Predators shape the behavior and distribution of prey organisms, driving their evolution and environmental impact. We studied relationships between interspecific competition and predation risk in Ponto-Caspian invasive gammarid crustaceans: Pontogammarus robustoides and Dikerogammarus villosus. We hypothesized that a stronger competitor D. villosus would displace P. robustoides from a preferred habitat, but the frequency of this displacement would be reduced in the presence of a higher fish predator, the racer goby. We studied gammarid preferences for stone or sand substrata in 24-h pairwise-choice tests in single-species or mixed-species treatments, at 2 densities (12 or 24 individuals per tank), in the presence or absence of a predator. D. villosus displaced P. robustoides from the stone habitat, even at the low density. As P. robustoides tested separately preferred stones at both densities, its displacement resulted from active avoidance of a stronger competitor even when the substratum was not actually a limited resource. The presence of a predator reduced the number of instances of displacement of P. robustoides by D. villosus, allowing the weaker species to stay on its preferred substratum. Nevertheless, the presence of D. villosus still increased fish predation on P. robustoides. The presence of a predator modifies relationships between prey organisms, allowing a weaker competitor to stay in its preferred habitat. Thus, antipredator responses of prey organisms may have cascading effects on the functioning of the community, affecting habitat choice and competition among species.

Racer goby is one of several PontoCaspian gobiids spreading throughout European rivers and concurrent with recent declines in threatened populations of a native species of similar biology, the European bullhead. Although suggestive of competitive interactions, evidence thereof is scarce, so we examined behavioural interactions between racer goby and bullhead (single specimens of each species together, also pairs of each species) under experimental conditions (shared space with two shelters) to determine whether the invader displaces the native species when food resources are limited. Food (live chironomids) was added to a single feeder at rates below satiation levels twice over 24 h (once in light and once in darkness), with fish behaviour (aggressive interactions: attacks and threatening) and feeding activity (time spent near or inside the feeder) recorded using video cameras and infrared illumination. Racer goby exhibited aggressive behaviour towards bullhead (mean = 2.5 aggressive events h -1 ), but rarely the inverse (threatening only, mean = 0.05 events h -1 ), significantly limiting bullhead foraging time (by 62 %) and being faster to reach food in the feeding time in 76 % of cases. Gobies were more aggressive during daylight (77 % of all aggressive events occurring in light), and both species spent more time on feeding activities in darkness (88 and 66 % of all time spent in the feeder by bullheads and gobies, respectively). However, the adverse impact of goby on bullhead was independent of light conditions. Our results suggest that under natural conditions, racer goby are likely to displace bullhead during feeding, with potential consequences for foraging efficiency.

We studied the attachment strength and aggregation behaviour of Dreissena polymorpha in the presence of large roach Rutilus rutilus ([180 mm total length) (efficient molluscivore), small roach (\110 mm) (unable to feed on zebra mussels) and perch Perca fluviatilis (not feeding on mussels). The intention was to check whether small (\10 mm) and large ([10 mm) mussels would respond specifically to fish capable of consuming them (i.e. large roach). After 1 day of exposure, we found no significant differences in mussel attachment strength. After 6 days in the presence of large roach, mussels were attached more strongly than in the other treatments. After a 1-day exposure to all kinds of fish, mussels were more aggregated than in the control treatment. After 6 days, the largest percentage of aggregated mussels was found in the presence of large roach, while the aggregation levels in the other treatments were lower and did not differ from one another. Perhaps, an initial response was a non-specific reaction to the presence of any fish, while a specific response to large roach appeared later. Thus, zebra mussels were able to recognize their potential predators. The observed behaviour of mussels may enhance their resistance to molluscivores in the field by limiting the access of predators to their potential prey (due to the increased aggregation of prey) and by increasing predator handling costs (due to the stronger attachment of prey).

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Because of the increasing anthropogenic pressure, the amount of solid waste discarded in water is growing considerably. It can affect aquatic organisms, modifying their distribution and abundance. This study was designed to investigate the effect of several types of solid waste (bottles, textiles, bundles of string, pieces of polythene and rubber) upon the taxonomic composition and biodiversity of macroinvertebrates inhabiting the near-shore zone (up to 1-m depth) of the Włocławek Reservoir (the River Vistula, central Poland). The epifauna developing on these materials was compared with organisms living on natural substrata: bare sandy bottom (from which the waste was collected), macrophytes and sandy bottom overgrown by plants. The communities inhabiting the artificial substrata (except strings) were much more diverse (in terms of taxon richness, Shannon-Wiener index and evenness) than the surrounding bare sandy bottom. The biodiversity on the waste was similar to that found on the macrophytes and bottom among them, located nearby. The most suitable materials appeared to be bottles, providing shelters in their interiors. The artificial substrata differed considerably from all natural habitats with respect to taxonomic composition, though they were more similar to the macrophytes than to the bottom sediments. Furthermore, the variability among the assemblages developing on the particular waste objects, even of the same type, was much larger than that observed on the macrophytes, indicating the patchy nature of the former habitat. Thus, discarded waste can constitute alternative habitats for invertebrates, especially when natural substrata are unsuitable (e.g. sandy bottom), but they cannot fully replace natural substrata, such as plants, in their habitat-forming role in ecosystems.

Pontogammarus robustoides has spread across Europe since the second half of the twentieth century. It is often regarded as a psammophilous and/ or phytophilous species. We studied preferences of P. robustoides for 5 mineral substrata of different grain sizes, 5 plant species and 3 artificial plants differing in leaf shape. We tested 10 gammarid individuals in 24-h pairwise choice tests in darkened tanks. Adults selected stones of 27 ± 4.8-mm grain diameter, rather than smaller or larger objects, whereas juveniles preferred particles of 15 ± 3.2-mm grain diameter. Structurally complex plants (natural and artificial) were most preferred for both age groups. Juveniles, unlike adults, preferred natural plants over artificial ones and mineral substrata. Adults preferred mineral materials over macrophytes and did not differentiate between natural and artificial plants. Juveniles did not avoid areas occupied by adults, but in their presence exhibited significantly stronger preferences for substrata providing them with suitable shelters. Habitats selected by gammarids in our study somewhat differed from those commonly used in the field, indicating their high plasticity. Different preferences of adults and juveniles may help them avoid competition and/or cannibalism in the field.

We conducted underwater surveys using SCUBA gear to examine habitat and microhabitat competition between the Ponto‐Caspian racer goby Babka gymnotrachelus and native European bullhead Cottus gobio to assess the potential for competitive displacement of the native species by the invading species. In summer, 88 surveys were made in a tributary of the River Vistula within defined benthic areas across the entire width of the river bed. The occurrence of fish by total length class (small: <6 cm; large: >6 cm) and environmental conditions (depth, water velocity, substratum type, plant cover, shelter type) was recorded. We found a substantial separation between the species and size classes in relation to substratum, shelter type and water velocity. European bullheads were limited to lotic areas with stony bottoms, whereas racer gobies also occupied lentic areas over sand or mud. European bullheads usually took refuge under stones in contrast to a wider range of shelters used by racer goby, including tree roots and rubbish. In general, the breadth of habitat used by the racer goby was wider than that of the European bullhead, although habitat overlap between the species was not statistically significant except for the type of shelter occupied by small fish, selecting smaller stones. An inverse relationship was observed between small European bullheads and all racer gobies in areas where they co‐occurred, suggesting that invader may be having an adverse effect on the distribution and habitat use of small native bullheads, particularly in areas of moderate water velocities over small stones and gravel.

Pulmonate snails inhabiting the near-shore zone of water bodies experience air exposure during water level fluctuations and some of them are known to survive long time in temporary basins. In laboratory, we investigated survival and behavioural defences (horizontal migrations and burrowing in substratum) of adult pulmonate snails Planorbarius corneus during substratum (sand and organic-rich) drying. We examined horizontal migrations at shorter and longer distances (24 and 48 cm), during gradual (8 days) or sudden water level drop rates. P. corneus was remarkably resistant to substratum drying, surviving 50 days on sand and more than 53 days on organic-rich bottom. The snails followed the retreating water level and grouped in the submerged parts of experimental tanks. This response was reduced on organic-rich bottom at a longer distance and with water level decreasing suddenly. Moreover, snails exposed to drying buried shallowly in both bottom types. Thus, P. corneus exhibited variable and efficient strategies enabling its survival during water level fluctuations, including active migrations following the retreating water level and burying in sediments.