations of this mussel, and the effects of this top-down interaction on native macroinvertebrates, we performed an in situ experiment in the Uruguay River. By comparing benthic communities of artificial substrates colonised in treatments allowing or excluding fish access, it was demonstrated that fish significantly reduce colonisation by L. fortunei (i.e. its density, biomass and body size). In the absence of mussels (before settlement), fish preyed upon the native macroinvertebrate community inhabiting artificial substrates, reducing their densities compared to the fish exclusion treatment. However, it was found that in the presence of golden mussel (after settlement), the total density of benthic macroinvertebrates, densities of scrapers, and densities of dipterans and gastropods strongly decreased in fish exclusion treatments, suggesting a strong effect of L. fortunei in reducing the native fauna. By preying on L. fortunei, fish favoured the persistence of native macroinvertebrates. Further studies focused on predatory fish species and their efficiency in removing L. fortunei may contribute to advancing towards using native fish as mitigating agents.

A mesocosm investigation of the effects of quagga mussels (Dreissena rostriformis bugensis) on Lake Michigan zooplankton assemblages

Abstract Dreissenid mussels are known to disrupt the base of the food web by filter feeding on phytoplankton; however, they may also directly ingest zooplankton thereby complicating their effects on plankton communities. The objective of this study was to quantify the effects of quagga mussel feeding on the composition and size structure of Lake Michigan zooplankton assemblages. Two mesocosm (six 946 L tanks) experiments were conducted in summer 2013, using quagga mussels and zooplankton collected near Beaver Island, MI, to examine the response of zooplankton communities to the presence and absence of mussels (experiment 1) and varying mussel density (experiment 2). Mesocosms were sampled daily and zooplankton taxa were enumerated and sized using microscopy and FlowCAM® imaging. In experiment 1, the presence of quagga mussels had a rapid negative effect on veliger and copepod nauplii abundance, and a delayed negative effect on rotifer abundance. In experiment 2, mussel density had a negative effect on veliger, nauplii, and copepodite abundance within 24 h. Multivariate analyses revealed a change in zooplankton community composition with increasing mussel density. Ten zooplankton taxa decreased in abundance and frequency as quagga mussel density increased: except for the rotifer Trichocerca sp., treatments with higher mussel densities (i.e., 1327, 3585, and 5389 mussels/m 2 ) had the greatest negative effect on small-bodied zooplankton (≤ 128 μm). This study confirms results from small-scale (≤ 1 L) experiments and demonstrates that quagga mussels can alter zooplankton communities at mesoscales (~ 1000 L), possibly through a combination of direct consumption and resource depletion.

Freshwater mussels (Anodonta anatina) reduce transmission of a common fish trematode (eye fluke, Diplostomum pseudospathaceum).

Recent results suggest that bivalves can play an important role in restraining the spread of various aquatic infections. However, the ability of mussels to remove free-living stages of macroparasites and reduce their transmission is still understudied, especially for freshwater ecosystems. We investigated the influence of the common freshwater mussel (Anodonta anatina) on the transmission of a trematode (eye fluke, Diplostomum pseudospathaceum), which frequently infects fish in farms and natural habitats. In our experiments, mussels caused a significant decrease (P < 0·001) in the abundance of trematode free-living stages, from 6520 to 1770 cercariae L-1 on average (about 4-fold in 2 h). Individual clearance rates of mussels were 0·6‒3·7 L per hour (mean 1·9). These tests were followed by experimental infections of rainbow trout (Oncorhynchus mykiss) with different doses of D. pseudospathaceum cercariae in the presence or absence of mussels. Exposure of fish to cercariae in the presence of mussels significantly (P < 0·05) reduced the infection intensities in fish (by 30-40%) at all exposure doses. Our results indicate that freshwater bivalves can markedly reduce local cercariae densities and could be useful in mitigation of trematodoses harmful to fish farming.

Exploration of the phycoremediation potential of Laminaria digitata towards diflubenzuron, lindane, copper and cadmium in a multitrophic pilot-scale experiment

The presence of contaminants in aquatic ecosystems can cause serious problems to the environment and marine organisms. This study aims to evaluate the phycoremediation capacity of macroalgae Laminaria digitata for pesticides (diflubenzuron and lindane) and toxic elements (cadmium and copper) in seawater in the presence or absence of mussels. The photosynthetic activity was monitored in the macroalgae to assess its “physiological status”. The results showed that the presence of algae decreased diflubenzuron concentration in mussels by 70% after 120 h of exposure. Additionally, this macroalgae was efficient to reduce lindane, Cu and Cd in seawater; even though it not was able to reduce these contaminants in mussels. The studied pollutants did not affect the physiological status of macroalgae. This study reveals that the application of phycoremediation with macroalgae can be an useful and effective mitigation strategy to remove/decrease contaminant levels from the aquatic environment.

Wave action and starvation modulate intra-annual variation in displacement, microhabitat selection, and ability to contact prey in the common sea star, Asterias rubens Linnaeus

The common sea star, Asterias rubens, is a major predator in the rocky subtidal zone in the northern Gulf of St. Lawrence and along the Atlantic coast of Newfoundland, Canada. Knowledge about the ecology of A. rubens in these highly seasonal systems is largely limited to studies of interspecific competition for prey. Two experiments in an oscillatory wave tank were used to test the hypothesis that wave action and starvation modulate displacement, microhabitat selection, and ability to contact mussel (Mytilus edulis) prey in A. rubens from southeastern Newfoundland. Both experiments were carried out in two seasons (winter and summer) to quantify intra-annual variation in the effects of wave action (three wave velocities: null [0.0ms−1], low [0.1ms−1], and high [0.2ms−1]) and starvation (two levels: fed and starved three weeks). Experiment 1, in which two sea stars were allowed to displace and make contact with six microhabitats typifying natural seabed heterogeneity, showed that A. rubens displaced up to 50% less with increasing wave velocity. Displacement was 120% higher in summer than winter and lower in starved than fed individuals in summer only. The sea star predominantly stayed on flat horizontal surfaces, did so more frequently in the presence than absence of waves, and spent more time in the two most uneven microhabitats in winter than summer regardless of wave velocity. Experiment 2, in which one sea star in the centre of a depression was allowed to move to the top of surrounding mounds, showed that more sea stars that left the centre of the depression reached the top of the mounds in summer than winter, in the absence than presence of waves, and when starved than fed. Sea stars that reached the top of mounds spent more time there in the presence than absence of mussels in summer only, when starved than fed in summer only, and in the presence than absence of mussels at null and low wave velocities. Measurements of wave height, sea temperature, and A. rubens abundance over six months at one subtidal site indicated that the sea star was >4 times more abundant at 8m than at 2m and 4m. This difference was unrelated to temperature and likely mainly caused by the wave environment. Collectively, results demonstrate that wave action, and to a lesser extent starvation, are key modulators of the inclination and ability of A. rubens to explore its environment and localize prey. In southeastern Newfoundland, the response of A. rubens to wave action and starvation is adaptable, being generally stronger in summer than winter, when sea temperature is markedly lower.

Dispersal abilities of riverine freshwater mussels influence metacommunity structure

Summary Historically, the importance of dispersal ability for the distribution of organisms has often been ignored, partly because of the difficulty of measuring it. Many unionid mussels, which have larvae that are obligate parasites (usually on fish), are endangered and the conservation and management of these mussels depend on knowledge of the main drivers of their distribution. Metacommunity theory predicts that limited dispersal should weaken the association of community composition with environmental factors. We tested this prediction by comparing the strength of association with environmental factors of (i) mussels with different dispersal abilities based on mobility of known host fish and (ii) mussels with different host infection strategies targeting fish with different mobility. Mussels with more mobile host fish showed a significantly stronger association with host fish presence and catchment (as a proximate measure for large‐scale differences in environmental conditions or as a spatial component) compared to mussels with less mobile host fish. Our results thus indicate that mussel distribution is more closely linked to host fish for high‐dispersal mussels, which suggests the potential for species sorting. Mussel species with weak dispersal capabilities show the opposite pattern and are therefore less able to colonise all suitable sites (species sorting with limited dispersal). Thus, the absence of mussels does not necessarily indicate unsuitable environmental conditions, but can also be caused by dispersal limitation. The impact of different host infection strategies was less clear, and differences in responses could be driven by differences in host fish or even habitat specialisation in addition to potential differences in dispersal abilities. A better knowledge of the dispersal via host fish of unionid mussels and their host infection strategies will be crucial to understanding their metacommunity dynamics, a necessary precursor for effective conservation practices.

Beyond food: a foundation species facilitates its own predator

Facilitation by foundation species can play a critical role in structuring ecological communities. As environmental stress increases, generally more organisms become dependent on the stress buffering provided by foundation species. As such, foundation species may even facilitate their own predators, an interaction that can influence the functioning and structure of the foundation species population and thereby the facilitated organisms. This work presents a case study on the blue mussels and sea stars, where we tested to what extent a foundation species (i.e. blue mussels) may facilitate its own predator (sea star) when exposed to a gradient of environmental stress (hydrodynamic forces). Amelioration of hydrodynamic stress by mussels facilitated sea stars, allowing them to persist on a soft bottom in highly dynamic environment, which would not be possible in the absence of mussels. Moreover, sea stars continue preying on mussels when environmental stress increases. The results suggest that a foundation species may interact with its own predator beyond the role of food source, by ameliorating environmental stress, creating an additional dependence link between the foundation species and the predator, which potentially has major implications for ecosystem structure and stability.

3D modelling of dreissenid mussel impacts on phytoplankton in a large lake supports the nearshore shunt hypothesis and the importance of wind-driven hydrodynamics

A three-dimensional hydrodynamic model (ELCOM) coupled with a biological model (CAEDYM) was calibrated with field data from Lake Simcoe (2008) and used to examine the expected impact of dreissenid mussels on the distribution of phytoplankton and nutrients, mussel energetic, and the interactions with hydrodynamic conditions. In accordance with the near-shore shunt hypothesis and the actual distribution of mussels, the model predicted a considerable impact of mussels on phytoplankton in nearshore areas (<15 m depth; −38 % phytoplankton). Horizontal advection could account for the lower (−15 % phytoplankton) offshore impact despite the absence of mussels within this region of the lake. These predictions were similar to observed long-term changes in phytoplankton biomass after the dreissenid mussel invasion. Simulated decreases in the initial mussel biomass by 50 % led to increased phytoplankton nearshore, increased mussel growth and decreased phosphorus excretion relative to ingestion. In contrast, increases in initial mussel biomass by 50 % resulted in minor changes in phytoplankton, since near-bottom depletion of phytoplankton caused growth and energy-limitation of mussels. The model, which was the first to capture metabolically-driven variations of mussel-mediated nutrient cycling, predicted a partial uncoupling of carbon and phosphorus cycling by mussels, driven by varying controls on mussel growth. A 60 % increase in the intensity of three storms during the summer had little net-effect on phytoplankton biomass, but reduced energy-limitation of mussels and increased mussel growth due to increased supply of phytoplankton. Our model demonstrates that near-bottom depletion of phytoplankton is an important process limiting the impact of mussels, and indicates that it affects the mussels’ cycling of energy and nutrients. It further suggests that stronger storm events could lead to further increase of flux of organic matter from the pelagic zone to the benthos, further altering energy pathways in the lake.

Bioaccumulation Experiments in Mussels Contaminated with the Food-Borne PathogenArcobacter butzleri: Preliminary Data for Risk Assessment

The aim of this study was to evaluate, at a laboratory scale, the ability of this microorganism to grow in seawater and bioaccumulate in mussels (Mytilus galloprovincialis) maintained in constantly aerated tanks, containing twenty litres of artificial seawater. Three concentrations of A. butzleri LMG 10828T were tested (about 5 × 106 CFU/mL, 5 × 104 CFU/mL, and 5 × 102 CFU/mL). Following contamination, enumeration of A. butzleri was performed from water and mussels each day, for up to 96 h. Three contamination experiments with artificial seawater in absence of mussels were also performed in the same manner. In the experiments with mussels, A. butzleri declined in water of approximately 1 log every 24 h from the contamination. In artificial seawater without mussels the concentration of A. butzleri remained on the same logarithmic level in the first 48 h and then decreased of about 1 log every 24 hours. In mussels, the concentration was approximately 2 log lower than the exposition level after 24 h from the contamination, and then it decreased exponentially of 1 log every 24 h. Our findings suggest that in the experimental conditions tested A. butzleri is neither able to effectively grow in seawater nor bioaccumulate in mussels, at least in the free and cultivable form.

Influence of zebra (Dreissena polymorpha) and quagga (Dreissena rostriformis) mussel invasions on benthic nutrient and oxygen dynamics

Although invasive zebra ( Dreissena polymorpha ) and quagga ( Dreissena rostriformis ) mussels are known to increase water column concentrations of nitrogen and phosphorus, the relative roles of direct nutrient release by the mussels and mussel-induced alterations to sediment fluxes are little understood. In short-term microcosm experiments comparing the presence and absence of mussels on sediments from Oneida Lake, New York, USA, both dreissenid species approximately doubled benthic oxygen consumption and fluxes of ammonium. The impact on soluble reactive phosphate (SRP) flux varied from 40% to 140% depending on whether mussels were located in the water column or directly on sediments. The additional flux was attributable directly to release by the mussels, with fluxes from the sediments remaining largely unchanged. However, mussels located directly on sediment surfaces released twice as much SRP as mussels located higher in the water column, with a molar N:P ratio as low as 4:1. The high rate of SRP release by mussels on sediment, possibly caused by mobilization of iron-bound phosphorus from sediment particles in the anoxic guts of the mussels, could contribute to observed increases in SRP, abundance of nuisance algae such as Cladophora , and abundance and toxicity of the cyanobacteria Microcystis in dreissenid invaded ecosystems.

Predicting habitat use and trophic interactions of Eurasian ruffe, round gobies, and zebra mussels in nearshore areas of the Great Lakes

The Laurentian Great Lakes have been subject to numerous introductions of nonindigenous species, including two recent benthic fish invaders, Eurasian ruffe (Gymnocephalus cernuus) and round gobies (Neogobius melanostomus), as well as the benthic bivalve, zebra mussel (Dreissena polymorpha). These three exotic species, or “exotic triad,” may impact nearshore benthic communities due to their locally high abundances and expanding distributions. Laboratory experiments were conducted to determine (1) whether ruffe and gobies may compete for habitat and invertebrate food in benthic environments, and (2) if zebra mussels can alter those competitive relationships by serving as an alternate food source for gobies. In laboratory mesocosms, both gobies and ruffe preferred cobble and macrophyte areas to open sand either when alone or in sympatry. In a 9-week goby–ruffe competition experiment simulating an invasion scenario with a limited food base, gobies grew faster than did ruffe, suggesting that gobies may be competitively superior at low resource levels. When zebra mussels were added in a short-term experiment, the presence or absence of mussels did not affect goby or ruffe growth, as few zebra mussels were consumed. This finding, along with other laboratory evidence, suggests that gobies may prefer soft-bodied invertebrate prey over zebra mussels. Studies of interactions among the “exotic triad”, combined with continued surveillance, may help Great Lakes fisheries managers to predict future population sizes and distributions of these invasive fish, evaluate their impacts on native food webs, and direct possible control measures to appropriate species.

Is the decline of freshwater mussel populations in a regulated coastal river in south‐eastern Australia linked with human modification of habitat?

Abstract The Hawkesbury‐Nepean River provides potable water for 5 million people living in the Sydney basin, and water for agricultural and horticultural production that meets most of Sydney's daily needs for fresh food. Anecdotal evidence indicated that numbers of freshwater mussels have seriously declined in much of the river over recent decades. A field survey revealed the presence of populations of three species of mussels, Hyridella depressa, Hyridella australis and Velesunio ambiguus in the river. Higher density mussel populations were most common in catchment areas with little human modification to the channel bed or adjacent vegetation communities. Levels of disturbance of riparian vegetation and, to a lesser degree, land use, were identified as being strongly associated with the absence of mussels from some reaches. Catchment geomorphology was also shown to be relevant to the abundance, population structure and suite of mussel species present in different geomorphic reaches of the river. Absence of mussels was noted from areas where they had been recorded in previous studies. These results demonstrate that mussel species are under threat in the catchment. As filter feeders with the ability to remove excess nutrients and bioaccumulate toxic substances, freshwater mussels play an important role in natural remediation processes in freshwater systems. The decline in mussel populations in the Hawkesbury‐Nepean River highlights concerns for the overall health of the river system, and supports the need to identify the subsidiary impacts of physical habitat modification in developing both riverine and riparian management strategies. Copyright © 2006 John Wiley &amp; Sons, Ltd.

INVERTEBRATE‐MEDIATED NUTRIENT LOADING INCREASES GROWTH OF AN INTERTIDAL MACROALGA1

Even in nitrogen‐replete ecosystems, microhabitats exist where local‐scale nutrient limitation occurs. For example, coastal waters of the northeastern Pacific Ocean are characterized by high nitrate concentrations associated with upwelling. However, macroalgae living in high‐zone tide pools on adjacent rocky shores are isolated from this upwelled nitrate for extended periods of time, leading to nutrient limitation. When high‐intertidal pools are isolated during low tide, invertebrate‐excreted ammonium accumulates, providing a potential nitrogen source for macroalgae. I quantified the influence of mussels (Mytilus californianus Conrad) on ammonium accumulation rates in tide pools. I then evaluated the effects of ammonium loading by mussels on nitrogen assimilation and growth rates of Odonthalia floccosa (Esp.) Falkenb., a common red algal inhabitant of pools on northeastern Pacific rocky shores. Odonthalia was grown in artificial tide pool mesocosms in the presence and absence of mussels. Mesocosms were subjected to a simulated tidal cycle mimicking emersion and immersion patterns of high‐intertidal pools on the central Oregon coast. In the presence of mussels, ammonium accumulated more quickly in the mesocosms, resulting in increased rates of nitrogen assimilation into algal tissues. These increased nitrogen assimilation rates were primarily associated with higher growth rates. In mesocosms containing mussels, Odonthalia individuals added 41% more biomass than in mesocosms without mussels. This direct positive effect of mussels on macroalgal biomass represents an often overlooked interaction between macroalgae and invertebrates. In nutrient‐limited microhabitats, such as high‐intertidal pools, invertebrate‐excreted ammonium is likely an important local‐scale contributor to macroalgal productivity.

Refugia and Local Controls: Benthic Invertebrate Dynamics in Lower Green Bay, Lake Michigan following Zebra Mussel Invasion

In many aquatic ecosystems benthic invertebrate abundance has increased following zebra mussel ( Dreissena polymorpha) invasion. We examine the impact of zebra mussel density on the abundance and distribution of benthic invertebrates and postulate refuge from predation as a mechanism for the increases we found in some taxa. Benthic invertebrates in zebra mussel druses and in adjacent sediment samples were surveyed at sites in six locations representing various trophic conditions in lower Green Bay. Mean invertebrate density and taxa richness were significantly higher in the druses than in the adjacent sediment. Species diversity in the druses was inversely correlated to turbidity over the study area. Sediment samples were dominated by oligochaetes and chironomids. Amphipods were the most abundant taxa in most, but not all, of the druse samples. Other taxa present included leeches, hydra, mayflies, and caddisflies. The effectiveness of druses as refuge from predation for amphipods was investigated under laboratory conditions with various predators (perch, round goby, and rusty crayfish). In mesocosms, predation losses averaged 75% lower where zebra mussels were present. In the absence of mussels, predation loss to perch and round goby was 100% and 66% to crayfish. We conclude that the increased abundance of other invertebrates in druses in lower Green Bay may be due to increased refugia, however the assemblage composition at any given site varies with local conditions.

Naturally low seston concentration and the net energy balance of the greenshell mussel (Perna canaliculus) at Island Bay, Cook Strait, New Zealand

This paper describes an investigation of the physiological energetics of the New Zealand greenshell mussel, Perna canaliculus (Gmelin, 1791), which is aimed at determining if mussels are absent from many shores of Cook Strait as a consequence of a negative energy balance resulting from ambient low seston quantity and quality. Seston characteristics and mussel physiological functions were measured under ambient summer conditions at Island Bay, a site in Cook Strait. Estimates of total particulate matter (TPM in mg litre−1), particulate organic matter (POM in mg litre−1), and particulate organic carbon (POC in |ig litre−1) were low but of a similar magnitude to values reported for many comparable temperate regions. However, seston % organic matter (% OM) values were consistently low (<25%) and resulted in negative net absorption efficiency (AE) values and concomitant negative scope for growth (SFG = net energy balance) values. We suggest that the ongoing costs of energy loss associated with extra‐and intracellular digestion (termed metabolic faecal loss) are greater than the energy derived from seston of such low organic matter. Our data indicate that ambient levels of seston % OM of ≤25% are insufficient to promote a positive net energy balance in P. canaliculus, and as such probably play a major role in explaining the almost complete absence of mussels from many Cook Strait shores.

Physical microhabitat requirements of freshwater pearl mussels, Margaritifera margaritifera (L.)

The spatial distribution patterns of freshwater pearl mussels, Margaritifera margaritifera (L.), in the River Kerry, north-western Scotland were investigated, together with their relationships to a number of physical parameters (distance from nearest bank, mean water depth, mean current velocity, substratum descriptors, aquatic vegetation cover). The mussels exhibited a highly contagious, non-random spatial distribution pattern. Adult and juvenile M. margaritifera were found to have broadly similar habitat `preferences', although adults were found over a wider range of physical conditions. Based on computed habitat suitability curves, optimum water depths of 0.3–0.4 m and optimum current velocities of 0.25–0.75 ms-1 at intermediate water levels were observed. River bed substratum characterisitics appear to be the best physical parameters for describing M. margaritifera habitat. Boulder-stabilised refugia, which contain enough sand for burrowing, are ideal microhabitats for juvenile mussels. Adults are able to tolerate silty or muddy conditions for unknown lengths of time, but juveniles are never found in this type of habitat. Substratum-based discriminant function models were used to predict the presence or absence of mussels, with a success rate of 76–92%.

Epibenthic fauna dredged from tidal channels in the Wadden Sea of Schleswig-Holstein: spatial patterns and a long-term decline

The epibenthic fauna dredged in the channels of the Wadden Sea of Schleswig-Holstein is dominated by crustaceans. Mean species richness is lower in a southern region (7.6±2.2 species haul−1) which is part of the Elbe estuary and lacks protective barrier islands. A northern region is more marine and is protected by a seaward barrier of high sands. Here mean species richness is 10.9±2.8. Within channels, there is no significant inshore-offshore gradient, and species number does not increase with depth. Stratified sampling near the island of Sylt revealed that abundance and diversity are high along the gentle upper slope (5 m depth) of channels when mussels are present, but low in the absence of mussels. Abundance and diversity are also low along the steeper slope below (at 10 and 15 m depth). In a channel with scattered stones and boulders, highest diversity (16.0±5.1) and dense epibenthic assemblages were encountered along the upper and middle slopes (5 and 10 m). Comparisons with historical surveys suggest that a decline of nearly fifty percent of all epifaunal species within the last hundred years may be attributed to fishery disturbances.

Environmental conditions and physiological tolerances of intertidal fauna in relation to shore zonation at Husvik, South Georgia

South Georgia, an extremely isolated island within the Antarctic Convergence, was covered by an extensive icecap until ~10,000 years ago. In consequence the depauperate intertidal fauna is of recent origin and consists almost entirely of brooders or direct developers which probably arrived as a result of rafting. Environmental conditions between the tidemarks are comparable with northern Norway and Greenland, so the absence of mussels and barnacles is due to isolation from the nearest feasible sources of colonization (the Falkland Islands and Magellan), and not to a hostile environment. Intertidal animals (eight species studied) have median upper lethal temperatures that are positively and linearly related to maximum height of distribution on the shore. Thermal niche width (median upper lethal temperature minus median lower lethal temperature) is also positively correlated with maximum height of distribution on the shore with species below mid-tide level having narrow niches in contrast to species above mid-tide level that have wide niches. There is no relationship between freezing resistance and position on the shore. Salinity and desiccation tolerances were also greater in animal species from the upper shore than in those from the lower shore. In the case of those species also studied elsewhere (Lasaea rubra (Mollusca: Bivalvia); Nacella condnna (Mollusca: Gastropoda) and Tigriopus sp. (Crustacea: Copepoda)), evidence is presented to show that no special adaptation is exhibited by South Georgian animals, and that upper zonation limits are controlled primarily by physical tolerance. This is particularly marked in the case of N. condnna which has similar thermal tolerances on the Antarctic Peninsula, on Signy Island and at South Georgia; in consequence it can only inhabit the extreme lower shore at South Georgia, yet penetrates to mid-tide level at Signy Island.