Native Olympia Oyster Research Articles

Exploring high intertidal refugia as an approach for the restoration of an intertidal oyster

Marine organisms frequently inhabit intertidal zones that serve as refuges from predation and competition but are not optimal physiologically. Restoration practitioners working with intertidal species may similarly have to consider whether restoration success will be greater where conditions are more benign (usually lower in the intertidal) or where negative biotic interactions are reduced (usually higher in the intertidal). In cases where a target species has greater desiccation tolerance than its enemies, restoration may be more successful higher in the intertidal zone, despite potential performance trade-offs. In many US West Coast estuaries, non-native drill species can decimate native oyster populations, posing a challenge to restoration. Given that native Olympia oysters Ostrea lurida should be better able to withstand tidal emersion than the non-native Atlantic oyster drill Urosalpinx cinerea, we explored using the high intertidal as a refuge from predation as a potential restoration technique. Using surveys and a field experiment, we investigated the recruitment, growth, and survival of oysters as well as drill abundance and predation over 3 tidal elevations. Oysters recruited and survived equally well at +0.1, +0.5, and +0.8 m mean lower low water, but juvenile oyster growth decreased with increasing elevation. In our experiment, predation on oysters was lower at the highest elevation than at low and mid elevations, but in natural populations there was a near complete absence of O. lurida at any elevation where U. cinerea was present. This suggests that a higher tidal elevation refuge is not a viable approach for oyster restoration in our study area.

The portfolio approach: a way forward for restoration of native Olympia oysters in a dynamic urban estuary

Native Olympia oysters (Ostrea lurida) occur in remnant natural populations throughout San Francisco Bay (SFB), California, United States, where managers have been working to restore oysters since 2002. Environmental conditions vary substantially along the estuarine gradient in the bay, such that oysters are exposed to different stressors in different locations and years. We monitored oysters that naturally recruited to experimental restoration substrates over 10 years at two sites on this gradient. We found that recruitment rates and adult densities varied over time and had different trajectories at the study locations. We found gradients in predation, space competition, and physical stressors between sites and along a tidal elevation span that corresponded with differences in adult densities. We documented mass mortality resulting from an extreme low‐salinity event at one site and subsequent recovery later that year. Our results, combined with data from natural populations at multiple sites, suggest that restoration outcomes within SFB could be improved by building and strengthening a connected portfolio of sites that are not simultaneously exposed to the same stressors. We recommend siting restoration projects across locations that represent a range of environmental conditions but that are sufficiently close for larval exchange. Unlike typical restoration approaches that use site selection to identify the best locations, the portfolio approach, just like the economic investment principle, would employ a diversity of environmental conditions. Because these environmental conditions can change over time, this approach can reap the greatest rewards over the long run precisely because of that diversity.

Shell cover, rugosity, and tidal elevation impact native and non-indigenous oyster recruitment: Implications for reef ball design

Estuaries have been armored with artificial habitat to protect coastal infrastructure from erosion, but armoring can have negative ecological impacts. Other shoreline protection strategies, such as eco-engineered seawalls and living shorelines, offer more natural, rugose substrata to native species while limiting coastal erosion. Concerns about recruitment of non-indigenous species (NIS) call into question whether structures can be engineered to foster native communities. In southern California, USA, we explored whether concrete reef balls that recruit native Olympia oysters, Ostrea lurida, could be engineered to increase recruitment of O. lurida and discourage recruitment of non-indigenous Pacific oysters, Magallana (formerly Crassostrea) gigas. We modified 15 × 15 cm concrete tiles with added shell cover and rugosity and deployed four treatments: two with surface shell (crushed, large fragments) and two without shell (smooth, rugose) at two sites in San Diego Bay and one site in Newport Bay at two tidal elevations (0.2 and 0.8 m MLLW) from May to September 2018. O. lurida generally recruited in highest abundance and percent cover to 0.2 m MLLW relative to 0.8 m MLLW; at 0.8 m MLLW, shelled treatments with high rugosity favored higher cover and abundance of O. lurida relative to unshelled, low-rugosity treatments. In contrast, M. gigas percent cover was generally higher on unshelled treatments relative to shelled treatments, and rugosity never had statistically significant positive effects on their abundance or cover. Recruitment strength and percent cover of both oyster species showed remarkable context-dependency but a generalized recommendation emerged across sites with strikingly different recruitment strengths: deployment of lower-elevation reef balls would favor O. lurida recruitment, and addition of shell fragments and rugosity will increase native O. lurida recruitment at higher tidal elevations. Shell additions and rugosity may also discourage non-indigenous M. gigas recruitment and cover in some contexts. Importantly, given the now-global distribution of M. gigas, resource managers elsewhere may wish to explore such eco-engineering strategies to control the spread and space domination of this and other NIS.

Proteomic changes associated with predator‐induced morphological defences in oysters

Inducible prey defences occur when organisms undergo plastic changes in phenotype to reduce predation risk. When predation pressure varies persistently over space or time, such as when predator and prey co-occur over only part of their biogeographic ranges, prey populations can become locally adapted in their inducible defences. In California estuaries, native Olympia oyster (Ostrea lurida) populations have evolved disparate phenotypic responses to an invasive predator, the Atlantic oyster drill (Urosalpinx cinerea). In this study, oysters from an estuary with drills, and oysters from an estuary without drills, were reared for two generations in a laboratory common garden, and subsequently exposed to cues from Atlantic drills. Comparative proteomics was then used to investigate molecular mechanisms underlying conserved and divergent aspects of their inducible defences. Both populations developed smaller, thicker, and harder shells after drill exposure, and these changes in shell phenotype were associated with upregulation of calcium transport proteins that could influence biomineralization. Inducible defences evolve in part because defended phenotypes incur fitness costs when predation risk is low. Immune proteins were downregulated by both oyster populations after exposure to drills, implying a trade-off between biomineralization and immune function. Following drill exposure, oysters from the population that co-occurs with drills grew smaller shells than oysters inhabiting the estuary not yet invaded by the predator. Variation in the response to drills between populations was associated with isoform-specific protein expression. This trend suggests that a stronger inducible defence response evolved in oysters that co-occur with drills through modification of an existing mechanism.

Mapping oysters on the Pacific coast of North America: A coast-wide collaboration to inform enhanced conservation.

To conserve coastal foundation species, it is essential to understand patterns of distribution and abundance and how they change over time. We synthesized oyster distribution data across the west coast of North America to develop conservation strategies for the native Olympia oyster (Ostrea lurida), and to characterize populations of the non-native Pacific oyster (Magallana gigas). We designed a user-friendly portal for data entry into ArcGIS Online and collected oyster records from unpublished data submitted by oyster experts and from the published literature. We used the resulting 2,000+ records to examine spatial and temporal patterns and made an interactive web-based map publicly available. Comparing records from pre-2000 vs. post-2000, we found that O. lurida significantly decreased in abundance and distribution, while M. gigas increased significantly. Currently the distribution and abundance of the two species are fairly similar, despite one species being endemic to this region since the Pleistocene, and the other a new introduction. We mapped the networks of sites occupied by oysters based on estimates of larval dispersal distance, and found that these networks were larger in Canada, Washington, and southern California than in other regions. We recommend restoration to enhance O. lurida, particularly within small networks, and to increase abundance where it declined. We also recommend restoring natural biogenic beds on mudflats and sandflats especially in the southern range, where native oysters are currently found most often on riprap and other anthropogenic structures. This project can serve as a model for collaborative mapping projects that inform conservation strategies for imperiled species or habitats.

Response of eelgrass (Zostera marina) to an adjacent Olympia oyster restoration project.

Recent restoration efforts for the native Olympia oyster, Ostrea lurida, are commonly motivated by potential return of oyster-associated ecosystem services, including increased water filtration. The potential impact of such restoration on another species of ecological concern, eelgrass, Zostera marina, is unclear, but has been hypothesized to be positive if oyster filter feeding increases light penetration to eelgrass. For two years after construction of an oyster restoration project, we assessed the response of adjacent eelgrass (impact) compared to control and reference eelgrass beds by monitoring changes in light intensity, eelgrass shoot density, biomass, leaf morphometrics, and epiphyte load. We observed lower light intensity consistently over time, including prior to restoration, near the constructed oyster bed relative to the control and one of the reference locations. We also observed minor variations between control and impact eelgrass morphology and density. However, the changes observed were not outside the range of natural variation expected in this system, based upon comparisons to reference eelgrass beds, nor were they detrimental. This limited impact to eelgrass may be due in part to the incorporation of a buffer distance between the restored oyster bed and the existing eelgrass bed, which may have dampened both positive and negative impacts. These findings provide evidence that Olympia oyster restoration and eelgrass conservation goals can be compatible and occur simultaneously.

A Comparison of the Juvenile Dungeness Crab Metacarcinus magister Habitat Provided by Contemporary Oyster Aquaculture Versus Historical Native Oysters in a U.S. West Coast Estuary

Oysters and seagrasses provide structurally complex estuarine habitat for fish and invertebrate species. On the U.S. West Coast, complex oyster habitat was historically provided by the native Olympia oyster Ostrea lurida but is now provided by the commercially cultured oyster Crassostrea gigas. Ostrea lurida is found in subtidal and low intertidal areas, whereas C. gigas is predominantly cultured at higher intertidal elevations, resulting in a potential shift in available habitat for other fish and invertebrates that use this intertidal habitat. This change in the available habitat and its use was examined for the juvenile Dungeness crab Metacarcinus magister, and results showed the following: (1) comparable crab densities in remnant and restored populations of O. lurida and cultured C. gigas in two estuaries, (2) generally higher crab densities in both of these shell habitats than those observed in eelgrass Zostera marina or open mud habitat, (3) contemporary juvenile crab density in intertidal areas of Willapa Bay was most influenced by distance from the estuary mouth (declining with increasing distance) but also declined with increasing tidal elevation, and (4) when extrapolated to the estuarine ecosystem scale using areal estimates of habitat coverage, historical habitat provided by O. lurida potentially produced three times more juvenile crabs than those currently produced in cultured C. gigas. Nonetheless, both intertidal oyster habitats contribute more to juvenile crab production than eelgrass or open unstructured mud, and the ecosystem services associated with the placement of native and commercial oyster beds should be considered when defining goals for and permitting both aquaculture and native oyster restoration in Willapa Bay and other U.S. West Coast estuaries. Managers should consider this shifting temporal baseline in intertidal habitat provision, but also conducting similar evaluations at this broader estuary scale when evaluating habitat value for other resources that use these habitats differently.

Spatial and Temporal Distribution of the Early Life History Stages of the Native Olympia Oyster Ostrea lurida (Carpenter, 1864) in a Restoration Site in Northern Puget Sound, Wa

For effective long-term restoration of depleted marine species, it is critical that populations are able to replenish themselves through reproduction. The Olympia oyster Ostrea lurida (Carpenter, 1864), the only native oyster species on the West Coast of North America, is the subject of a number of restoration efforts across its range. Information about the distribution of its early life history stages is critical for the design of effective and connected restoration networks. In this study, spawning adults (brooders), planktonic larvae, and settlers were mapped in Fidalgo Bay, Anacortes, WA, an active restoration site in the Salish Sea. Sampling was conducted weekly from May through August 2013 at five intertidal and three subtidal stations. Brooding adults, planktonic larvae, and settlers were sampled. Environmental variables measured were temperature, relative water flux (a proxy for flow), tidal height, and submersion time of traps and shell strings. A low percentage of individuals sampled were brooding (maximum 13%) throughout the sampling period, but abundance peaked in early July. Small planktonic larvae (153–243 µm) were present throughout the sampling period, but peaked in mid-July, whereas large larvae (243–333 µm) were mostly only found in mid-July. Overall, there was high variability in the distribution of larvae and settlement among various locations or depths throughout Fidalgo Bay. Planktonic larvae and settlers were much more abundant near conspecifics and at a trestle bridge that could have affected hydrodynamics and, therefore, settlement. In addition, settlement increased with increasing water flow in the intertidal, but not in the subtidal, zone. More settlement occurred at –0.3 m below mean lower low water (MLLW) than at stations above or below this elevation, indicating that postlarvae prefer to settle at an intermediate elevation band in the intertidal zone. Temperature was the most important variable measured, with an association between the increasing presence of planktonic larvae and settlement with increasing temperature. In sum, prerecruitment processes appear to be very important in determining the spatial distribution of Olympia oysters in Fidalgo Bay. These results suggest that restoration of Olympia oysters is improved by the addition of appropriate substrates in sites that contain conspecifics and are just below MLLW.

Particle Processing by Olympia Oysters Ostrea lurida and Pacific Oysters Crassostrea gigas

The native Olympia oyster (Ostrea lurida) has been the subject of few detailed feeding studies compared with many other species of bivalve mollusk. More information on this species’ feeding activity and behavior are needed to better understand its historical ecological role in Pacific Northwest (PNW) estuaries, compared with that of the widely farmed, well-studied, non-native Pacific oyster (Crassostrea gigas). In this study, the feeding physiology and particle processing behaviors of O. lurida and C. gigas were examined in both controlled laboratory experiments and in situ trials that spanned the wet and dry seasons of the PNW. It was hypothesized that O. lurida would have lower filtration and particle processing rates than those of C. gigas. Results supported this hypothesis in that filtration and absorption rates of the Pacific oyster were significantly greater than those of O. lurida under laboratory conditions. In the field, C. gigas was also found to filter, absorb, and deposit organic material at significantly greater rates than O. lurida. These observations explain the previously reported greater growth rates of C. gigas relative to those of O. lurida. Furthermore, after applying data collected here to recent modeling efforts, it estimated that C. gigas could potentially remove between 1.2 and 3.6-fold and deposit between 1.6- and 4.6-fold more organic suspended material than comparable, historic populations of O. lurida in the PNW.

Spatially Explicit Estimates of In Situ Filtration by Native Oysters to Augment Ecosystem Services during Restoration

US west coast populations of the native Olympia oyster Ostrea lurida declined precipitously in the late nineteenth and early twentieth centuries and were often replaced by the non-native Pacific oyster (Crassostrea gigas) by the aquaculture industry. Recovery of native oyster ecosystem services derived from their suspension feeding activities (termed “filtration services” (FS)) often serves as a powerful incentive for restoration of populations of O. lurida along the US west coast despite uncertainty about the potential effects of their filtration activities on concentrations of suspended particulate matter. Here, we provide an improved FS model for O. lurida and C. gigas in Yaquina Bay, OR, that is based on both in situ feeding behavior and the complex hydrodynamics of the estuary. The total area and the order of locations chosen for oyster restoration in Yaquina Bay were examined to determine how oyster FS could be maximized with limited resources. These modeling efforts quantified estimates showing (1) native oysters, if restored in Yaquina Bay to historic levels, may contribute nearly an order of magnitude greater FS than previously estimated; (2) C. gigas contributes significantly greater FS than O. lurida, especially during the wet season; (3) FS provided by either species is highly dependent upon seasonal river forcing and salinity; (4) spatial variation in FS arises from the hydrodynamics of the system, uneven oysters distributions, and upstream pre-filtering. We found that spatially explicit models demonstrated the benefits of prioritizing restoration to areas with the greatest FS potential, rather than placing oysters randomly within historic habitats. Directing restoration in this manner used between 75% (dry season) and 60% (wet season) less of the restored area needed to achieve comparable FS with randomly placed oysters.

Ecophysiology of the Olympia Oyster, Ostrea lurida, and Pacific Oyster, Crassostrea gigas

The native Olympia oyster, Ostrea lurida, was once abundant in many US Pacific Northwest (PNW) estuaries, but was decimated by human activity in the late nineteenth early to twentieth centuries. Having been the subject of only few modern, detailed studies, a dearth of basic physiological information surrounded O. lurida and how it compared to the now dominant, non-native Pacific oyster, Crassostrea gigas. Utilizing laboratory and in situ studies in Yaquina Bay, OR, we explored the clearance rates of both species across a wide range of conditions. Pacific oysters not only had greater size-specific clearance rates than Olympia oysters, but also had a lower optimum temperature. Clearance rates for both species were reduced at lower salinity, at lower organic content, and at higher turbidity. Clearance rate models were constructed for each species using three approaches: (1) a single mechanistic model that incorporated feeding response functions of each species to the effects of temperature, salinity, turbidity, and seston organic content based on laboratory studies; (2) another additive model in which the number and type of response functions from laboratory studies were allowed to vary; and (3) a statistical model that utilized environmental data collected during in situ feeding trials. Clearance rate models that correlated feeding activity with in situ environmental data were found to often better predict oyster clearance rates (based on Adj R 2) for both species in Yaquina Bay, OR, than mechanistic, additive models based on laboratory feeding response functions; however, in situ correlative models varied in accuracy by species and season. This work represents important first steps towards better understanding the physiological ecology of the native Olympia oyster and how it differs from introduced and now dominant Pacific oyster.

Scales of recruitment variability in warming waters: Comparing native and introduced oysters in Hood Canal, Washington, USA

AbstractThe effect of climate change on natural oyster recruitment has the potential to disrupt many of the ecosystem services oysters provide. Due to the temperature‐sensitivity of reproduction, oyster recruitment may shift as water temperatures rise. A biological imprint of climate change was revealed in a multi‐decadal time series of recruitment of non‐native Pacific oysters (Crassostrea gigas) in the main stem of Hood Canal, Washington, USA, extracted from historic fishery documents. Water in July and August warmed significantly from 1945 to 1995 (0.028 ± 0.004°C per year [±SE]) and accounted for an increase in Pacific oyster recruitment (7% per year, 0.028 ± 0.006 spat per year on log scale [±SE]); recruitment also strongly tracked inter‐annual variability in summer water temperature. Methods used to collect historical data were repeated in 2013–2015 when recruitment of both Pacific oysters and native Olympia oysters (Ostrea lurida) were recorded in main stem and lower Hood Canal. Both historic and modern data show large variation within and between years for temperature as well as recruitment. The modern data add information regarding spatial variation, in that recruitment patterns in the two regions of Hood Canal were decoupled. As temperatures continue to increase, non‐native Pacific oysters are likely to be favored over Olympia oysters, which recruit earlier at lower temperatures and presently contribute less than half of total oyster recruits. Future recruitment, however, may be limited by environmental factors other than temperature, a point indicated particularly in Hood Canal where many subtidal species already respond strongly to gradients in dissolved oxygen.

Experimental test of oyster restoration within eelgrass

Abstract Both seagrasses and oysters are foundation species valued for their wide range of ecosystem services, but their space competition sets a constraint on joint benefits. A reserve for native Olympia oysters (Ostrea lurida) was established in lower Hood Canal (Washington State, USA) more than a century ago but is now devoid of that species and dominated by native eelgrass (Zostera marina). This situation sets up a conservation conflict because management activities for one species are at odds with the protection of another. In experimental enhancement plots, Olympia oysters were outplanted at low density, which successfully maintained eelgrass density and production. One method was used in 2013 (seeded cultch, 8% cover) and two additional methods in 2015 (anchored cultch and single oysters, the latter at 4% cover). For all outplant methods, oysters experienced 99% annual mortality, associated with the attraction of non‐native and native predators. Shell cover remained steady for a year and then declined rapidly, as shell accumulation did not exceed sedimentation rates. Eelgrass per se does not preclude Olympia oysters, given that the two species were observed to co‐occur at a coastal estuarine site (Willapa Bay, Washington). However, even when socio‐political constraints on restoration activities were overcome, ecological constraints remained from predation. Competition between these two protected species was avoided, but it may be the case that top‐down control on oysters was particularly acute owing to low oyster density and/or the environmental conditions of eelgrass beds.

Upstream—Downstream Shifts in Peak Recruitment of the Native Olympia Oyster in San Francisco Bay During Wet and Dry Years

Understanding the conditions that drive variation in recruitment of key estuarine species can be important for effective conservation and management of their populations. The Olympia oyster (Ostrea lurida) is native to the Pacific coast of North America and has been a target of conservation efforts, though relatively little information on larval recruitment exists across much of its range. This study examined the recruitment of Olympia oysters at biweekly to monthly intervals at four sites in northern San Francisco Bay from 2010 to 2015 (except 2013). Mean monthly temperatures warmed at all sites during the study, while winter (January–April) mean monthly salinity decreased significantly during a wet year (2011), but otherwise remained high as a result of a drought. A recurring peak in oyster recruitment was identified in mid-estuary, in conditions corresponding to a salinity range of 25–30 and >16 °C at the time of settlement (April–November). Higher average salinities and temperatures were positively correlated with greater peak recruitment. Interannual variation in the timing of favorable conditions for recruitment at each site appears to explain geographic and temporal variation in recruitment onset. Higher winter/spring salinities and warmer temperatures at the time of recruitment corresponded with earlier recruitment onset within individual sites. Across all sites, higher winter/spring salinities were also correlated with earlier onset and earlier peak recruitment. Lower winter salinities during 2011 also resulted in a downstream shift in the location of peak recruitment.

Spatial and temporal variability of contaminants within estuarine sediments and native Olympia oysters: A contrast between a developed and an undeveloped estuary

Chemical contaminants can be introduced into estuarine and marine ecosystems from a variety of sources including wastewater, agriculture and forestry practices, point and non-point discharges, runoff from industrial, municipal, and urban lands, accidental spills, and atmospheric deposition. The diversity of potential sources contributes to the likelihood of contaminated marine waters and sediments and increases the probability of uptake by marine organisms. Despite widespread recognition of direct and indirect pathways for contaminant deposition and organismal exposure in coastal systems, spatial and temporal variability in contaminant composition, deposition, and uptake patterns are still poorly known. We investigated these patterns for a suite of persistent legacy contaminants including polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) and chemicals of emerging concern including pharmaceuticals within two Oregon coastal estuaries (Coos and Netarts Bays). In the more urbanized Coos Bay, native Olympia oyster (Ostrea lurida) tissue had approximately twice the number of PCB congeners at over seven times the total concentration, yet fewer PBDEs at one-tenth the concentration as compared to the more rural Netarts Bay. Different pharmaceutical suites were detected during each sampling season. Variability in contaminant types and concentrations across seasons and between species and media (organisms versus sediment) indicates the limitation of using indicator species and/or sampling annually to determine contaminant loads at a site or for specific species. The results indicate the prevalence of legacy contaminants and CECs in relatively undeveloped coastal environments highlighting the need to improve policy and management actions to reduce contaminant releases into estuarine and marine waters and to deal with legacy compounds that remain long after prohibition of use. Our results point to the need for better understanding of the ecological and human health risks of exposure to the diverse cocktail of pollutants and harmful compounds that will continue to leach from estuarine sediments over time.

Local adaptation in an estuarine foundation species: Implications for restoration

Understanding the extent of local adaptation is critical for conservation and restoration planning in an era of environmental change. Adaptive differentiation among populations may mediate a species' response to environmental stress, yet these evolutionary processes are seldom studied in marine systems. We used native Olympia oysters (Ostrea lurida) as a model organism for studying local adaptation with applications to marine restoration. We tested whether populations of oysters in California estuaries are locally adapted to their home sites and to stressful low salinity events, which are predicted to increase in frequency with climate change. We spawned oysters from three sites in San Francisco Bay and raised their offspring under common laboratory conditions. These oysters were then reciprocally transplanted among the three field sites. At two of the three sites, oysters of local origin tended to survive better than those from other populations, suggesting that local adaptation may occur even within a single estuary. In a subsequent experiment, we raised oysters from two sites in San Francisco Bay and one site in Tomales Bay through two generations under common conditions and then subjected these oysters to a low salinity challenge in the laboratory. Oysters with the highest survival originated from the site with the lowest field salinity, suggesting that natural selection may favor stress tolerant phenotypes in certain regions. As interest grows in restoring heavily impacted native species, our results indicate that considering local adaptation may be essential to deciding how and where to conserve and restore species faced with changing conditions.

The History and Decline of OstreaLuridain Willapa Bay, Washington

ABSTRACT With an annual production of 1500 metric tons of shucked oysters, Willapa Bay, WA currently produces more oysters than any other estuary in the United States. This production is mainly composed of the Pacific oyster (Crassostrea gigas), rather than the native Olympia oyster (Ostrea lurida). Historically, Willapa Bay was home to vast Olympia oyster beds, which formed the foundation of a major extractive fishery in the late 1800s. Yet the historical baseline extent of this habitat is poorly understood as it was first documented following decades of exploitation and was therefore based on a shifted baseline. An extensive and thorough literature review was undertaken to ascertain whether oyster beds mapped as “cultivated” in the 1890s were in fact originally wild beds. The most complete harvest statistics to date have been presented for the Olympia oyster in Willapa Bay (from 1849 to 2011) to provide useful historical insights into the expansion and collapse of the Olympia oyster fishery and discuss ...

Ocean acidification increases the vulnerability of native oysters to predation by invasive snails

There is growing concern that global environmental change might exacerbate the ecological impacts of invasive species by increasing their per capita effects on native species. However, the mechanisms underlying such shifts in interaction strength are poorly understood. Here, we test whether ocean acidification, driven by elevated seawater pCO₂, increases the susceptibility of native Olympia oysters to predation by invasive snails. Oysters raised under elevated pCO₂ experienced a 20% increase in drilling predation. When presented alongside control oysters in a choice experiment, 48% more high-CO₂ oysters were consumed. The invasive snails were tolerant of elevated CO₂ with no change in feeding behaviour. Oysters raised under acidified conditions did not have thinner shells, but were 29-40% smaller than control oysters, and these smaller individuals were consumed at disproportionately greater rates. Reduction in prey size is a common response to environmental stress that may drive increasing per capita effects of stress-tolerant invasive predators.