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Pearly mussels (Unionoidea) live in lakes, rivers, and streams around the world. These bivalves play important roles in freshwater ecosystems and were once both culturally and economically valuable as sources of food, pearls, and mother-of-pearl. Today, however, hundreds of species of these mussels are extinct or endangered. David L. Strayer provides a critical synthesis of the factors that control the distribution and abundance of pearly mussels. Using empirical analyses and models, he assesses the effects of dispersal, habitat quality, availability of fish hosts, adequate food, predators, and parasites. He also addresses conservation issues that apply to other inhabitants of fresh waters around the globe and other pressing issues in contemporary ecology.

Predators affect their prey through consumptive and non-consumptive effects (NCEs). Regarding marine NCEs, predator chemical cues (e.g., exudates) can trigger various anti-predator responses in prey that can vary in magnitude according to the intensity of the perceived cues. Specimens of the mussel Perumytilus purpuratus were exposed to seawater that held different densities of the predatory snail Acanthina monodon as a proxy for predator cue intensity. The response of mussels was quantified in terms of valve gape size (VGS), clearance rate (CR), and oxygen consumption rate (OCR). The results showed that mussels reduced VGS, CR, and OCR as a function of the intensity of the predator exudates to which they were exposed. These results suggest that mussels are not only able to detect predators through their chemical exudates but can also detect the intensity of the predatory signal and thus respond physiologically (CR, OCR) and behaviorally (VGS) in accordance to the potential predation risk. These responses may have cascading effects at the community level, given that mussels are foundation species.

Intestinal microbes are essential participants in host vital activities. The composition of the microbiota is closely related to the environmental factors. Predator presence may impact on intestinal microbiota of prey. In the present study, stone crab Charybdis japonica was used as potential predator, an external stress on mussel Mytilus coruscus , to investigate the intestinal microbiota alteration in M. coruscus . We set up two forms of predator presence including free crab and trapped crab, with a blank treatment without crab. The composition of intestinal microbiota in mussels among different treatments showed significant differences by 16S rRNA techniques. The biodiversity increased with trapped crab presence, but decreased with free crab presence. Neisseria , the most abundant genus, fell with the presence of crabs. Besides, the Arcobacter , a kind of pathogenic bacteria, increased with free crab presence. Regarding PICRUTs analysis, Environmental Information Processing, Genetic Information Processing and Metabolism showed differences in crab presence treatments compared with the blank, with a bit higher in the presence of free crab than trapped crab. In conclusion, trapped crab effects activated the metabolism and immunity of the intestinal flora, but free crabs made mussels more susceptible to disease and mortality, corresponding to the decreased biodiversity and the increased Arcobacter in their intestine.

Spatially complex habitats provide refuge for prey and mediate many predator–prey interactions. Increasing anthropogenic pressures are eroding such habitats, reducing their complexity and potentially altering ecosystem stability on a global scale. Yet, we have only a rudimentary understanding of how structurally complex habitats create ecological refuges for most ecosystems. Better informed management decisions require an understanding of the mechanisms underpinning the provision of physical refuge and this may be linked to prey size, predator size and predator identity in priority habitats. We tested each of these factors empirically in a model biogenic reef system. Specifically, we tested whether mortality rates of blue mussels (Mytilus edulis) of different sizes differed among: (i) different forms of reef structural distribution (represented as ‘clumped’, ‘patchy’ and ‘sparse’); (ii) predator species identity (shore crab, Carcinus maenas and starfish, Asterias rubens); and (iii) predator size. The survival rate of small mussels was greatest in the clumped experimental habitat and larger predators generally consumed more prey regardless of the structural organisation of treatment. Small mussels were protected from larger A. rubens but not from larger C. maenas in the clumped habitats. The distribution pattern of structural objects, therefore, may be considered a useful proxy for reef complexity when assessing predator–prey interactions, and optimal organisations should be considered based on both prey and predator sizes. These findings are essential to understand ecological processes underpinning predation rates in structurally complex habitats and to inform future restoration and ecological engineering practices.

The global loss of marine ecosystem engineers has caused an unprecedented decline in biodiversity. Although wild shellfish habitats have been shown to support biodiverse ecosystems, little is known about how biodiversity is altered by restored shellfish habitats, particularly mussels. To explore the biodiversity response to restored mussel habitats we deposited mussels on the seafloor in 1.5 × 1.5 m plots across a gradient of benthic environments. To understand a holistic community response, this study looks at the response of three faunal classifications over 1 year: infauna, epifauna, and pelagic fauna, compared with adjacent control plots (no mussels). The restored mussel habitats recorded 42 times more demersal fish than control areas, while macroalgae and mobile benthic invertebrates had over a twofold increase in abundance. Overall, the addition of mussels to the seafloor resulted in a general reduction of infaunal abundance and biodiversity, but an increase in epifaunal and pelagic faunal abundances, specifically from those species that benefit from benthic habitat complexity and an increase in food availability. From a management perspective, we highlight location-specific differences to consider for future restoration efforts, including environmental conditions and potential observed factors such as nearby sources of species, particularly predators, and relevant demersal fish ranges. Ultimately, measuring biodiversity responses in small-scale studies will serve as a valuable guide for larger scale restoration efforts and this study recommends considerations to enhance biodiversity outcomes in restored mussel habitats.

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.

The transplanting of juvenile and adult mussels onto soft sediments is an emerging technique for the ecological restoration of the biogenic habitat formed by mussels. While these habitats are often found within estuarine systems, the spatial suitability of these environments for restoration is poorly described. The dynamic and variable environmental conditions characteristic of estuaries could represent challenges to the persistence of restored mussel beds. To assess whether there are spatial differences in mussel responses to transplantation within an estuarine environment, six experimental mussel beds of adult green-lipped mussels (Perna canaliculus) were established along an environmental gradient in a small estuarine harbour in northern New Zealand. Transplanted mussel beds were sampled immediately after installation and again at 3 and 9 months later. Minor differences in the density, length and condition index of mussels were identified among the six sites over the course of the study; however, their responses were typically similar across sites. These results suggest that these mussels have the capacity to establish themselves within estuarine environments and that their subsequent performance once transplanted onto the seafloor appears to be determined by other site-specific factors, such as the presence of predators and the degree of exposure to storm waves.

ABSTRACT Aim Freshwater mussels are considered among the most at‐risk taxa in the world. As such, comprehensive monitoring assessments of what abiotic and biotic factors influence mussel occupancy will be vital for guiding effective conservation. Here, we analysed vertebrate and mussel environmental DNA (eDNA) metabarcoding data to explore the influence of biotic (i.e., host fish diversity, predator presence, and community composition) and abiotic (i.e., drainage size, forest cover, and stream order) factors on freshwater mussel populations. Location This study utilised water samples and tactile survey data collected from streams throughout Fort Johnson, Louisiana. Methods We first evaluated the effectiveness of eDNA metabarcoding for characterising freshwater communities based on previous conventional tactile surveys. Next, we used eDNA metabarcoding analysis for freshwater mussels and vertebrate species alongside GIS‐derived satellite remote sensing data to assess how various biotic and abiotic variables impact freshwater mussel eDNA occupancy. Results Our eDNA metabarcoding survey largely agreed with both historical and contemporary surveys on Fort Johnson, while uniquely detecting Louisiana pigtoe (Pleurobema riddellii), a proposed threatened species under the US Endangered Species Act. We also found that eDNA detections and occupancy had strong seasonal variation, with increased read abundance and diversity in the spring. Vertebrate, fish, and predator diversity (as a function of habitat quality) were strongly predictive of mussel occupancy, supporting the concept of land managers focusing on the entire ecosystem for mussel conservation. Lastly, we found that percent forest cover and drainage basin size influenced mussel eDNA occupancy, informing habitat associations for mussel species of interest (i.e., the mussels occupied larger drainage sizes and perennial streams). Conclusions Our results demonstrate that combining eDNA metabarcoding of target and non‐target species with occupancy modelling can provide insights into the ecology of freshwater mussels and is a useful tool to improve their conservation and management.

The influence of environmental change on species interactions will affect population dynamics and community structure in the future, but our current understanding of the outcomes of species interactions in a high-CO2 world is limited. Here, we draw upon emerging experimental research examining the effects of ocean acidification on coastal molluscs to provide hypotheses of the potential impacts of high-CO2 on predator-prey interactions. Coastal molluscs, such as oysters, mussels, and snails, allocate energy among defenses, growth, and reproduction. Ocean acidification increases the energetic costs of physiological processes such as acid-base regulation and calcification. Impacted molluscs can display complex and divergent patterns of energy allocation to defenses and growth that may influence predator-prey interactions; these include changes in shell properties, body size, tissue mass, immune function, or reproductive output. Ocean acidification has also been shown to induce complex changes in chemoreception, behavior, and inducible defenses, including altered cue detection and predator avoidance behaviors. Each of these responses may ultimately alter the susceptibility of coastal molluscs to predation through effects on predator handling time, satiation, and search time. While many of these effects may manifest as increases in per capita predation rates on coastal molluscs, the ultimate outcome of predator-prey interactions will also depend on how ocean acidification affects the specified predators, which also exhibit complex responses to ocean acidification. Changes in predator-prey interactions could have profound and unexplored consequences for the population dynamics of coastal molluscs in a high-CO2 ocean.

Although predator and prey species often interact over broad geographic ranges, little consideration has been given to whether environmentally-driven variation in prey traits might create a landscape of selection that shapes the evolution of predator traits. Here, we investigate whether predatory intertidal snails (the dogwhelk Nucella canaliculata ) vary in their drilling capacity in association with prey defenses (mussel shell thickness) over ~ 1000 km of coastline in the northeast Pacific. We analyzed mussels ( Mytilus californianus ) collected over two decades and found that shells from central Oregon were historically ~ 20% thicker than those from California. We tested the drilling capacity of dogwhelks raised under common-garden laboratory conditions and found that dogwhelks from California could drill mussels 3.4 times thicker than those from Oregon. Thus, dogwhelks with stronger drilling ability are associated with thinner mussel shells. Although these findings appear counterintuitive at first, they are consistent with strong prey-driven selection on the predator in California, and a region of weak selection in Oregon where mussels may be too thick to favor drilling. Our results also suggest that this selective landscape may be changing; consistent with recent studies of ocean acidification, mussels collected in 2019 were thinner than in prior decades, particularly in Oregon. Overall, our study highlights the importance of studying species interactions within an explicit geographic context of shifting selection pressures.