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 small step or a giant leap: Accounting for settlement delay and dispersal in restoration planning.

Understanding larval duration and hence dispersal potential of the European oyster Ostrea edulis is crucial to inform restoration strategies. Laval duration has an obligatory period of maturity to pediveliger (when larvae are ready to settle), but also an unknown period until metamorphosis is triggered by a settlement cue. The extent to which larvae can prolong the pediveliger period and delay metamorphosis has not been studied. Here we show that O. edulis larvae can delay metamorphosis for a period of 11 days, while retaining the capability to settle in high proportions when presented with a suitable settlement cue. O. edulis larvae are likely to be able to delay metamorphosis even further, since 80% of larvae in the control treatment were still alive when the experiment was terminated at day 14. The results indicate the ability of O. edulis larvae to more than double pelagic duration and probably further delay metamorphosis. We discuss these findings in the context of larval mortality, and the importance of O. edulis' larval settlement requirements for dispersal potential, recruitment success and connectivity of restoration sites.

Ephemeral detection of Bonamia exitiosa (Haplosporida) in adult and larval European flat oysters Ostrea edulis in the Solent, United Kingdom.

The haplosporidian parasite Bonamia exitiosa was detected using PCR in four adult and six larval brood samples of the European flat oyster Ostrea edulis from the Solent, UK. This represents the second reported detection of this parasite along the south coast of England. Adult oysters were collected and preserved from seabed populations or restoration broodstock cages between 2015 and 2018. The larvae within brooding adults sampled during 2017 and 2018 were also preserved. Molecular analysis of all samples was performed in 2019. The DNA of B. exitiosa was confirmed to be present within the gill tissue of one oyster within the Portsmouth wild fishery seabed population (n = 48), sampled in November 2015; the congeneric parasite Bonamia ostreae was not detected in this individual. This is the earliest record of B. exitiosa in the Solent. Concurrent presence of both B. ostreae and B. exitiosa, determined by DNA presence, was confirmed in the gill and heart tissue of three mature individuals from broodstock cages sampled in October 2017 (n = 99), two from a location on the River Hamble and one from the Camber Dock in Portsmouth Harbour. B. exitiosa was not detected in the November 2018 broodstock populations. A total of six larval broods were positive for B. exitiosa, with five also positive for B. ostreae. None of the brooding adults were positive for B. exitiosa suggesting that horizontal transmission from the surrounding environment to the brooding larvae is occurring. Further sampling of broodstock populations conducted by the Fish Health Inspectorate at the Centre for Environment, Fisheries and Aquaculture Science in June 2019 did not detect infection of O. edulis by B. exitiosa. These findings together suggest that the pathogen has not currently established in the area.

Calcium carbonate unit realignment under acidification: A potential compensatory mechanism in an edible estuarine oyster.

Ocean acidification (OA) is well-known for impairing marine calcification; however, the end response of several essential species to this perturbation remains unknown. Decreased pH and saturation levels (Ω) of minerals under OA is projected to alter shell crystallography and thus to reduce shell mechanical properties. This study examined this hypothesis using a commercially important estuarine oyster Magallana hongkongensis. Although shell damage occurred on the outmost prismatic layer and the undying myostracum at decreased pH 7.6 and 7.3, the major foliated layer was relatively unharmed. Oysters maintained their shell hardness and stiffness through altered crystal unit orientation under pH 7.6 conditions. However, under the undersaturated conditions (ΩCal ~ 0.8) at pH 7.3, the realigned crystal units in foliated layer ultimately resulted in less stiff shells which indicated although estuarine oysters are mechanically resistant to unfavorable calcification conditions, extremely low pH condition is still a threat to this essential species.

Conservation and restoration of a keystone species: Understanding the settlement preferences of the European oyster (Ostrea edulis).

The European oyster Ostrea edulis is a keystone species that is internationally recognised as 'threatened and declining' in the NE Atlantic by OSPAR and several nations have consequently adopted strategies for its conservation and restoration. Understanding the settlement behaviour of O. edulis larvae is crucial to inform these strategies. We compared the efficiency of several treatments in triggering settlement. The most effective settlement occurred with the presence of conspecifics: 100% settled in <23 h. Marine stones with habitat-associated biofilms induced 81% settlement that started after a 45 h delay. Sterile shells and terrestrial stones did not induce more settlement than control treatments. These results indicate that O. edulis larvae are gregarious and finely-tuned to settle in response to cues which are indicative of their adult habitat requirements. The role of chemical cues in mediating settlement, and the importance of this to restoration, are discussed.

Consistent differences in fitness traits across multiple generations of Olympia oysters.

Adaptive evolution and plasticity are two mechanisms that facilitate phenotypic differences between populations living in different environments. Understanding which mechanism underlies variation in fitness-related traits is a crucial step in designing conservation and restoration management strategies for taxa at risk from anthropogenic stressors. Olympia oysters (Ostrea lurida) have received considerable attention with regard to restoration, however there is limited information on adaptive population structure. Using oysters raised under common conditions for up to two generations (F1s and F2s), we tested for evidence of divergence in reproduction, larval growth, and juvenile growth among three populations in Puget Sound, Washington. We found that the population with the fastest growth rate also exhibited delayed and reduced reproductive activity, indicating a potential adaptive trade-off. Our results corroborate and extend upon a previous reciprocal transplant study on F1 oysters from the same populations, indicating that variation in growth rate and differences in reproductive timing are consistent across both natural and laboratory environments and have a strongly heritable component that cannot be entirely attributed to plasticity.

Analysis of large-scale spatio-temporal trends of Ostrea puelchana beds in Northern Patagonian gulfs, Argentina.

Wild oyster populations have declined severely worldwide, however fluctuations of South Atlantic populations are poorly documented. We explored the changes in the abundance of Ostrea puelchana population of Northern Patagonia, Argentina, by linking data from paleontological, archaeological and informal sources, with time series data from fishing, ecological and studies of oyster pathology. The present work is the first study which includes a South Atlantic time series concerning oyster beds. The focal area for this study is the San Matías Gulf (SMG, 40° 50'-42° 15' S, 63° 5'-65° 10' W). Populations of O. puelchana were inferred from sub-fossil deposits (>700 years ago) throughout the gulf, but were documented in surveys a century ago only in the NW coast. The population has declined in the last decades. However, new populations have established recently in the NE and southern regions of the gulf. A Bonamia exitiosa epizootic was coincident with the declining trend of the abundance provided by the time series, suggesting that beds declined as a consequence of parasite infections. Dredging fisheries for scallops took place in the 1970s and 1980s on the NE coast of the gulf, in areas adjacent to the NE oyster beds. We proposed that fishing activities might have had a low impact on oyster beds, since NE beds expanded and increased during that period. The southward expansion of oyster population at latitudes beyond the historical distribution range might reflect long-term adequate environmental conditions for larval survival on the NE and S of the SMG.

Larval carry-over effects from ocean acidification persist in the natural environment.

An extensive body of work suggests that altered marine carbonate chemistry can negatively influence marine invertebrates, but few studies have examined how effects are moderated and persist in the natural environment. A particularly important question is whether impacts initiated in early life might be exacerbated or attenuated over time in the presence or absence of other stressors in the field. We reared Olympia oyster (Ostrea lurida) larvae in laboratory cultures under control and elevated seawater pCO2 concentrations, quantified settlement success and size at metamorphosis, then outplanted juveniles to Tomales Bay, California, in the mid intertidal zone where emersion and temperature stress were higher, and in the low intertidal zone where conditions were more benign. We tracked survival and growth of outplanted juveniles for 4 months, halfway to reproductive age. Survival to metamorphosis in the laboratory was strongly affected by larval exposure to elevated pCO2 conditions. Survival of juvenile outplants was reduced dramatically at mid shore compared to low shore levels regardless of the pCO2 level that oysters experienced as larvae. However, juveniles that were exposed to elevated pCO2 as larvae grew less than control individuals, representing a larval carry-over effect. Although juveniles grew less at mid shore than low shore levels, there was no evidence of an interaction between the larval carry-over effect and shore level, suggesting little modulation of acidification impacts by emersion or temperature stress. Importantly, the carry-over effects of larval exposure to ocean acidification remained unabated 4 months later with no evidence of compensatory growth, even under benign conditions. This latter result points to the potential for extended consequences of brief exposures to altered seawater chemistry with potential consequences for population dynamics.

The potential role of an unregulated coastal anthropogenic activity in facilitating the spread of a non-indigenous biofoulant.

Despite an exponential rise in anthropogenically-mediated transfers of non-indigenous species during the last 150 years, several coastal anthropogenic activities remain unregulated under current legislation frameworks. This study investigates the potential role of commercial periwinkle (Littorina littorea) harvesting as an unregulated facilitator of both small- and large-scale geographic range expansion of an invasive oyster epibiont (Ostrea chilensis) within the Menai Strait (North Wales, UK) and beyond. The frequency of oyster-fouled periwinkles was greatest in areas of high adult oyster abundance and restricted to large, market-sized periwinkles (>20 mm) inhabiting the low shore. Active efforts by commercial collectors to reject oyster-fouled periwinkles were found to be inadequate, with oysters of all sizes observed within collected hauls. Whilst the survival of fouled and unfouled periwinkles was comparable under post-collection refrigerated conditions, a significant decrease in both mobility and flesh content was associated with the presence of oyster epibionts. Survival of all but the smallest oyster epibionts under post-collection refrigerated conditions enhances the possibility of accidental non-indigenous oyster transfers. Better interventions during both initial visual inspection and post-griddling stages are recommended, as well as the development of techniques that kill off all non-indigenous epibionts, whilst leaving the freshness and marketability of the periwinkles uncompromised.

Variance in the reproductive success of flat oyster Ostrea edulis L. assessed by parentage analyses in natural and experimental conditions.

In order to document further the phenomena of variance in reproductive success in natural populations of the European flat oyster Ostrea edulis, two complementary studies based on natural and experimental populations were conducted. The first part of this work was focused on paternity analyses using a set of four microsatellite markers for larvae collected from 13 brooding females sampled in Quiberon Bay (Brittany, France). The number of individuals contributing as the male parent to each progeny assay was highly variable, ranging from 2 to more than 40. Moreover, paternal contributions showed a much skewed distribution, with some males contributing to 50-100% of the progeny assay. The second part of this work consisted of the analysis of six successive cohorts experimentally produced from an acclimated broodstock (62 wild oysters sampled in the Quiberon Bay). Allelic richness was significantly higher in the adult population than in the temporal cohorts collected. Genetic differentiation (F(st) estimates) was computed for each pair of samples and all significant values ranged from 0.7 to 11.9%. A limited effective number of breeders (generally below 25) was estimated in the six temporal cohorts. The study gives first indications of the high variance in reproductive success as well as a reduced effective size, not only under experimental conditions but also in the wild. Surprisingly, the pool of the successive cohorts, based on the low number of loci used, appeared to depict a random and representative set of alleles of the progenitor population, indicating that the detection of patterns of temporal genetic differentiation at a local scale most likely depends on the sampling window.

Molecular detection and quantification of the protozoan Bonamia ostreae in the flat oyster, Ostrea edulis.

Bonamia ostreae is an intracellular protozoan which is recognized as a cause of mortality in European populations of flat oysters (Ostrea edulis). Based on the recent characterization of actin genes of B. ostreae, specific primers were designed for real-time PCR using SYBR Green chemistry. Specificity was demonstrated by the unique melting temperature peak observed in positive samples and by the lack of amplification in samples of oysters infected by closely related parasites, including Bonamia exitiosa. A calibration curve using a cloned template was defined to estimate copy number. The assay had a 6 log- dynamic range, mean inter- and intra-assay variation coefficients of <1% and a minimum detection limit of 50 gene copies per reaction. Using infected oyster samples as templates, the assay was at least 10-fold more sensitive than conventional PCR. The quantitative assay was applied to test 132 oysters, and results were compared with the heart imprint method. There was a strong correlation between both techniques, and the results showed that the real-time PCR assay should be useful for studies of the ecology of B. ostreae and its host-parasite relationship.