Predator-induced behavioural defences in two competitive invasive species: the zebra mussel and the quagga mussel

A global synthesis of predation on bivalves

Predation is a dominant structuring force in ecological communities. In aquatic environments, predation on bivalves has long been an important focal interaction for ecological study because bivalves have central roles as ecosystem engineers, basal components of food webs, and commercial commodities. Studies of bivalves are common, not only because of bivalves' central roles, but also due to the relative ease of studying predatory effects on this taxonomic group. To understand patterns in the interactions of bivalves and their predators we synthesised data from 52 years of peer-reviewed studies on bivalve predation. Using a systematic search, we compiled 1334 studies from 75 countries, comprising 61 bivalve families (N = 2259), dominated by Mytilidae (29% of bivalves), Veneridae (14%), Ostreidae (8%), Unionidae (7%), and Dreissenidae and Tellinidae (6% each). A total of 2036 predators were studied, with crustaceans the most studied predator group (34% of predators), followed by fishes (24%), molluscs (17%), echinoderms (10%) and birds (6%). The majority of studies (86%) were conducted in marine systems, in part driven by the high commercial value of marine bivalves. Studies in freshwater ecosystems were dominated by non-native bivalves and non-native predator species, which probably reflects the important role of biological invasions affecting freshwater biodiversity. In fact, while 81% of the studied marine bivalve species were native, only 50% of the freshwater species were native to the system. In terms of approach, most studies used predation trials, visual analysis of digested contents and exclusion experiments to assess the effects of predation. These studies reflect that many factors influence bivalve predation depending on the species studied, including (i) species traits (e.g. behaviour, morphology, defence mechanisms), (ii) other biotic interactions (e.g. presence of competitors, parasites or diseases), and (iii) environmental context (e.g. temperature, current velocity, beach exposure, habitat complexity). There is a lack of research on the effects of bivalve predation at the population and community and ecosystem levels (only 7% and 0.5% of studies respectively examined impacts at these levels). At the population level, the available studies demonstrate that predation can decrease bivalve density through consumption or the reduction of recruitment. At the community and ecosystem level, predation can trigger effects that cascade through trophic levels or effects that alter the ecological functions bivalves perform. Given the conservation and commercial importance of many bivalve species, studies of predation should be pursued in the context of global change, particularly climate change, acidification and biological invasions.

Differential effects of ocean acidification and warming on biological functioning of a predator and prey species may alter future trophic interactions

Independently, ocean warming (OW) and acidification (OA) from increased anthropogenic atmospheric carbon dioxide are argued to be two of the greatest threats to marine organisms. Increasingly, their interaction (ocean acidification and warming, OAW) is shown to have wide-ranging consequences to biological functioning, population and community structure, species interactions and ecosystem service provision. Here, using a multi-trophic experiment, we tested the effects of future OAW scenarios on two widespread intertidal species, the blue mussel Mytilus edulis and its predator Nucella lapillus. Results indicate negative consequences of OAW on the growth, feeding and metabolic rate of M. edulis and heightened predation risk. In contrast, Nucella growth and metabolism was unaffected and feeding increased under OAW but declined under OW suggesting OA may offset warming consequences. Should this differential response between the two species to OAW, and specifically greater physiological costs to the prey than its predator come to fruition in the nature, fundamental change in ecosystem structure and functioning could be expected as trophic interactions become disrupted.

What we know and don't know about the invasive zebra (Dreissena polymorpha) and quagga (Dreissena rostriformis bugensis) mussels

We summarized existing knowledge on Dreissena polymorpha (the zebra mussel) and D. r. bugensis (the quagga mussel), including data on their taxonomy, systematics, evolution, life cycle, reproduction, feeding, growth and longevity, population dynamics, interspecific competition, habitat requirements, and distribution within and among waterbodies. We analyzed the history of spread of both species and the major pathways and vectors of their spread in Europe and North America. Special consideration was given to their ecological and economic impacts and their natural enemies, like waterfowl, fishes, and parasites, as well as the prevention of their introduction, early detection, control, and containment. We also outlined the most salient ecosystem services provided by zebra and quagga mussels, including water purification, nutrient recycling, culling the effects of eutrophication, biomonitoring, and their role as a food resource for fish and waterfowl. Finally, we identified major knowledge gaps and key studies needed to better understand the biology, ecology, and impacts of these aggressive freshwater invaders. Our review indicates that much crucial information on the quagga mussel is still missing, including key life history parameters, like spawning cues, fecundity, and longevity, particularly for the profundal zone of deep lakes.

Ecosystem services provided by the exotic bivalves Dreissena polymorpha, D. rostriformis bugensis, and Limnoperna fortunei

The ecosystem services approach to conservation is becoming central to environmental policy decision making. While many negative biological invasion-driven impacts on ecosystem structure and functioning have been identified, much less was done to evaluate their ecosystem services. In this paper, we focus on the often-overlooked ecosystem services provided by three notable exotic ecosystem engineering bivalves, the zebra mussel, the quagga mussel, and the golden mussel. One of the most significant benefits of invasive bivalves is water filtration, which results in water purification and changes rates of nutrient cycling, thus mitigating the effects of eutrophication. Mussels are widely used as sentinel organisms for the assessment and biomonitoring of contaminants and pathogens and are consumed by many fishes and birds. Benefits of invasive bivalves are particularly relevant in human-modified ecosystems. We summarize the multiple ecosystem services provided by invasive bivalves and recommend including the economically quantifiable services in the assessments of their economic impacts. We also highlight important ecosystem disservices by exotic bivalves, identify data limitations, and future research directions. This assessment should not be interpreted as a rejection of the fact that invasive mussels have negative impacts, but as an attempt to provide additional information for scientists, managers, and policymakers.

Non-consumptive effects of a predatory snail (Acanthina monodon) on a dominant habitat-forming mussel species (Perumytilus purpuratus)

Predators can influence prey through direct consumption as well as through non-consumptive effects (NCEs). NCEs usually occur mediated by behavioral changes in the prey upon detection of predator cues. Such changes may involve reduction of feeding with a variety of physiological consequences. We evaluated NCEs from an intertidal predatory snail (Acanthina monodon) on a dominant habitat-forming mussel species (Perumytilus purpuratus) from the southeastern Pacific coast. We tested whether A. monodon exerts negative NCEs on clearance rate, oxygen consumption rate, biodeposit production, and between-valve gap size in P. purpuratus. We found that waterborne predator cues triggered a decrease in these variables except biodeposit production. However, the organic content of the biodeposits increased in the presence of predator cues. The snail's physical contact with the mussels strengthened the negative NCEs on between-valve gap size. Since P. purpuratus is a dominant filter-feeder and foundation species in rocky intertidal habitats, predator NCEs on this species might indirectly influence ecosystem-level processes and community structure.

Ocean acidification alters anti-predator responses in a competitive dominant intertidal mussel

Widespread intertidal mussels are exposed to a variety of natural and anthropogenic stressors. Even so, our understanding of the combined influence of stressors such as predation risk and ocean acidification (OA) on these species remains limited. This study examined the response of the purple mussel (Perumytilus purpuratus), a species distributed along Pacific southeastern rocky shores, to the effects of predation risk and OA. Using a laboratory 2 × 2 cross design, purple mussels were either devoid or exposed to predator cues from the muricid snail Acanthina monodon, while simultaneously exposing them to current (500 ppm) or projected OA conditions (1500 ppm). The response of purple mussels to these factors was assessed using growth, calcification, clearance, and metabolic rates, in addition to byssus production. After 60 d, the presence of predator cues reduced mussel growth in width and length, and in the latter case, OA enhanced this response making the effects of predator cues more severe. Calcification rates were driven by the interaction between the two stressors, whereas clearance rates increased only in response to OA, likely explaining some of the growth results. Mussel byssus production also increased with pCO₂ but interacted with predation risk: in the absence of predator cues, byssus production increased with OA. These results suggest that projected levels of OA may alter and in some cases prevail over the natural response of purple mussels to predation risk. Considering the role played by this mussel as a dominant competitor and ecosystem engineer in rocky shores, these results have community-wide implications.

Attachment of zebra and quagga mussel adhesive plaques to diverse substrates

Like marine mussels, freshwater zebra and quagga mussels adhere via the byssus, a proteinaceous attachment apparatus. Attachment to various surfaces allows these invasive mussels to rapidly spread, however the adhesion mechanism is not fully understood. While marine mussel adhesion mechanics has been studied at the individual byssal-strand level, freshwater mussel adhesion has only been characterized through whole-mussel detachment, without direct interspecies comparisons on different substrates. Here, adhesive strength of individual quagga and zebra mussel byssal plaques were measured on smooth substrates with varying hydrophobicity-glass, PVC, and PDMS. With increased hydrophobicity of substrates, adhesive failures occurred more frequently, and mussel adhesion strength decreased. A new failure mode termed 'footprint failure' was identified, where failure appeared to be adhesive macroscopically, but a microscopic residue remained on the surface. Zebra mussels adhered stronger and more frequently on PDMS than quagga mussels. While their adhesion strengths were similar on PVC, there were differences in the failure mode and the plaque-substrate interface ultrastructure. Comparisons with previous marine mussel studies demonstrated that freshwater mussels adhere with comparable strength despite known differences in protein composition. An improved understanding of freshwater mussel adhesion mechanics may help explain spreading dynamics and will be important in developing effective antifouling surfaces.

The combined effects of climate change stressors and predatory cues on a mussel species

In order to make adequate projections on the consequences of climate change stressors on marine organisms, it is important to know how impacts of these stressors are affected by the presence of other species. Here we assessed the direct effects of ocean warming (OW) and acidification (OA) along with non-consumptive effects (NCEs) of a predatory crab and/or a predatory snail on the habitat-forming mussel Perumytilus purpuratus. Mussels were exposed for 10-14 weeks to contrasting pCO₂ (500 and 1400 μatm) and temperature (15 and 20 °C) levels, in the presence/absence of cues from one or two predator species. We compared mussel traits at sub-organismal (nutritional status, metabolic capacity-ATP production-, cell stress condition via HSP70 expression) and organismal (survival, oxygen consumption, growth, byssus biogenesis, clearance rates, aggregation) levels. OA increased the mussels' oxygen consumption; and OA combined with OW increased ATP demand and the use of carbohydrate reserves. Mussels at present-day pCO₂ levels had the highest protein content. Under OW the predatory snail cues induced the highest cell stress condition on the mussels. Temperature, predator cues and the interaction between them affected mussel growth. Mussels grew larger at the control temperature (15 °C) when crab and snail cues were present. Mussel wet mass and calcification were affected by predator cues; with highest values recorded in crab cue presence (isolated or combined with snail cues). In the absence of predator cues in the trails, byssus biogenesis was affected by OA, OW and the OA × OW and OA × predator cues interactions. At present-day pCO₂ levels, more byssus was recorded with snail than with crab cues. Clearance rates were affected by temperature, pCO₂ and the interaction between them. The investigated stressors had no effects on mussel aggregation. We conclude that OA, OW and the NCEs may lead to neutral, positive or negative consequences for mussels.

Multiple factors regulate filtration by invasive mussels: Implications for whole-lake ecosystems

The quagga mussel (Dreissena rostriformis bugensis) is a filter-feeding invasive species that has re-engineered many freshwater ecosystems worldwide. High clearance rates (CRs) and dense populations underpin their ecological impacts. CRs, however, are highly variable, as are environmental factors that regulate them. Despite their widespread distribution in Europe and North America, knowledge of how multiple environmental factors regulate CRs of quagga mussels remains limited. We investigated quagga mussel CRs under varying conditions including water temperature, food availability, habitat depth, flow velocity, and duration of incubation in chambers with both static and flowing water. We found that CR was positively related to water temperature and initial food concentration in static chambers. When coupled with limited food concentration, cold water (7.5 °C), due to a deep-water upwelling event, produced very low CR (~ 10× lower) compared to warmer water (12-24 °C) (0.47 vs. 3.12-5.84 L g^-1 DW h^-1). Mussels from deeper water (20 m) had CRs that were ~ 3.5× higher than from shallower depths (2-10 m) and CRs were inversely affected by total mussel dry weight. Flow rates from 1 to 22 cm s^-1 generated a unimodal pattern of CR with an optimal flow velocity of 6-12 cm s^-1 (~ 2× higher than suboptimal CRs). Enhanced flow velocity (22 cm s^-1), reflective of storm conditions in shallow waters, significantly increased the closing/reopening activity of mussel valves relative to lower velocities (1-12 cm s^-1). Incubation time had a strong negative effect (~ 2-4× reduction) on CRs likely reflecting refiltration in static chambers versus food saturation of mussels in flowing chambers, respectively. Our findings highlight how multiple factors can influence quagga mussel CRs by factors of 2-10. Given widespread habitat heterogeneity in large aquatic ecosystems, whole-lake estimates of mussel impacts should include multiple regulatory factors that affect mussel filtration.

Zebra or quagga mussel dominance depends on trade-offs between growth and defense-Field support from Onondaga Lake, NY

Two invasive mussels (zebra mussel, Dreissena polymorpha and quagga mussel D. rostriformis bugensis) have restructured the benthic habitat of many water bodies in both Europe and North America. Quagga mussels dominate in most lakes where they co-occur even though zebra mussels typically invade lakes first. A reversal to zebra mussel over time has rarely been observed. Laboratory experiments have shown that quagga mussels grow faster than zebra mussels when predator kairomones are present and this faster growth is associated with lower investment in anti-predator response in quagga mussels than zebra mussels. This led to the hypothesis that the dominance of quagga mussels is due to faster growth that is not offset by higher vulnerability to predators when predation rates are low, as may be expected in newly colonized lakes. It follows that in lakes with high predation pressure, the anti-predatory investments of zebra mussels should be more advantageous and zebra mussels should be the more abundant of the two species. In Onondaga Lake, NY, a meso-eutrophic lake with annual mussel surveys from 2005 to 2018, quagga mussels increased from less than 6% of the combined mussel biomass in 2007 to 82% in 2009 (from 3 to 69% by number), rates typical of this displacement process elsewhere, but then declined again to 11-20% of the mussel biomass in 2016-2018. Average total mussel biomass also declined from 344-524 g shell-on dry weight (SODW)/m2 in 2009-2011 to 34-73 g SODW/m2 in 2016-2018, mainly due to fewer quagga mussels. This decline in total mussel biomass and a return to zebra mussel as the most abundant species occurred as the round goby (Neogobius melanostomus) increased in abundance. Both the increase to dominance of quagga mussels and the subsequent decline following the increase in this molluscivorous fish are consistent with the differences in the trade-off between investment in growth and investment in defenses of the two species. We predict that similar changes in dreissenid mussel populations will occur in other lakes following round goby invasions, at least on the habitats colonized by both species.

Experimental evidence for the adaptive response of aquatic invertebrates to chronic predation risk

As acute stress induced by predation risk can generate significant oxidative damage, prey organisms are forced to balance their defence reaction and the cost of activating the cellular defence system. Stress tolerance differs significantly among species; therefore predator pressure indirectly shapes the community structure. To test adaptation abilities of amphipod crustaceans (Dikerogammarus villosus and Gammarus jazdzewskii) we exposed them to acute (35 min.) and chronic (1 or 7 days) predation risk (the Eurasian perch). We measured respiration (related to metabolic rate), cellular defence systems (antioxidant enzyme (catalase) activity and heat shock protein (Hsp70) concentration), and the level of oxidative damage (thiobarbituric acid reactive substances (TBARS) concentration). Both amphipods increased their respiration rate in the presence of predation cues, irrespective of the duration of their pre-exposure to danger. This increase in D. villosus was initiated more quickly (immediately vs. after 10 min. of the test) and lasted for a longer time (20 vs. 10 min.) than in G. jazdzewskii. However, only G. jazdzewskii after a short exposure to predation risk exhibited an increase in its catalase activity, Hsp70 concentration and oxidative damage. No changes in these parameters were exhibited by D. villosus or after a chronic exposure of G. jazdzewskii to predation cues. Our results show that prey organisms are able to reconfigure their physiology to maintain increased metabolic rate under prolonged predator pressure and, at the same time, reduce oxidative damage as well as costs related to anti-oxidant defence.

Field biomonitoring using the zebra mussel Dreissena polymorpha and the quagga mussel Dreissena bugensis following immunotoxic reponses. Is there a need to separate the two species?

The zebra mussel, Dreissena polymorpha constitutes an extensively used sentinel species for biomonitoring in European and North American freshwater systems. However, this invasive species is gradually replaced in freshwater ecosystem by Dreissena bugensis, a closely related dreissenid species that shares common morphological characteristics but possess some physiological differences. However, few are known about differences on more integrated physiological processes that are generally used as biomarkers in biological monitoring studies. Declining of zebra mussel populations raises the question of the sustainability of using one or both species indifferently to maintain the quality of environmental pollution monitoring data. In our study, we performed a field comparative study measuring immune-related markers and bioaccumulation of PCBs, PAHs and PBDEs in sympatrically occurring mussel populations from three sites of the St. Lawrence River. For tested organisms, species were identified using RFLP analysis. Measurement of bioaccumulated organic compounds indicated a higher accumulation of PCBs and PBDEs in D. bugensis soft tissues compared to D. polymorpha while no differences were noticed for PAHs. Results of hemocytic parameters highlighted that differences of hemocyte distributions were associated to modulations of phagocytic activities. Moreover, marked differences occurred in measurement of hemocytic oxidative activity, indicating divergences between the two species for ROS regulation strategies. This physiological characteristic may deeply influence species responses facing environmental or pollution related stress and induce bias if the two species are not differentiated in further biomarker or bioaccumulation measurement-based studies.

Keep calm and don't stop growing: Non-consumptive effects of a sympatric predator on two invasive Ponto-Caspian gammarids Dikerogammarus villosus and Pontogammarus robustoides

Predators shape prey populations by elimination of individuals (consumptive effects) and by inducing modifications in prey behaviour, physiology or morphology (NCE—non-consumptive effects). Due to the resource allocation to defence, decreased feeding and higher stress, the costs of predator NCEs can be considerable. Therefore, the resistance to NCEs may be crucial for population growth and interspecific competition. We tested the resistance of Ponto-Caspian gammarids Dikerogammarus villosus and Pontogammarus robustoides to NCEs imposed by their predator, the racer goby Babka gymnotrachelus. As D. villosus is often avoided by predators in the presence of alternative food, we hypothesised that it would bear lower behavioural and physiological costs of anti-predator responses. We tested gammarid feeding in short-time experiments (2–4 h) with food (chironomid larvae) located at various distances from the stony shelter (to enforce food searching, Experiment I) or in the direct gammarid proximity (no searching needed, Experiment II). Moreover, we checked the predator effect on gammarid growth in a 2-week Experiment III. Both gammarids exposed to predators reduced feeding efficiency outside the shelter (Experiment I). Contrary to our expectations, the response of D. villosus was stronger. When food was provided in their direct proximity (Experiment II), the feeding of both species was unaffected by predators, indicating that a shelter supplied with food can reduce predator NCEs. The growth of P. robustoides was reduced in the presence of predators (Experiment III), whereas that of D. villosus was unaffected. Although D. villosus has a more effective defence strategy than P. robustoides, it bears similar or even higher behavioural costs of NCEs. However, it exhibits the higher resistance to the long-term predator presence, sustaining its growth rate under such conditions. This may be one of the factors contributing to the great invasion success of D. villosus, currently taking place in European fresh waters.

Happy together? Avoidance of conspecifics by gregarious mussels

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.

Will the Displacement of Zebra Mussels by Quagga Mussels Increase Water Clarity in Shallow Lakes during Summer? Results from a Mesocosm Experiment

Zebra mussels (Dreissena polymorpha) are known to increase water clarity and affect ecosystem processes in invaded lakes. During the last decade, the conspecific quagga mussels (D. rostriformis bugensis) have displaced zebra mussels in many ecosystems including shallow lakes such as Oneida Lake, New York. In this study, an eight-week mesocosm experiment was conducted to test the hypothesis that the displacement of zebra mussels by quagga mussels leads to further decreases in phytoplankton and increases in water clarity resulting in increases in benthic algae. We found that the presence of zebra mussels alone (ZM), quagga mussels alone (QM), or an equal number of both species (ZQ) reduced total phosphorus (TP) and phytoplankton Chl a. Total suspended solids (TSS) was reduced in ZM and ZQ treatments. Light intensity at the sediment surface was higher in all three mussel treatments than in the no-mussel controls but there was no difference among the mussel treatments. There was no increase in benthic algae biomass in the mussel treatments compared with the no-mussel controls. Importantly, there was no significant difference in nutrient (TP, soluble reactive phosphorus and NO₃^-) levels, TSS, phytoplankton Chl a, benthic algal Chl a, or light intensity on the sediment surface between ZM, QM and ZQ treatments. These results confirm the strong effect of both mussel species on water clarity and indicate that the displacement of zebra mussel by an equivalent biomass of quagga mussel is not likely to lead to further increases in water clarity, at least for the limnological conditions, including summer temperature, tested in this experiment.