Dirk Platvoet

Invading predatory crustacean Dikerogammarus villosus eliminates both native and exotic species.

As the tempo of biological invasions increases, explanations and predictions of their impacts become more crucial. Particularly with regard to biodiversity, we require elucidation of interspecific behavioural interactions among invaders and natives. In freshwaters in The Netherlands, we show that the invasive Ponto–Caspian crustacean amphipod Dikerogammarus villosus is rapidly eliminating Gammarus duebeni, a native European amphipod, and Gammarus tigrinus , until now a spectacularly successful invader from North America. In the laboratory, survival of single (unguarded) female G. duebeni was significantly lower when male D. villosus were free to roam as compared with isolated within microcosms. In addition, survival of paired (guarded) female G. duebeni was significantly lower when male D. villosus as compared with male G. duebeni were present. D. villosus killed and consumed both recently moulted and, unusually, intermoult victims. Survival of G. tigrinus was significantly lower when D. villosus were free to roam as compared with isolated within microcosms and, again, both moulted and intermoult victims were preyed upon. Male D. villosus were significantly more predatory than were females, while female G. tigrinus were significantly more often preyed upon than were males. Predation by D. villosus on both species occurred over a range of water conductivities, an environmental feature previously shown to promote amphipod coexistence. This predatory invader is predicted to reduce further the amphipod diversity in a range of freshwater habitats in Europe and North America.

The predatory impact of the freshwater invader Dikerogammarus villosus on native Gammarus pulex (Crustacea: Amphipoda); influences of differential microdistribution and food resources

Predation between invading and native species can produce patterns of exclusion and coexistence. Dikerogammarus villosus , a Ponto-Caspian amphipod species, has invaded many central European freshwaters in the past decade, replacing native Gammarus amphipod species. For instance, the arrival of D. villosus in Holland has been accompanied by the decline of Gammarus duebeni and G. tigrinus populations within invaded systems. This study examined what may happen when D. villosus eventually encounters native Dutch populations of Gammarus pulex , and how factors such as microhabitat and food resource availability could contribute to a future species replacement or coexistence. A laboratory simulation of a lake/pooled area of river indicated that G. pulex and D. villosus differed in distribution within the same habitat, and showed that although the distribution of the native differed in the presence of the invader, the presence of the native had no effect on the distribution of the invader. Gammarus pulex suffered severe intraguild predation (IGP) from D. villosus in mixed species treatments with no reciprocal predation of D. villosus by G. pulex . This IGP occurred regardless of whether no alternative food resource was available (91% of the G. pulex population eliminated after 7 days), or alternative foods/prey were available to excess, such as leaf material (85%), chironomids (77%) or fish food flakes (74%). We conclude that although differential microdistribution of the two species could promote coexistence, the presence of alternative foods/prey resources, merely slow the rate of IGP and replacement of the native by the invader. Our study joins one of an increasing number emphasizing the potential damaging impacts of D. villosus on native communities.

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.

Environmental and morphological factors influencing predatory behaviour by invasive non-indigenous gammaridean species

Predatory behaviour seems to be more frequent in invasive gammaridean species than in native ones. This results in the exclusion of other, mostly native gammaridean species and a change in benthic communities. The present study analysed the influence of environmental factors (water temperature) and morphological factors (sex, body parts involved in catching and holding prey) on the predatory behaviour of Dikerogammarus villosus. A diet study of invasive relatives of D. villosus showed that predation intensity is especially high in spring and summer, that is, at increasing and high temperatures. Experiments with D. villosus in climate rooms at various temperatures, using the native Gammarus fossarum as prey, showed that the average predation rate by both sexes gradually increased over the temperature range from 5 to 30°C. Natural mortality during the experiments was negligible compared to losses due to predation. At each temperature, the predation rate by females was lower than that by males. Males showed a steep allometric growth of body parts involved in the process of catching and holding prey, compared to females at increasing body size in a number of measurements. This may explain the difference in predatory behaviour between males and females, which plays a role in intraguild predation a supposed mechanism for species displacement.

Direct and indirect effects of species displacements; an invading freshwater amphipod can disrupt leaf litter processing and shredder efficiency

Abstract Invasive species may threaten the fundamental role played by native macroinvertebrate shredders in determining energy flow and the trophic dynamics of freshwater ecosystems. Functionally, amphipods have long been regarded as mainly shredders, but they are increasingly recognized as major predators of other macroinvertebrate taxa. Furthermore, intraguild predation (IGP) between native and invasive amphipods underlies many species displacements. We used laboratory mesocosms to investigate what might happen to shredders and leaf-litter processing in water bodies invaded by the highly predatory Ponto-Caspian amphipod Dikerogammarus villosus, which is spreading rapidly throughout Europe and may soon invade the North American Great Lakes. The leaf-shredding efficiency of D. villosus was significantly lower than that of 3 Gammarus species (2 native and 1 invasive) that D. villosus has either already displaced or may be currently displacing in The Netherlands. In addition, D. villosus was a major predator of all of these native and invasive amphipod shredders and of a common isopod shredder Asellus aquaticus. Leaf processing in Gammarus and Asellus mesocosms declined rapidly in the presence of D. villosus and ceased altogether within 4 d because by then, all potential shredders had been killed and consumed. Furthermore, the shredding efficiency of surviving amphipods and isopods declined significantly within 2 d of the release of D. villosus, a result indicating that predator-avoidance behavior may override leaf processing. We discuss the implications of these direct and indirect effects of D. villosus invasions and species displacements on community structure and litter processing in aquatic ecosystems.

Feeding on micro-algae in the invasive Ponto-Caspian amphipod Dikerogammarus villosus (Sowinsky, 1894)

Feeding on micro-algae is shown in the invasive Ponto-Caspian amphipod Dikerogammarus villosus. Compared with controls, males, females and juveniles of this species significantly reduced the concentration in suspension of unicellular micro-algae. Juveniles had higher concentrations of algae in the cardiac gut than adults. The presence of these algae in the mid- and hindgut was also recorded. This feeding behaviour was filmed and the mechanisms involved are described and discussed. We comment on the use of the Functional Feeding Group (FFG) concept to classify feeding in amphipods. The role of being a feeding-generalist in aiding the invasion process is also discussed.

Potential roles for differential body size and microhabitat complexity in mediating biotic interactions within invasive freshwater amphipod assemblages.

Cannibalism and intraguild predation (IGP) are common amongst freshwater amphipod crustacean as- wsemblages, particularly between individuals of different body size, with IGP of smaller by larger species. The decline of Gammarus tigrinus populations in mainland Europe has been accompanied by the arrival of the Ponto- Caspian invader Dikerogammarus villosus and previous studies have implicated IGP of G. tigrinus by the larger D. villosus, as the principal driving force in this replacement. We examined how factors such as microhabitat and body size may mediate both cannibalism within G. tigrinus populations and IGP by D. villosus and thus contribute to fi eld patterns of coexistence and exclusion. A fi eld survey of an invaded Dutch lake indicated that G. tigrinus and D. villosus differed in distribution, with D. villosus being the numerically dominant amphipod (80-96 %) on the rocky boulder substrate of the shoreline and G. tigrinus being the dominant amphipod (100 %) in the crushed shell/sand matrix immediately adjacent to this. Laboratory microcosm experiments indicated that G. tigrinus cannibalism, particularly of smaller by larger size classes, may be common. In addition, although D. villosus predation of all G. tigrinus size classes was extreme, the smallest size classes suffered the highest predation. Indeed, when exposed to D. villosus, predation of larger G. tigrinus was lowest when smaller G. tigrinus were also present. Increasing mi- crohabitat complexity from a simple bare substrate littered with Dreissena polymorpha zebra mussels to a crushed shell/sand matrix signifi cantly reduced both cannibalism and IGP. Our study emphasizes the need to consider both life history stages and habitat template, when considering the impacts of biotic interactions and it also emphasizes that complex, interacting factors may be mediating the range expansion of D. villosus.

‘Killer Shrimps’, Dangerous Experiments and Misguided Introductions: How Freshwater Shrimp (Crustacea: Amphipoda) Invasions Threaten Biological Water Quality Monitoring in the British Isles

Abstract In 2010, the ‘killer shrimp’ Dikerogammarus villosus (Crustacea: Amphipoda) invaded the British Isles. Past research from central Europe has shown this eastern European shrimp invader to be a ‘voracious omnivore’, highly predatory of a wide range of freshwater macroinvertebrate taxa and also fish fry. It can become ‘super-abundant’ within invaded sites, greatly dominating native assemblages in terms of numbers and biomass. Although the vast majority of past research has focused on the negative impacts of D. villosus invasion on native biodiversity, we consider the usually overlooked implications for biological water quality monitoring and ecological assessment. We show how past invasions of other freshwater shrimp in the British Isles, such as Gammarus pulex and Crangonyx pseudogracilis, have undermined the ability of biotic indices to reliably reflect changes in water quality. Within such invasions, more pollution tolerant invaders can replace more sensitive natives and invaders can be highly predatory of other macroinvertebrate taxa which contribute to biotic indices. We predict the impacts of the D. villosus invasion will be greater than any previous shrimp invasion of the British Isles and indeed potentially of any other freshwater macroinvertebrate invasion thus far. As it spreads throughout the British Isles, we predict this species will have drastic deleterious impacts on native macroinvertebrate assemblages, especially in its preferred rocky/stony habitats. We consider ways forward for future biological water quality monitoring and ecological assessment within D. villosus invaded watercourses.

Is Gammarus tigrinus (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.

The freshwater Amphipoda of the Falkland Islands

All four species of freshwater amphipods recorded in the literature from the Falkland Islands were found again during a recent expedition. Hyalella curvispina Shoemaker has been previously recorded, but under several incorrect names. Hyalella neonoma nom. nov. is proposed for the preoccupied name H. patagonica, the commonest species in our collections. Falklandella obtusa Schellenberg and F. cuspidata Schellenberg are completely redescribed, and the latter is removed to a new genus, Praefalklandella. According to their external morphology, the last two genera belong to the Crangonyctoidea, but the calceoli discovered on both antennae of the male of Praefalklandella are surprisingly of the ‘gammarid’ type not the ‘crangonyctid’ type.