Detection of chemical cues of predators and food resources is a key for the behaviour of many species in aquatic ecosystems. We checked whether predator origin and diet, containing potential food and/ or alarm signals, affect the behaviour of omnivorous prey partly sharing their diet with a top predator. We conducted y-maze experiments to study the responses of invasive omnivorous and cannibalistic amphipods (Dikerogammarus villosus and Pontogammarus robustoides) to chemical cues of fish predators, injured amphipods and chironomid larvae (common amphipod food). As the predators, we used the goby Babka gymnotrachelus (sympatric with the amphipods) and piranha Pygocentrus nattereri (allopatric to the amphipods). The fish were either starved or fed amphipods or chironomids. D. villosus preferred predators fed chironomids and conspecifics as well as crushed conspecifics and chironomids, but avoided both hungry predators. Thus, D. villosus may perceive the presence of a top predator as an indication of both food resource and predation risk. P. robustoides avoided predators fed both amphipods (particularly conspecifics) and their alarm substances. The responses of both species depended more on the predator diet than on its species, which is likely to facilitate the recognition of allopatric predators and survival in newly invaded areas.

The ability to select appropriate substratum in a patchy environment is crucial for food availability, reproductive success, anti‐predator defence and resistance to other environmental dangers. Therefore, even euryecious invasive species may exhibit clear substratum preferences, contributing to their success in novel areas. We used 24‐h pairwise‐choice experiments to study substratum preferences of the highly invasive freshwater Ponto‐Caspian gammarid Dikerogammarus villosus (Amphipoda, Crustacea). We tested its preferences for stony substrata of various grain diameters (2–124 mm), macrophytes differing in structural complexity (Myriophyllum spicatum, Ceratophyllum demersum, Potamogeton perfoliatus, Potamogeton nodosus, Stuckenia pectinata) and zebra mussels. We hypothesized that: (1) gammarids would select substrata adjusted to their body size and offering maximum protection; (2) mussels would be the most preferred substratum except in high densities, which lead to deterioration of water quality; (3) large gammarids would displace smaller conspecifics to worse substratum. Our results showed that large (>9 mm) and small (<7 mm) gammarids preferred stones >6 cm and between 6 and 8 cm, respectively, with relatively large empty spaces among particles (4–9 cm). Small gammarids selected complex plants, particularly M. spicatum, whereas large individuals also preferred less complex forms with large leaves (Pot. nodosus, Pot. perfoliatus). Small gammarids exhibited higher affinity for natural macrophytes than large individuals. All gammarids preferred stones over plants. Contrary to most of the published data, gammarids selected stones more often than dense zebra mussel colonies and did not discriminate between low density mussel and stone habitats. Oxygen depletion seems to be the factor distracting gammarids from mussel habitats. Thus, they seem to select mussel colonies only when alternative hard substrata are missing. Differences in substratum preferences between size classes were not pronounced, but small individuals switched to the less preferred habitat in the presence of adults, probably avoiding cannibalism and competition.

Responsiveness to biotic factors is crucial for the survival of sessile aquatic animals. They cannot escape from danger, but developed a number of defences against predation, usually delayed in time. We checked the initial defence of the freshwater byssate zebra mussel, Dreissena polymorpha, associated with valve gaping. We tested the effect of chemical signals: fish predator scent (the roach Rutilus rutilus), conspecific alarm cue and a mixture of both, as well as a mechanical stimulus: the presence of an amphipod (Dikerogammarus villosus) mechanically irritating mussels. The alarm cues and amphipod presence made mussels spend more time with closed/narrowly open valves, which can be related to decreasing detection probability by reduced infochemical excretion and/or protecting soft tissues in the presence of an imminent threat. In contrast, reactions to the predator scent alone were much weaker. Moreover, the fish scent mixed with alarm substance induced weaker responses than the alarm substance alone. Thus, the fish infochemical might mask the presence of the alarm cue components, potentially benefiting the predator. A variety of defences exhibited by mussels demonstrates the importance of the predation cue type (direct/indirect, chemical/mechanical, originating from conspecifics/ predators/mixed) for the behaviour of sessile animals.

Macrobiofouling is an important phenomenon in the aquatic environment, resulting in economic losses and environmental changes, including negative impact on hard‐shelled animals. A freshwater invasive byssate bivalve, the zebra mussel Dreissena polymorpha (Dreissenidae), strongly affects bivalves from the family Unionidae by fouling their shells. We tested potential mechanisms explaining variable fouling of different unionids (four native species: Anodonta anatina, A. cygnea, Unio pictorum, U. tumidus and the non‐native Sinanodonta woodiana) by the zebra mussel. We conducted a field survey (unionids collected at a sandy and muddy site in a dam reservoir), field experiment (unionids exposed without sediments in suspended baskets to dreissenid settlement) and laboratory experiment (a multiple choice test with and without the possibility of unionid burrowing into sandy sediments). In the survey, zebra mussel density was highest on A. anatina and S. woodiana, intermediate on U. pictorum and lowest on A. cygnea and U. tumidus. In the field experiment, A. anatina and S. woodiana were more fouled than Unio spp. In the laboratory, zebra mussels less often attached to U. pictorum and, when unionids could burrow themselves, also to S. woodiana. However, no unionid species was positively selected in the presence of stone substrata. The percentage of zebra mussels attached directly to unionids (compared to those attached to conspecifics) in the field survey was negatively related to the overall zebra mussel biomass. Zebra mussel fouling negatively affected the biomass of an allopatric S. woodiana on muddy bottom. Dreissenids overgrow different unionid species to variable extent, not only due to the differences in their exposed surface area, but, as our experimental results show, also because of their active substratum selection. Moreover, mussels seem to prefer unionid surface over conspecific shells, the latter being fouled only when overall dreissenid fouling is heavy. As unionids often constitute the main source of hard substratum for zebra mussels in waterbodies, the species composition of a unionid assemblage may affect dreissenid success by offering them variable substratum quality.

Zebra mussel Dreissena polymorpha is a Ponto-Caspian species invasive in Europe and North America, with great environmental impact. It lives byssally attached to hard substrata in large aggregations, which is often explained by its preferences for conspecifics, though direct evidence for such preferences has been rather limited so far. We studied the reactions of zebra mussels to conspecifics, hypothesizing that they may either be attracted to one another or form aggregations only in the absence of alternative attachment sites. In Experiment 1, we tested mussel tendency to detach from existing druses depending on druse size (2–25 individuals) and substratum type (soft: sand; hard: glass). Mussels detached significantly more often on the hard substratum and from larger druses compared to soft substratum and smaller druses, respectively. This indicates that mussels tended to avoid conspecifics at high density, particularly when alternative substratum was available. In Experiment 2, we tested the responses of single mussels to distant (3 or 15 cm) conspecifics (0, 3, 15 individuals per 2.5 l tank) on the sandy substratum. The presence of conspecifics, regardless of their distance and density, resulted in single unattached mussels staying more often in their initial positions. Mussels did not move preferentially towards or away from the conspecifics. Thus, even on unsuitable substratum mussels were not attracted by conspecifics and probably exhibited an avoidance reaction by reducing their movement. This suggests that dense mussel aggregations are formed due to the lack of available alternative attachment sites rather than due to their preferences for conspecifics.