Behavioural differences and interactions between two sessile bivalves forming mixed-species assemblages

Friends or enemies: Multi-species interactions among biofoulers, endoparasites and their gastropod hosts

Parasites are a crucial factor that shapes the functioning of communities throughout the world, as are gregarious macrofoulers in aquatic ecosystems. However, little is known about the effects of three-way interactions between macrofoulers, endoparasites and their hosts. We predict that macrofouling and parasite infection may act (i) independently of each other, (ii) synergistically, increasing their final negative impact on the host or (iii) antagonistically, the former weakening the negative impact of the latter. We investigated multiway relationships between an invasive freshwater filter-feeding macrofouler (the zebra mussel), digenean endoparasite and their gastropod host, Viviparus viviparus. Furthermore, we checked the recruitment of mussels in living gastropods versus their empty shells. We sampled living V. viviparus and their empty shells with attached dreissenids from a Polish dam reservoir. We counted and weighed attached mussels and determined wet weight, shell height and sex of gastropods. Then we dissected the molluscs to look for digenean larvae and gastropod embryos. We use these parameters to look for reciprocal associations between mussel fouling, parasitic infection and gastropod size and fertility, as well as to infer the most likely mechanisms of the observed relationships. Dreissenid overgrowth was associated with reduced fertility and size of viviparids, but also with a lower prevalence of digenean metacercariae (Leucochloridiomorpha sp.). We did not observe a negative influence of these digeneans on their gastropod hosts. In addition, large living viviparids and their empty shells were equally used as substrates by dreissenids, but small living gastropods were more fouled than shells of the corresponding size. A trade-off exists in the studied system: filter-feeding macrofoulers may bring some profits for their host, reducing the pressure of waterborne parasites (which may be crucial in the case of pathogenic species/life stages), although at the cost of the reduced growth and fertility of the host. Furthermore, mussels attached to mollusc hosts can exert a cascading effect on the reduced prevalence of digeneans in their final hosts, including those of medical or veterinary importance.

Importance of substratum quality for potential competitive niche overlap between native and invasive unionid mussels in Europe

Infaunal freshwater mussels are highly threatened and declining worldwide. One of the potential threats to mussels consists of biological invasions. We intended to investigate the habitat overlap and behavioural differences between native (Unio pictorum, Unio tumidus, Anodonta anatina, Anodonta cygnea) and invasive (Asian Sinanodonta woodiana) unionid bivalves to determine potential sources of competition. Furthermore, we investigated differences between S. woodiana from the established population in artificially heated waters and from the recent population in a natural thermal regime. We used pairwise choice tests on mud, medium, coarse and very coarse sand, mixture of medium and coarse sand, fine, medium and coarse gravel, and observed mussel locomotion and burrowing in preferred and non-preferred substrata. All species generally preferred fine-grained materials. The widest preference range was exhibited by S. woodiana (both populations), whereas A. cygnea was the most selective. The preferences of the cold-water population of S. woodiana were shifted towards coarser materials compared to conspecifics from the heated waters, and highly overlapped with the preferences of the native species. Anodonta cygnea most often moved horizontally and spent the shortest time deeply burrowed. Both Unio species were deeply burrowed for the largest amount of time and the horizontal locomotion of U. tumidus was the lowest among the test species. Sinanodonta woodiana, especially from the heated water population, exhibited relatively weak locomotion (compared to A. cygnea) and burrowing (compared to Unio spp. and A. anatina). Deep burrowing was more common on fine-grained materials. Our results suggest that the native mussels can be threatened by S. woodiana due to their overlapping habitat preferences, potentially hindering habitat separation. However, mobile native mussels may be capable of migrating and avoiding competition. Accumulating knowledge of the biology and ecology of freshwater mussels could contribute to the creation and improvement of conservation plans to protect these threatened animals.

Dreissenids’ breaking loose: differential attachment as a possible driver of the dominance shift between two invasive mussel species

Ponto-Caspian dreissenids are notorious freshwater invaders. Recently, widespread observations show a dominance shift from the early invader, Dreissena polymorpha, to its successor, Dreissena bugensis. These observations likely reflect congeneric species differences in physiological and behavioural traits. Here, we assessed the mussel attachment strength, attachment rate, and the mode of byssal failure as trait differences that could potentially contribute to dominance shifts. The attachment traits were measured in field and laboratory experiments. Fouling plates were deployed in the Rhine-Meuse river delta and dreissenids were collected and acclimatised in 60 L non-aerated freshwater tanks. Attachment strength was positively correlated with shell size. The attachment strength of D. bugensis was significantly greater compared to slower growing D. polymorpha individuals of a dreissenid field assemblage. This corresponded to the superior byssal thread morphology of D. bugensis (i.e. higher number and two times wider byssal threads). Moreover, our results indicated that byssal threads of D. bugensis are stronger than those of D. polymorpha, as the latter ruptured more often. Additionally, D. bugensis had a significantly lower attachment rate than D. polymorpha. Having a greater attachment strength gives D. bugensis an advantage when it comes to withstanding currents and predators. On the other hand, not being attached allows an individual to actively move around. This would allow them to move away from fast changing unfavourable environmental conditions. These attachment traits indicate competitive benefits for D. bugensis over D. polymorpha, therefore possibly contributing to the dominance shifts.