Strategies of a successful new invader in European fresh waters: fecundity and reproductive potential of the Ponto-Caspian amphipod Dikerogammarus villosus in the Austrian Danube, compared with the indigenous Gammarus fossarum and G. roeseli

1. Fecundity of a Dikerogammarus villosus population at Spitz was studied in the Austrian Danube during the 3‐year period 2002–2004. Ovigerous females were absent in October and November, and extremely scarce in December when the reproductive season started again slowly. From January to September pre‐copulatory pairs and egg‐carrying females were present. The reproductive cycle lasted for 9–10 months. 2. Various pigmentation phenotypes of D. villosus have been described in the literature. However, no significant differences were found between the reproductive variables studied here and several colour morphs. Mating was size‐assortative; mean body length of males was about 1.3 times greater than that of their potential mates, and the wet weight was approximately twice as heavy. 3. The relationship between the number of embryos per clutch and the wet weight of females was described by a 3‐parameter power equation. The population mean was 43 eggs with a range of five to 194 eggs. Eighty‐two specimens from 1359 D. villosus females had more than 100 eggs: the smallest of these females was 12 mm long (30 mg) wet weight, and the largest, which was 18 mm long (91 mg), had 194 eggs in embryonic development stage 4. 4. Numbers of embryos in developmental stages 2 (early egg stage) and 7 (newly hatched neonates) differed significantly with body wet weight of ovigerous females (P < 0.05). For an average female in the range 10–12 mm (20–30 mg) the number of juveniles in the brood pouch was 74% of the number of stage 2 eggs. This value can be interpreted as the survival rate of eggs. 5. The overall mean egg volume (EV, ±95% CL) of stage 2 eggs of D. villosus was 0.05 ± 0.001 mm³, and EV increased significantly at each stage of development. At stage 6, egg volume had increased by a factor of 2.6, and averaged 0.13 ± 0.001 mm³. In comparison, G. fossarum and G. roeseli had significantly larger eggs in all developmental stages. 6. Mean egg size of D. villosus (0.063 mm³) was maximal in January. For D. villosus (and G. roeseli) the minimum mean egg size occurred in September. In contrast to G. fossarum and G. roeseli, a second peak in egg size was not observed for D. villosus, and egg size fell more or less successively from January to September. 7. A simple index of fecundity was calculated from the number of stage 2 eggs divided by the female's wet weight. The highest values were observed in April and May, when females from the overwintering generation grew to their maximum body size. Thus the release of a large number of neonates corresponds with the availability of plentiful food and rising water temperatures for juvenile growth in the spring. The lowest value occurred in December. In June the small females of a summer generation appeared, with a naturally low fecundity. 8. The relationship between brood development time and water temperature was studied in the laboratory at a series of constant temperatures. At 16 °C, mean brood development time was 14 days for D. villosus, compared with about 3 weeks for the indigenous species. At 10 °C, mean brood development time was 24 days in D. villosus, compared with 40 days in G. fossarum and 44 days in G. roeseli. At 4 °C it was 1.8 and 3.5 times longer in G. fossarum and G. roeseli. 9. The number of offspring produced by a single clutch from a large female D. villosus is considerably higher than the total numbers produced by the indigenous freshwater gammarids, such as G. fossarum, G. roeseli and G. pulex, during their life‐spans of 1.5–2 years in seven to nine successive broods. Only one or two large ovigerous D. villosus would probably be enough to start a new population. A potentially high reproductive capacity, comparatively small eggs, optimal timing to release the maximum number of neonates per female in April/May, and a long reproductive cycle, together with rapid development of eggs, rapid growth to sexual maturation, short life span, tolerance to a wide range of environmental conditions, and exceptional predatory capabilities, all give the invasive Ponto‐Caspian gammarid an opportunity to become globally distributed in freshwater ecosystems of the temperate climate zone.     

A local gammarid uses kairomones to avoid predation by the invasive crustaceans Dikerogammarus villosus and Orconectes limosus

Since the 1980s the Ponto-Caspian gammarid Dikerogammarus villosus has spread throughout Europe while displacing native species and is predicted to invade further continents. After it was introduced into Europe in the 1890s the North American crayfish Orconectes limosus spread throughout Europe and served as a vector to displace native crayfish as well. In Lake Constance (Germany) the previously dominant gammarid Gammarus roeselii is subjected to both of these invasive crustaceans. In our experiments both species placed predation pressure on G. roeselii. Kairomone perception tests in a Y-maze revealed the capability of the gammarids G. roeselii and D. villosus to perceive and avoid the scent of the predator crayfish O. limosus. Both species also avoided the kairomone of the other gammarid, but did not avoid the scent of their own species. This taxa specific behavior suggests that taxa specific signals are used. This behavior can help the gammarids avoid shelters previously occupied by predators.     

Potential alteration of cross‐ecosystem resource subsidies by an invasive aquatic macroinvertebrate: implications for the terrestrial food web

相似文献   

Effectiveness of zebra mussels to act as shelters from fish predators differs between native and invasive amphipod prey

Biological invasions cause organisms to face new predators, but also supply new anti-predator shelters provided by alien ecosystem engineers. We checked the level of anti-predator protection provided to three gammarid species by an invasive Ponto-Caspian zebra mussel Dreissena polymorpha, known for its habitat modification abilities. We used gammarids differing in their origin and level of association with mussels: Ponto-Caspian aliens Dikerogammarus villosus (commonly occurring in mussel beds) and Pontogammarus robustoides (not associated with mussels), as well as native European Gammarus fossarum (not co-occurring with dreissenids). The gammarids were exposed to predation of two fish species: the racer goby Babka gymnotrachelus (Ponto-Caspian) and Amur sleeper Perccottus glenii (Eastern Asian). This set of organisms allowed us to check whether the origin and level of association with mussels of both prey and predators affect the ability of gammarids to utilize zebra mussel beds as shelters. We tested gammarid survival in the presence of fish and one of five substrata: sand, macrophytes, stones, living mussels and empty mussel valves. D. villosus survived better than its congeners on all substrata, and its survival was highest in living dreissenids. The survival of the other gammarids was similar on all substrata. Both fish species exhibited similar predation efficiency. Thus, D. villosus, whose affinity to dreissenids has already been established, utilizes them as protection from fish predators, including allopatric predators, more efficiently than other amphipods. Therefore, the presence of dreissenids in areas invaded by D. villosus is likely to help the invader establish itself in a new place.     

There is more than one ‘killer shrimp’: trophic positions and predatory abilities of invasive amphipods of Ponto‐Caspian origin

1. Biological invasions are regarded as one of the greatest threats to biological diversity. One of the macroinvertebrate groups with the largest number of invasive species in fresh water are gammarid amphipods. Their omnivorous (including predatory) feeding behaviour may facilitate their spread and establishment in new areas. 2. Dikerogammarus villosus, the ‘killer shrimp’, is a well‐known example of a Ponto‐Caspian gammarid that is a very effective predator and successful coloniser in Europe. There are, however, other invasive Ponto‐Caspian amphipods, which have spread successfully in Northern, Central and Western Europe. Our aim here was to test whether two of such invaders (Pontogammarus robustoides and Dikerogammarus haemobaphes) are also more predacious than a native species (Gammarus fossarum). 3. Stable isotope analysis (δ¹⁵N and δ¹³C) of Ponto‐Caspian amphipods coexisting in a reservoir demonstrated that the trophic positions of P. robustoides and D. haemobaphes were similar to that of D. villosus. Echinogammarus ischnus and Chelicorophium curvispinum occupied the lowest position in the food web, while the native Gammarus fossarum (collected from another waterbody) had an intermediate trophic position. 4. Stomach content analysis of P. robustoides, D. haemobaphes and G. fossarum collected in the field, as well as laboratory feeding experiments, was used to compare diet and feeding preferences among the two invasive and one native species. All three species were omnivorous and predacious. However, the two invasive species (P. robustoides and D. haemobaphes) were more effective predators than G. fossarum and showed a clear preference for animal prey and tissue. 5. Pontogammarus robustoides and D. haemobaphes may, like D. villosus, also be called ‘killer shrimps’ and could have a similar impact as invaders of European freshwater and brackish waterbodies.     

Molecular markers for systematic identification and population genetics of the invasive Ponto‐Caspian freshwater gammarid Dikerogammarus villosus (Crustacea,Amphipoda)

The Ponto‐Caspian amphipod, Dikerogammarus villosus, is an invasive species of many European rivers. First, we show that size difference of nrDNA ITS1 allows discriminating D. villosus from Dikerogammarus bispinosus, a closely related but morphologically hardly distinguishable species. Second, we present two types of polymorphic markers for D. villosus, three microsatellites and two single nucleotide polymorphisms (SNPs) of mtDNA COI gene, which were scored by polymerase chain reaction‐single strand conformational polymorphism (PCR‐SSCP). These markers will be very useful in studying population genetics of D. villosus.     

Potential effects of the invasive ‘killer shrimp’ (Dikerogammarus villosus) on macroinvertebrate assemblages and biomonitoring indices

1. Water quality monitoring data from 10 watercourses and laboratory mesocosm studies were used to assess the potential impacts of the crustacean amphipod invader Dikerogammarus villosus on resident macroinvertebrate assemblage structure in Central European fresh waters. 2. The presence of D. villosus was associated with a decline in the prevalence of many native species, pollution sensitive as well as pollution tolerant, and changes in biotic indices, despite the trends of improved water quality coinciding with the invasion period. A general increase in the prevalence of other invaders was also noted. The potential impacts of D. villosus were substratum dependent, differing between stone, concrete and sand‐dominated sites. 3. Mean Multimetric Macroinvertebrate Index Flanders (MMIF) values were marginally lower when D. villosus was present (P < 0.06), as opposed to when other amphipod species or no amphipods were present, despite the improved water quality. Mesocosm studies showed that several macroinvertebrate taxa were completely eliminated in treatments with D. villosus, oligochaete worms, Caenidae mayfly, chironomids and tipulids being particularly vulnerable to D. villosus predation. Biological Monitoring Working Party (BMWP) scores were lower in mesocosms with D. villosus as opposed to the native Gammarus pulex or no amphipods at all. 4. We predict that resident macroinvertebrate assemblages in both Central Europe and Britain will come under increasing pressure as D. villosus invasions progress. Consequently, macroinvertebrate biotic indices, such as the MMIF or BMWP, may need to be revised to account for changes in taxa sensitivities to water quality as well as increased predation and competition.     

Mutual predation between and cannibalism within several freshwater gammarids: Dikerogammarus villosus versus one native and three invasives

Dikerogammarus villosus, a Ponto-Caspian species, is known to be a particularly successful invader, and is currently the prevailing invasive gammarid in Southern German large waters. Field observations suggest that D. villosus has replaced the native G. pulex and the invasive D. haemobaphes, also a Ponto-Caspian species, in some reaches of the German Danube. Dikerogammarus villosus is also believed to be the reason why Echinogammarus berilloni, a Mediterranean species, and Dikerogammarus bispinosus, a further Ponto-Caspian gammaridean species, could not build significant populations in Southern German rivers. Because intraguild predation (IGP) is regarded as a major force leading to species displacements, we hypothesized that superiority in predation by D. villosus is responsible for the disappearance or limited occurrence of several native and invasive gammarideans in many reaches of Southern German waters. To test this hypothesis, we conducted a series of laboratory IGP experiments with 1:1 combinations in which D. villosus was allowed to prey on other gammarids and vice versa. We also studied the extent of cannibalism within species. Dikerogammarus villosus was a stronger predator than G. pulex and E. berilloni. However, D. villosus was not stronger in predation than its relatives D. haemobaphes and D. bispinosus, although results with the combinations of D. villosus and D. bispinosus were less conclusive, especially in the female–female combination. Cannibalism rate was high in D. haemobaphes compared with other species. We conclude that superiority in IGP experiments could explain the disappearance of G. pulex and the missing or limited occurrence of E. berilloni since the arrival of D. villosus in Southern German rivers. However, the results of IGP experiments do not explain why D. haemobaphes was displaced by D. villosus in many places and why D. bispinosus could not build significant populations in Southern German waters. Possibly specific combinations between mutual predation and cannibalism or factors other than predation could have affected displacement and occurrence patterns of Dikerogammarus species in Southern German waters.     

Interference competition among native and invader amphipods

Aquarium experiments were used to study indications of interference competition, such as substratum choice shifts, swimming activities and mortality of invasive and indigenous gammarids in each other's presence. The more recent invaders Gammarus tigrinus and Dikerogammarus villosus were more likely to prefer stone substratum, whereas the native Gammarus pulex and an earlier invader Gammarus roeseli were found more frequently in the water layer. Sand was the least likely substratum to be chosen by any of the species. G. pulex and G. roeseli did not alter their substratum preference in each other's presence. In the presence of D. villosus, G. pulex shifted towards smaller stones and increased its swimming activities, whereas D. villosus did not change its behaviour in the presence of G. pulex. These shifts may indicate interference competition, with D. villosus being the stronger competitor. The greatest shifts in substratum preference arose when one species had occupied a substratum before the other one was introduced, especially when D. villosus was already present before G. pulex was introduced, possibly indicating pre-emptive competition. Swimming activities of G. pulex increased in the presence of D. villosus, whereas D. villosus spent little time swimming. Mortality was comparable between the different experiments without any indication of predation. The effect of Intra Guild Predation (IGP) may not be reflected adequately by short-time experiments as moults occurred seldom during the experiments. Although no IGP was observed during our experiments, habitat shifts occurred, which may indicate that competitive interactions are apparent before IGP starts. Such shifts may serve to avoid intraguild competition.     

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.     

Ontogenetic shift in the trophic role of the invasive killer shrimp Dikerogammarus villosus: a stable isotope study

The introduction of the amphipod Dikerogammarus villosus in European fresh waters is to date recognized as a threat to the integrity of invaded communities. Predation by D. villosus on native benthic invertebrates is assumed as the key determinant of its ecological impact, yet available information describe the species as a primary consumer as well as a carnivore depending on local conditions. Here, we assessed the trophic position (TP) of D. villosus in Lake Trasimeno, a recently invaded lentic system in central Italy, using the CN isotopic signatures of individuals captured in winter spanning two orders of magnitude in body size. TP estimations were compared with those characterizing the native amphipod Echinogammarus veneris and other representative invertebrate predators. On average, D. villosus showed a trophic position higher than E. veneris, and comparable with that of odonate nymphs. An in-depth analysis revealed that large-sized individuals had a trophic position of 3.07, higher than odonates and close to that of the hirudinean predator Erpobdella octoculata, while small-sized specimens had a trophic position of 2.57, similar to that of E. veneris (2.41). These findings indicate that size-related ontogenetic shifts in dietary habits may per se vary the nature of the interaction between Dikerogammarus villosus and native invertebrates from competition to predation. Information collated from published isotopic studies corroborated the generality of our results. We conclude that intra-specific trophic flexibility may potentially amplify and make more multifaceted the impact of the species on other invertebrate species in invaded food webs.

     

Evaluation of the use of Water Framework Directive typology descriptors, reference sites and spatial scale in macroinvertebrate stream typology

The Rhine ecosystem is highly influenced by anthropogenic stresses from pollution, intensive shipping and increased connectivity with other large European rivers. Canalization of the Rhine resulted in a reduction of heterogeneity to two main biotopes: sandy streambeds and riverbanks consisting of groyne stones. Both biotopes are heavily subjected to biological invasions, affecting the rivers food web structure. The Ponto-Caspian amphipods, Chelicorophium curvispinum and Dikerogammarus villosus, have exerted the highest impact on this food web. The filterfeeding C. curvispinum dominated the Rhine food web on the stones in 1998, swamping the stone substrata with mud. However, in 2001 it decreased in numbers, most likely due to top-down regulation caused by increased parasitic and predatory pressure of other more recently invaded Ponto-Caspian species. D. villosus showed a fast population increase after its invasion and particularly influenced the macroinvertebrate community on the stones by predaceous omnivory. This species seemed to have maintained its predatory level after its population established. Effects of these mass invaders on the macroinvertebrate community of sandy streambeds in the Rhine are unclear. Here, low densities of macroinvertebrates were observed with the Asiatic clam, Corbicula fluminea, as most abundant species. Stable isotope values of food webs from the stones and sand in 2001 were similar. Aquatic macrophytes are nearly absent and the food web is fuelled by phytoplankton and particulate organic matter, originating from riparian vegetation as indicated by similar δ¹³C values. Omnivores, filter-, deposit-, and detritus-feeders are the primary and secondary macroinvertebrate consumers and function as keystone species in transferring energy to higher trophic levels. Invaders comprise 90% of the macroinvertebrate numbers, and can be considered ecosystem engineers determining the functional diversity and food web structure of the Rhine by either bottom-up or top-down regulation.     

Reproductive biology of Dikerogammarus haemobaphes: an invasive gammarid (Crustacea: Amphipoda) colonizing running waters in Central Europe

Dikerogammarus haemobaphes is a Ponto-Caspian gammarid that has invaded vast areas in Central and Western Europe. Our paper is a first presentation of its life history features in an invaded region. The study was conducted in the Vistula River in Poland from autumn 2003 to autumn 2005 in two sites differing in hydrological conditions with one being water reservoir. The results showed that the reproductive period lasted 8 months from April till October in both sites. Three generations per 1 year were observed: autumn (overwintering), spring and summer. Ten cohorts per year were distinguished. The individuals from the reservoir were much bigger than those from the other site. The fecundity of those specimens was also higher and they laid 52 eggs per clutch in average in comparison with 37 eggs in the river itself. The strong relationship between the number of embryos (in developmental stage 2) per clutch and the length of females was noticed. The overall mean egg size of stage 2 of D. haemobaphes was 0.430 ± 0.029 mm which is smaller than noted for native species such as Gammarus fossarum. A potentially high reproductive capacity, comparatively small eggs, short time of eggs’ development, fast reaching sexual maturation, short life span, tolerance to a wide range of environmental conditions, all promote the invasion of this Ponto-Caspian gammarid in freshwater ecosystems of the temperate climate zone.     

How to be an invasive gammarid (Amphipoda: Gammaroidea)–comparison of life history traits

About six life history and two ecological traits of gammarid species occurring in Central European waters were compared in order to identify the characters of successful invader. The species were (1) natives: Gammarus fossarum, G. pulex, G. lacustris, G. varsoviensis, G. balcanicus, G. leopoliensis, G. roeselii, and (2) aliens: Gammarus tigrinus, Chaetogammarus ischnus (=Echinogammarus ischnus), Pontogammarus robustoides, Obesogammarus crassus, Dikerogammarus haemobaphes, D. villosus. Generally the alien species were characterised by a combination of large brood size, high partial fecundity, early maturation and by appearance of higher number of generations per year. Also, these species presented higher tolerance towards severe environmental conditions, i.e. elevated salinity and human degradation of the environment. The above features seem to facilitate the colonisation of new areas and competition with native species -- a phenomenon that has been currently observed in various parts of Europe.     

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.     

Co‐variation between the intensity of behavioural manipulation and parasite development time in an acanthocephalan–amphipod system

Pomphorhynchus laevis, a fish acanthocephalan parasite, manipulates the behaviour of its gammarid intermediate host to increase its trophic transmission to the definitive host. However, the intensity of behavioural manipulation is variable between individual gammarids and between parasite populations. To elucidate causes of this variability, we compared the level of phototaxis alteration induced by different parasite sibships from one population, using experimental infections of Gammarus pulex by P. laevis. We used a naive gammarid population, and we carried out our experiments in two steps, during spring and winter. Moreover, we also investigated co‐variation between phototaxis (at different stages of infection, ‘young’ and ‘old cystacanth stage’) and two other fitness‐related traits, infectivity and development time. Three main parameters could explain the parasite intra‐population variation in behavioural manipulation. The genetic variation, suggested by the differences between parasite families, was lower than the variation owing to an (unidentified) environmental factor. Moreover, a correlation was found between development rate and the intensity of behavioural change, the fastest growing parasites being unable to induce rapid phototaxis reversal. This suggests that parasites cannot optimize at the same time these two important parameters of their fitness, and this could explain a part of the variation observed in the wild.     

The very hungry amphipod: the invasive <Emphasis Type="Italic">Dikerogammarus villosus</Emphasis> shows high consumption rates for two food sources and independent of predator cues

The invasion of the Ponto–Caspian amphipod Dikerogammarus villosus in European rivers is assumed to reduce macroinvertebrate diversity and to alter ecosystem functions. D. villosus shows an extraordinarily flexible feeding behavior including the ability to use various food sources. On the other hand, its response to predation risk seems to depend on environmental factors. To evaluate the ecological function of D. villosus, we estimated the daily food consumption for different food sources and analyzed potential effects of predator avoidance behavior on feeding. D. villosus consumption of willow leaves or chironomid larvae was quantified in 24-h laboratory experiments with and without kairomones of the European bullhead (Cottus gobio). Consumption rates were estimated based on gut content and gut evacuation rate under semi-natural laboratory conditions enabling the animals to feed over the whole time of the evacuation rate experiment. We observed very high evacuation rates and consequently high consumption rates up to 89% of body weight per day. Consumption rates differed significantly between food sources: D. villosus ingested more leaves than chironomid larvae. In contrast, predator cues did not affect the feeding of D. villosus. This might be explained by its strong refuge affinity and probably benefits its successful invasion. A comparison of the estimated consumption rates with results of an own consumption experiment (and other studies) under more artificial conditions indicated that more natural conditions result in higher consumption rates. Consequently, feeding rates from highly artificial experiments should be used with great caution to assess the ecosystem function of D. villosus.     

Effect of salinity on the distribution of Ponto-Caspian gammarids in a non-native area – environmental and experimental study

The native area of gammarids from the so-called ‘Caspian complex’, Pontogammarus robustoides (G.O. Sars, 1894), Obesogammarus crassus (G.O. Sars, 1894), Dikerogammarus haemobaphes (Eichwald, 1841) and D. villosus (Sowinsky, 1894), is associated with brackish waters. Over the last several decades they have colonized the European inland waters and part of the brackish Baltic Sea. It is believed that anthropogenic increase in the salinity of inland waters facilitated their expansion. However, the influence of salinity on the dispersal of gammarid species outside their native area is not fully understood. We tested the hypothesis that salinity was a major factor in determining distribution, based on the abundance of Gammaridae in three coastal areas of low salinity (brackish Baltic), i.e. 0.3, 3.4 and 7.3 PSU, successfully inhabited by them. Additionally, for the first time, the effect of water salinity on the osmoregulatory capacity of O. crassus was examined under laboratory conditions, for the salinities given above. The experiments showed that similarly as in the case of other Caspian complex species, salinity values of about 7 PSU create better conditions for osmoregulation in O. crassus than lower salinities (i.e. 0.3 and 3.4 PSU). In the environmental part of the study, we observed that only D. villosus achieved a significantly higher abundance in the area of 7.3 PSU. Thus, we concluded that in the range of 0.3–7.3 PSU, salinity is not a key factor governing the distribution of Ponto-Caspian gammarids.     

Influence of substrate preference and complexity on co-existence of two non-native gammarideans (Crustacea: Amphipoda)

Substrate choice, swimming activity and risk to predation by burbot (Lota lota) of the well established Gammarus roeselii and the invader Dikerogammarus villosus were studied in mixed and single-species aquarium experiments. We used stones, gravel and aquatic weeds (Elodea, Chara) as substrates. We hypothesized that both species have different substrate preferences and that substrate affects the predation risk. We also assumed that presence of D. villosus influences substrate preference and predation risk of G. roeselii since the invader is known to affect the behavior of other gammarids. Adults of D. villosus in single species experiments and juveniles in mixed and single species experiments were evenly distributed over the different substrates but adults in mixed species experiments were more likely to prefer stone substrate. In contrast, adults and juveniles of G. roeselii clearly preferred aquatic weeds independent of the presence/absence of the invader. Both species preferred substrates with fissured surface over substrates with smooth surface. Gammarus roeselii was observed swimming more often than D. villosus in the open water but its swimming activity was lower when its preferred substrate was present compared with its swimming activity if non-preferred substrates were present. Predation rate of burbot on D. villosus was comparatively low and independent of the substrate. Burbot consumed many more G. roeselii than D. villosus, both in mixed and single species experiments. But when the preferred substrate of G. roeselii (weeds) was used in the experiments, predation rate of burbot on G. roeselii was somewhat lower than that when non-preferred substrates were present. The results of the experiments support our hypothesis that the gammarids studied here have different substrate preferences and that presence of the preferred substrate can affect predation risk. However, there is no evidence that presence of D. villosus affected substrate choice or predation risk in G. roeselii. We consider that differences in use of spatial niches permit co-existence of G. roeselii and D. villosus in the wild when substrates are diverse. The fact that G. roeselii than D. villosus is more often observed swimming in the open water may explain its higher risk of being captured by fish.