The Gammaridae and Corixidae of an inland saline lake from 1975 to 1978

When observations began the population of Watch Lane Flash consisted of Gammarus duebeni and Sigara lateralis. Gammarus tigrinus was then introduced; probably during a transfer of water from a nearby canal. G. tigrinus rapidly increased in numbers and at the same time G. duebeni and S. lateralis decreased to zero. There followed a partial recolonisation by corixids but the species present were Sigara concinna and Sigara dorsalis. Throughout the study period corixid populations were stable in two immediately adjacent water bodies, one of which also contained G. duebeni. Thus the changes described were associated with the presence of G. tigrinus. The reasons for the success of G. tigrinus and its effects on other gammarids and corixids are discussed.     

Intraguild predation and species exclusions in amphipods: the interaction of behaviour, physiology and environment

1. Data from field surveys, laboratory experiments and computer simulations of community dynamics revealed that a novel interaction among intraguild predation, physiological adaptation and environment may explain the complex distributions of two putatively competing aquatic amphipods. 2. Gammarus pulex and G. tigrinus both thrive in fresh and oligohaline waters in western Europe. However, the native European G. pulex excludes the invading North American G. tigrinus from freshwaters of relatively low conductivity, whereas the reverse occurs at higher conductivities. Additionally, there is much spatio-temporal fluctuation in the patterns of coexistence of these species. 3. Laboratory experiments in The Netherlands and Ireland revealed that mutual predation of moulting individuals occurred frequently between these species. However, predation frequencies were differentially in favour of G. pulex under the ionic conditions to which this species is physiologically adapted (freshwater). On the other hand, predation was not differential under the ionic conditions to which G. tigrinus is physiologically adapted (oligohaline water). 4. A mathematical model, which extends the logistic equation to include mutual intraguild predation, simulated interactions over a range of values of relevant population parameters. This indicated that G. pulex would be excluded when balanced instantaneous rates of mutual predation were combined with the known greater reproductive output of G. tigrinus. However, this reproductive advantage is overcome by any relatively small bias in the instantaneous rate of predation favouring G. pulex, leading to the exclusion of G. tigrinus. This occurs even when the reproductive advantage to G. tigrinus is relatively large. Moreover, the model generated ‘switches’ in species dominance that are determined by the relative values of reproductive rate and mutual predation. The time taken to ‘switch’ may explain the transient periods of apparent coexistence of these species observed in the field. 5. The complex community dynamics of such species may thus be understood in terms of variation in the intensity of species interactions mediated by behavioural, physiological and environmental factors.     

Recent drastic changes in the gammarid fauna (Crustacea, Amphipoda) of the Vistula River deltaic system in Poland caused by alien invaders

During the last decade of 20th century, the nonindigenous gammarid species Gammarus tigrinus, Dikerogammarus haemobaphes, Pontogammarus robustoides and Obesogammarus crassus invaded the lower Vistula River and its deltaic, partly brackish regions. G. tigrinus, an oligohaline North‐American species, was introduced to western Europe in the 1950s; the remaining three species are oligohaline/freshwater Ponto‐Caspian species. All these species are now invading central and western Europe using the network of man‐made canals connecting different European river systems. In the Vistula River, the native European freshwater gammarid species Gammarus pulex and G. varsoviensis were replaced in the 1920s by the Ponto‐Caspian Chaetogammarus ischnus (syn. Echinogammarus ischnus), which in turn has been outnumbered by the more recent invasions of D. haemobaphes and P. robustoides. In brackish waters, the native Atlantic‐boreal species Gammarus zaddachi and Gammarus duebeni are replaced or at least outnumbered by G. tigrinus, P. robustoides and O. crassus. Possible invasion routes are discussed.     

The survival and growth of Gammarus tigrinus Sexton (Crustacea: Amphipoda) in relation to salinity and temperature

Field collections from an inland saline lake indicated that a rapid increase in numbers of a population of Gammarus tigrinus was positively correlated with high summer temperatures and salinities. The laboratory experiments on growth are an attempt to determine whether high summer temperatures or optimum salinity were primarily responsible for the rapid increase in numbers. Furthermore, G. tigrinus was not found in either of two adjacent lakes; one non-saline, the other at a higher salinity. The laboratory experiments on survival are an attempt to determine whether the restricted distribution of G. tigrinus was related to salinity. The implications of the field and laboratory studies are discussed and compared with other data from Europe and North America.     

Invasions of alien gammarid species and retreat of natives in the Vistula Lagoon (Baltic Sea, Poland)

During the last decades of the twentieth century, the alien gammarid species Gammarus tigrinus, Dikerogammarus haemobaphes, Pontogammarus robustoides and Obesogammarus crassus invaded the lower Vistula River and its deltaic, partly brackish regions. In brackish waters of the Vistula Lagoon the native Atlantic-boreal species Gammarus zaddachi and Gammarus duebeni have been replaced or at least outnumbered by the aliens. As compared to our earlier studies, through the years 1998–2004 we could observe nearly total decline of the native gammarid populations along the coasts of the Lagoon, and overdomination of the North-American G. tigrinus in most places. Possible reasons for the observed phenomena are e.g. increasing pollution and eutrophication of the Lagoon accompanied by competition between the native and the alien species.     

The population ecology of gammarus tigrinus (sexton) in the reed beds of the Tjeukemeer

The population ecology of Gammarus tigrinus (Sexton) was studied in the Tjeukemeer during 1969 and 1970. G. tigrinus reaches very high densities — up to 24,000/m² in parts of the study area. In 1970, the summer densities were 2–21/2 times greater than in 1969. Individuals do not grow to such large sizes in the summer as at other times of the year. Females begin to carry eggs in March or April and reproduction ceases in November. Large females have larger broods than smaller animals and the average size of the brood varies with the time of year. The egg incubation period and growth rate are dependent upon temperature. At summer temperatures females became sexually mature after about four weeks and the egg incubation period is about io days. The entire population is turned over about three times during the year. A combination of rapid growth rate, early onset of sexual maturity and high fecundity are probably responsible for the rapid spread of G. tigrinus throughout much of the Netherlands.     

Die Veränderung populationsdynamischer Parameter bei Gammarus tigrinus Sexton (Crustacea: Amphipoda) als Ausdruck subletaler Effekte durch die Wechselwirkung von Temperatur und cadmiumkontaminiertem Futter

Changes in the Population Dynamics of Gammarus tigrinus SEXTON (Crustacea: Amphipoda) as Expression of Sublethal Effects by Reciprocal Interactions of Temperature and Cadmium Enriched Food The population dynamics of Gammarus tigrinus SEXTON were studied in a flow-through system under semi-natural conditions over a period of 9 months. Elevated temperatures and Cadmiumenriched food alter lifespan of generations, growth rate, number of individuals and time to reach maturity, even before elevated Cadmium concentrations can be detected in the animals. Reciprocal interactions of Cadmium intake via food and/or water can be modified by the experimental design, because it depends on the experimental animals, on the Cadmium concentration and on the temperature regime with constant or cyclic conditions.     

Interference competition between alien invasive gammaridean species

The relative abundances of gammaridean species in the river Rhine have profoundly changed since the invasion of Dikerogammarus villosus in 1994/1995. This study tested whether these changes in gammaridean dominance could have been determined by interspecific competition and unequal mortality, for example by intraguild predation (IGP). Single and two species tests have been carried out in aquariums provided with all substrata present in the main channel of the Rhine. Changes in substratum choice, increased swimming activity and increased mortality of a species were used as indicators of interspecific competition during interaction between gammaridean species. Interspecific competition and mortality between the most abundant invasive gammaridean species in the Rhine, viz. Gammarus tigrinus, Echinogammarus ischnus and Dikerogammarus villosus were tested. In single-species experiments, G. tigrinus and D. villosus showed similar preferences for a stony substratum, whereas E. ischnus mostly occupied the water column. The two-species aquarium experiments indicated direct interference competition for substratum and unequal mortality between G. tigrinus and D. villosus, with D. villosus being the stronger competitor. Competitive stress was influenced by population density, was size-dependent and varied between the different types of substratum due to substratum choice. G. tigrinus did not show any behaviour indicative of interference competition in the presence of E. ischnus, and neither did E. ischnus or D. villosus in the presence of any of the other gammarideans. Swimming in the water layer may already enable E. ischnus to minimise its encounters with the stone-dwelling D. villosus and G. tigrinus. To maximise the encounters between E. ischnus and D. villosus, a fish (Lepomis gibbosus) was added to occupy the water layer during the aquarium experiments. E. ischnus showed a higher mortality in the presence of both D. villosus and fish, probably due to increased stress, as shelter opportunities to escape the predators had been minimised. The study shows that interference competition between gammaridean species can explain the replacement of the North American invader G. tigrinus by D. villosus in the river Rhine. E. ischnus and D. villosus both Ponto-Caspian invaders did not show interference competition in our experiments and co-exist in the Rhine.     

Exotic and indigenous freshwater gammarid species: physiological tolerance to water temperature in relation to ionic content of the water

Bioinvasions by closely related species often lead to niche competition between exotic and indigenous species. The outcome of this competition is partly determined by differences in physiological tolerance of the competing species to the environmental conditions of the colonised habitat. Physiological tolerance of the invading gammarid species Gammarus tigrinus, Echinogammarus ischnus and Dikerogammarus villosus and the indigenous gammarid species Gammarus pulex, Gammarus roeseli and Gammarus fossarum from Dutch waters was studied in the laboratory by comparing their pleopod beats at rest at different water temperatures, which reflect the gammarid's oxygen consumption. Pleopod beat frequencies increased from a minimum ventilatory activity of 0 beats per minute at 1 °C to maximum activity of up to 300 beats per minute at temperatures between 25 °C and 35 °C. At the state of maximum activity, a further increase in temperature was followed by a strong decrease in pleopod beat frequency, indicating acute stress, and subsequently mortality. Frequency response patterns of invading and indigenous gammarids were found to be highly similar, indicating a wide tolerance to temperature for all species. The tolerance of D. villosus, however, was reduced in brook water, indicating a lower competitive ability in relatively ion-poor water. G. tigrinus survived at higher temperatures in the more ion-rich, polluted waters than the indigenous gammarids, indicating a wider physiological tolerance and thus a higher competitive ability in these waters.     

Invader–invader interactions in relation to environmental heterogeneity leads to zonation of two invasive amphipods, Dikerogammarus villosus (Sowinsky) and Gammarus tigrinus Sexton: amphipod pilot species project (AMPIS) report 6

As biological invasions continue, interactions occur not only between invaders and natives, but increasingly new invaders come into contact with previous invaders. Whilst this can lead to species replacements, co-existence may occur, but we lack knowledge of processes driving such patterns. Since environmental heterogeneity can determine species richness and co-existence, the present study examines habitat use and its mediation of the predatory interaction between invasive aquatic amphipods, the Ponto-Caspian Dikerogammarus villosus and the N. American Gammarus tigrinus. In the Dutch Lake IJsselmeer, we found broad segregation of D. villosus and G. tigrinus by habitat type, the former predominating in the boulder zone and the latter in the soft sediment. However, the two species co-exist in the boulder zone, both on the short and longer terms. We used an experimental simulation of habitat heterogeneity and show that both species utilize crevices, different sized holes in a plastic grid, non-randomly. These amphipods appear to optimise the use of holes with respect to their ‘C-shape’ body size. When placed together, D. villosus adults preyed on G. tigrinus adults and juveniles, while G. tigrinus adults preyed on D. villosus juveniles. Juveniles were also predators and both species were cannibalistic. However, the impact on G. tigrinus of the superior intraguild predator, D. villosus, was significantly reduced where experimental grids were present as compared to absent. This mitigation of intraguild predation between the two species in complex habitats may explain the co-existence of these two invasive species.     

Population dynamics of alien gammarid species in the River Odra estuary

During the period of 1988–2000, three alien gammarid species, i.e. Gammarus tigrinus Sexton, 1939, Pontogammarus robustoides G. O. Sars, 1894, and Dikerogammarus haemobaphes (Eichwald, 1841) were recorded in the Odra estuary. This study, the first record of G. tigrinus and P. robustoides in Poland, focuses on the distribution, abundance, and biomass as well as on population structure of the three new species found in the coastal zone of the estuary. G. tigrinus and P. robustoides were first sampled in the Szczecin Lagoon (mid-part of the estuary) in 1988. The densities of both species were high from the time they were first recorded and exceeded 10 000 ind. m^–2, while the biomass was 25 and 140 g m^–2, respectively. G.tigrinus occurred at higher densities in northern and eastern parts of the Lagoon, while P. robustoides was particularly abundant in the south-eastern part, most heavily affected by River Odra discharge. In 1999, the third gammarid, D. haemobaphes, was recorded in the southern part of the estuary. Both in 1999 and 2000, the species was found in qualitative samples (dredge collections of Dreissena polymorpha) taken from the western branch of the river. As estimated from quantitative samples, densities and biomass in the eastern branch were rather low and did not exceed 150 ind. m^–2 and 2.7 g m^–2, respectively. The Szczecin Lagoon population of G. tigrinus produces at least two generations within a year: a summer one and an overwintering one. The latter is dominated by older and larger specimens. The species may reproduce in the Lagoon from April until November.     

The ecology of the Corixidae (Hemiptera: Heteroptera) in the Corrib catchment,Ireland

Two surveys were carried out in the Corrib catchment (Ireland) to determine the physical and chemical factors that govern the distribution of Corixidae. Of the twenty one species recorded, five species, Sigara scotti (Fieber), S. distincta (Fieber), S. fossarum (Leach), S. fallenoidea (Hungerford) and Cymatia bonsdorffii (Sahlberg) comprised 82% of the numerical total. Although individual species occurred in chemically diverse sites species assemblages and changes in the relative abundance of the majority of species and in species richness and diversity were evident in the progression from hard to soft water. High altitude soft water sites had a much lower number of species compared with chemically similar low altitude sites. In Lough Corrib, the main body of water in the catchment, most species were abundant only in sheltered areas with mud substrates and high percentage vegetation cover. Species diversity was high in these areas and either C. bonsdorffii, S. fossarum, S. fallenoidea or Callicorixa praeusta (Fieber) dominated numerically. However, S. scotti, S. dorsalis (Leach) and Arctocorisa germari (Fieber) were more abundant in exposed areas with sand, gravel or mearl substrates. In temporary ponds and in lotic water species diversity was also high but species composition (mainly C. praeusta, Sigara nigrolineata (Fieber), Corixa punctata (Illiger), C. panzeri (Fieber), C. bonsdorffii) was different to that of temporally stable lentic habitats.     

Is <Emphasis Type="Italic">Gammarus tigrinus</Emphasis> (Crustacea,Amphipoda) becoming cosmopolitan through shipping? Predicting its potential invasive range using ecological niche modeling

While the intensity of global shipping has increased dramatically over the last decades, species exchange between continents has likewise intensified. Ballast water of ships is recognized playing a major role in this process. Many of the larger sea ports have become bridgeheads for invasions. Ecological niche modeling is used to investigate the potential invasive range and high invasive risk ports of the North American amphipod Gammarus tigrinus. Sixty-two occurrences of G. tigrinus in its native range (North America) and 34 environmental data sets were compiled. Data on dispersal distances were used via ecological niche modeling to analyze the invasive potential of G. tigrinus. The invasive risk of large ports was analyzed according to modeling result, as well as their salinity in the main oceanic routes of the world. G. tigrinus had a rapid range extension on the British Isles and in the rest of Western Europe. Now it is invading the countries surrounding the Baltic Sea. Worldwide it has a vast potential invasive range. It has a high invasive risk for many large ports along the main oceanic routes, among which the ports of Shanghai, Buenos Aires and Montevideo have the highest invasive risk. G. tigrinus may become cosmopolitan through shipping, and this possibility is increasing. Particular emphasis should be placed on preventing human-mediated dispersal. Ports may be the first places G. tigrinus invades. This study can identify high invasive risk ports, especially those at risk of introduced North America species. More importantly, the water of large ports should be monitored regularly for exotic aquatic organisms that may survive temporarily or permanently.     

Colonization patterns and impacts of the invasive amphipods Chelicorophium curvispinum and Dikerogammarus villosus in the IJsselmeer area, The Netherlands

During the 1980s, Chelicorophium curvispinum and Dikerogammarus villosus colonized the lakes of the IJsselmeer area in The Netherlands. With the arrival of C. curvispinum the indigenous Apocorophium lacustre retreated to a small, sheltered bay with particular microconditions. The arrival of D. villosus is probably linked to a decrease of Gammarus tigrinus and possibly the disappearance of the gastropod Theodoxus fluviatilis. However, the occurrence of D. villosus may be restricted to stones along the banks of the lakes, and even there, G. tigrinus seems to remain stable at lower population densities. In samples from the beds of Dreissena polymorpha on the bottom of the lakes, D. villosus was virtually absent and densities of C. curvispinum were relatively low. G. tigrinus was here the single gammaridean species. Other changes in densities of invertebrates, like strong decreases in densities of Tricladida, Hirudinea, Asellidae, D. polymorpha and several gastropods, differ in details of timing from the arrival of the invaders, and are at least partly linked to other factors. In the River IJssel, the main source of invaders to the lakes, similar decreases in species abundance followed exceptionally high discharge events, but preceded both a relatively strong decrease of chlorophyll a levels and the arrival of D. villosus. This suggests that this arrival was not a direct cause of the recorded declines, although it may have prevented recovery.     

Food selection in three leaf-shredding stream invertebrates

Leaf disks (Betula papyrifera) were conditioned for two weeks by six species of aquatic hyphomycetes. Mass losses of the leaves were determined, and their concentrations of protein (extracted at pH 7, 10 and 12.8), phenolics (Folin-Ciocalteu and BSA-precipitation), lipids, and ergosterol (as indicator of fungal biomass) were measured. Enzymatic activities of the culture filtrates against cellulose, xylan and pectin were estimated. Gammarus tigrinis, Pycnopsyche guttifer and Tipula caloptera were given a choice of the six leaf/fungus combinations. G. tigrinus and P. guttifer consistently preferred some combinations over others; T. caloptera appeared to feed randomly. There were no significant correlations between consumption and any of the measured characteristics of leaf disks. With G. tigrinus and P. guttifer, the sequence of preference could be reproduced by extracting mycelia with non-polar solvents and applying the extracts to unconditioned leaf disks. Consumption of extract-coated disks was lower than consumption of conditioned disks. Numbers of endosymbiotic gut bacteria increased from G. tigrinus to P. guttifer to T. caloptera; diet diversity showed the opposite trend.     

The role of ion regulation in the control of the distribution of Gammarus tigrinus (Sexton) in salt-polluted rivers

Fluctuating salinities at different sites on the German salt-polluted rivers Werra and Weser were compared with extracellular ion levels of specimens of Gammarus tigrinus (Sexton; Amphipoda, Crustacea), collected at the same sites. G. tigrinus regulated haemolymph concentrations of inorganic anions (Cl⁻, SO²⁻ ₄, PO³⁻ ₄) and cations (Na⁺, K⁺, Mg²⁺, Ca²⁺) during fluctuations of salt pollution in the upper Weser. This capacity to regulate varying levels of salt pollution in the upper Weser, correlated well with the distribution of the brackish amphipods in this river ecosystem. G. tigrinus tolerated periods of Na⁺ and Cl⁻ stress (>380 mmol l⁻¹) without compensating these maxima by regulating extracellular Na⁺ and Cl⁻. However, during such bursts of Na⁺ and Cl⁻ stress in Werra and Weser, the ability to regulate extracellular [K⁺] at river water K⁺ stress of ≥6.0 mmol l⁻¹ may explain why this brackish species has been more successful in these rivers than its competitors like Gammarus pulex. The present investigation demonstrates that the water salinity affects the [NO⁻ ₃] in the haemolymph of G. tigrinus. With increasing hypo-osmotic stress the animals accumulate increasing amounts of NO⁻ ₃. A simultaneous increase in stream water [NO⁻ ₃] causes an additional accumulation of NO⁻ ₃ in the haemolymph. The high extent of accumulation indicates that active ion transport systems may be involved. The accumulation of NO⁻ ₃ in the haemolymph has low physiological consequences to G. tigrinus, but when hypo-osmotically stressed under anoxic conditions, nitrite formed by the reduction of nitrate may have an adverse affect on the metabolism of G. tigrinus. Accepted: 4 October 1999     

Above- and belowground patterns in a subalpine grassland-shrub mosaic

Many mountain pastures consist of a mosaic of grassland and shrub communities. Ongoing changes in mountain agriculture have affected the balance between the two elements of the mosaic. In order to understand the consequences of these changes for ecosystem functioning, we studied patterns in vegetation, root structure and soil properties along transects of varying grassland-to-shrub proportions. Our hypothesis was that differences in the vegetation aboveground are accompanied by differences belowground, related to soil properties and depth. The research was conducted at a subalpine site in the Trentino region (South-eastern Alps), consisting of Nardus stricta grasslands alternating with shrub patches of Rhododendron ferrugineum. Our investigation showed that the composition of vegetation was mainly governed by R. ferrugineum cover and less by soil properties. Plant species richness peaked at low to intermediate degrees of shrub cover and composition between transects became more similar with increasing shrub cover. Where R. ferrugineum cover was higher, Hemicryptophytes caespitosae were replaced by Nano-phanerophytes with consequences for belowground structures. At increasing shrub cover, root length density decreased, especially in the top soil, while root weight density remained stable and C content increased insignificantly. We discuss that theses structural changes along the gradient of R. ferrugineum cover affect a number of ecosystem services. The presented evidence suggests that maintaining grasslands with a low cover of R. ferrugineum balances a number of services, namely plant species diversity, carbon stabilization in soil and the prevention of soil erosion.     

Survival in northern microrefugia in an endemic Carpathian gammarid (Crustacea: Amphipoda)

Gammarus leopoliensis (Crustacea: Amphipoda) is considered a north‐eastern Carpathian endemic species and therefore can be regarded as an appropriate model for testing the hypothesis of Quaternary glacial survival in northern microrefugia. However, 250 km south, the south‐western Carpathians harbour populations that resemble phenotypically both G. leopoliensis and Gammarus kischineffensis, a similar species distributed east of the Carpathians. We used maximum‐likelihood and Bayesian methods to evaluate the phylogenetic relationships of these three taxa based on mitochondrial and nuclear markers, and quantitatively compared diversity patterns, phylogeography and divergence times among north‐eastern and south‐western Carpathian taxa. Results indicate that G. leopoliensis and the south‐western populations form together a strongly supported group (G. leopoliensis s.l.) which, along with G. kischineffensis, belongs to the Gammarus balcanicus clade. This group contains 12 lineages mainly of Pliocene age. G. leopoliensis consists of two widely distributed and recently expanded allopatric sister lineages that diverged from the southern ones ca. 4 Ma, indicating long‐term survival in northern microrefugia. The southern lineages are micro‐endemic and display a scattered distribution, suggesting a more ancient, relict pattern. We conclude that the contrasting diversity patterns between the disjunct distributional areas of G. leopoliensis s.l. reflect differential survival of lineages across the latitudinal gradient, offering a promising system for comparing the evolutionary ecology of lineages persisting in latitudinally disconnected microrefugia. These results fill an important gap in the knowledge of European gammarid biogeography and reveal that all Carpathian Gammarus taxa are ancient and diverse species complexes.     

Effects of species‐specific interactions with predation risk on the relative species sensitivities to a pesticide in water boatmen (Corixidae)

There is increasing concern that synergistic interactions between stressors may result in accelerated biodiversity loss. Yet, the prevalence and magnitude of these interactions remain one of the largest uncertainties in projections of future ecological change. Synergistic interactions between pesticide stress and predation risk are receiving increasing attention because they indicate that standard pesticide tests in the absence of predation risk may underestimate effects that occur under natural conditions. We questioned whether synergisms (or interactions in general), by differently shaping individual species’ sensitivities, can modulate species sensitivity rankings (SSRs) in survival and growth rate. Using laboratory and outdoor mesocosm experiments with five species of water boatmen (Hemiptera: Corixidae) we studied mortality and growth rate over seven days of exposure. We evaluated the presence and nature of interactions between the pesticide endosulfan and predation risk and tested to what extent this affected the SSRs to endosulfan. The combined exposure to the pesticide and predation risk resulted in synergistic effects for survival (Sigara lateralis) and growth rate (Sigara iactans) and in an antagonistic effect for growth rate (Hesperocorixa linnaei and female Sigara striata). These results suggest that standard tests may underestimate the pesticide effect in a natural predator environment for some species. While the effects of these interactions on SSRs were not strong and SSRs remained largely similar in the absence and presence of predation risk, some obvious species rank shifts occurred for growth rate which may potentially affect community structure through changed competitive strength. The study of SSRs and their dependency on biotic stressors may provide a simple conceptual and predictive framework to increase our understanding of how stressors like pesticides may differentially affect community structure in the absence and presence of another stressor.     

Differential predatory and interference interactions between native and invasive freshwater amphipods and a co-occurring mysid (Crustacea)

Animal introductions can often have unexpected and complex consequences for both native and invader species. Freshwater crustaceans such as Gammarus spp. (amphipods) and Mysis relicta (an ‘opossum shrimp’) have frequently come into contact because of deliberate and accidental introductions. However, there remains poor understanding of mechanisms leading to the co-existence and/or exclusion among amphipods and mysids. Here, we examined predatory and interference interactions between native (Gammarus duebeni celticus) and invasive (Gammarus tigrinus and Crangonyx pseudogracilis) amphipods and the native M. relicta in Britain’s largest expanse of freshwater, Lough Neagh. Laboratory mesocosm experiments simulating near-shore/mid-lough benthic habitats showed that both Gammarus species, but not C. pseudogracilis, preyed on M. relicta, which itself preyed on C. pseudogracilis. Further, M. relicta micro-distribution and habitat use changed because of interference from G. d. celticus and to a lesser extent G. tigrinus, with C. pseudogracilis having no such impact. In smaller microcosms, predation of M. relicta adults and juveniles by Gammarus spp. was significant. Although predation of Gammarus spp. by M. relicta was low, adult C. pseudogracilis were killed by M. relicta and its predation of juvenile C. pseudogracilis was high. We discuss the concurrence of these laboratory interactions with known field patterns of co-existence amongst these species.