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Shallow coastal environments are becoming more susceptible to marine heatwaves, particularly in Atlantic Canada, which is experiencing higher rates of ocean warming compared with the global average. Understanding bivalves' tolerance to heatwaves in Atlantic Canada is especially important, given the magnitude of bivalve aquaculture in this region. In this study, the physiological state of four commercially important bivalve species—blue mussel (Mytilus edulis Linnaeus, 1758), eastern oyster (Crassostrea virginica (Gmelin, 1791)), soft shell clam (Mya arenaria Linnaeus, 1758), and hard clam (Mercenaria mercenaria (Linnaeus, 1758))—was observed during a naturally occurring heatwave in Prince Edward Island, Canada. The metabolism, feeding physiology, and scope for growth of each species were compared. Results from this study, and comparisons with the literature, show interspecific variability in the metabolic rate and feeding physiology between species, but generally oysters, soft shell clams, and hard clams were less impacted during the heatwave compared with the blue mussels. Additionally, only the scope for growth of mussels declined throughout this experiment, which may be explained by cumulative heat stress, as temperatures experienced during this heatwave exceeded the thermal tolerance limits of mussels. [ABSTRACT FROM AUTHOR]

Sustained periods of anoxia, driven by eutrophication, threaten coastal marine systems and can lead to mass mortalities of even resilient animals such as bivalves. While mortality rates under anoxia are well‐studied, the specific mechanism(s) of mortality are less clear. We used a suite of complementary techniques (LT50, histology, 16S rRNA amplicon sequencing, and valvometry) to show that the proliferation of anaerobic bacteria within eastern oysters (Crassostrea virginica) accelerates mortality rate under anoxic conditions. Manipulative laboratory experiments revealed that oyster survival under anoxic conditions was halved when bacteria were present compared to when they were excluded by the broad‐spectrum antibiotic chloramphenicol. Histological assessments supported this mechanism and showed infiltration of bacteria in oysters that were not treated with antibiotics compared to a general lack of bacteria when oysters were treated with antibiotics. 16S rRNA amplicon sequencing failed to identify any particular genera of bacteria responsible for mortality, rather a diversity of endogenous anaerobic and/or sulfate‐reducing bacteria were common among oysters. In addition, monitoring of oyster valve gaping behavior in the field revealed that oysters showed remarkable valve closure synchrony when first exposed to anoxia. However, oysters periodically opened throughout anoxia/hypoxia in both the lab and field, suggesting that the infiltration of exogenous bacteria from the environment may also influence mortality rates under natural settings. Coupled with previous studies, we posit that mass mortality events in a wide range of coastal bivalves are likely the result of co‐morbidity from asphyxiation and bacterial processes. [ABSTRACT FROM AUTHOR]

In North America, studies regarding effects of CO2-induced low pH in bivalve aquaculture are largely restricted to the US Pacific coast. Studies on species from the northwest Atlantic are lacking. Furthermore, information on the roles of intergenerational exposure and biological sex in bivalve responses to low pH, particularly in an aquaculture-specific context, is scant. We tested if short-term (1 month) exposure to CO2-induced reductions in pHNBS affected the reproductive development of male and female eastern oysters (Crassostrea virginica) during hatchery-specific reproductive conditioning and whether maternal and/or paternal exposure influenced larval responses. Reduced pH (7.5–7.7) increased the rate of reproductive development in both males and females. There was no indication of intergenerational effects; adult pH conditions did not affect early larval development. In contrast, low pH conditions experienced by gametes during spawning, fertilization, and embryo incubation (48 h) resulted in higher larval survival (+6–8% from control), reduced shell height (−2 to 3 µm), and increased deformities (abnormal shell shape; +3–5%). We suggest that local adaptation to acidic land runoff may account for the positive effects of low pH observed in this study. Bioeconomic assessments are now needed to understand the implications of reduced pH on aquaculture operations in these regions of Atlantic Canada. [ABSTRACT FROM AUTHOR]

Mudworm ( Polydora websteri) parasitism is known to result in unsightly mud blisters in eastern oysters ( Crassostrea virginica), resulting in reduced product quality and increased vulnerability to illness and environmental stress. While typically a concern only for bottom-grown oysters, an abnormal severe outbreak of P. websteri in surface-cultured oysters in New Brunswick, Canada, was reported in 2013, along with an anecdotally reported concurrent increase in siltation. Although heavier loads of silt are reported to increase P. websteri infestations in oysters, studies report mixed effects of siltation to this regard. Here, we report the results of a field experiment testing the effect of siltation on P. websteri recruitment in surface-grown oysters at an aquaculture site in Richibucto, New Brunswick. Live oysters of similar size were deployed at the aquaculture site and were left to collect P. websteri recruits under one of two siltation treatments (high vs. low) for 50 days. Results suggested that P. websteri recruitment correlated with metrics of oyster size (shell weight, length, width, depth and surface area). When P. websteri counts were standardized for oyster shell morphometry, P. websteri recruitment was significantly higher in oysters exposed to the high siltation treatment, accumulating approximately 1.5× as many P. websteri as oysters exposed to the low siltation treatment. Our results suggest that higher amounts of siltation on surface-cultured oysters can result in increased rates of P. websteri parasitism. Enhanced cleaning regimes may help to alleviate the impacts of P. websteri in surface-grown oysters, although other mitigation strategies exist. [ABSTRACT FROM AUTHOR]

This study aims at better understanding of the physiological changes that occur during the winter months in Eastern oysters (Crassostrea virginica) living at their northernmost distributional limit (48°N). It was conducted in northeastern New Brunswick, Canada (47°49'N, 64°46'W). Oysters were held in Vexar® bags on the bottom of a bay and collected before (December) and during (January-April) the ice-cover period. Winter mortality was <5%. A significant decrease in the concentration of lipids stored in the digestive gland (from 42.0 to 22.5mg/g wet tissue) indicated that the animals had access to oxygen, probably through minimal shell openings. Proteins were also used, although less heavily (decrease from 68.0 to 59.1mg/g wet tissue), leading to an accumulation of ammonia in the intrapallial fluid (up to 0.83 μM), which is modest considering the length of time the animals lived as almost closed systems. In contrast, carbohydrates appear to play a minor role in winter energetics. Low pH values of intrapallial fluid were noted in late winter (c. 7.0) as compared with those measured at the onset of winter and in spring (c. 7.7). The animals used more energy substrates in late than in early winter, which might be partly related to the necessity to intensify detoxification mechanisms as toxic metabolites accumulate. Despite the fact that oysters do not filter-feed at temperatures <0 °C, the mass of the digestive gland increased by a factor of 1.5 during early winter, presumably as the result of nutrient transfer from other tissues. The ability of C. virginica to use energy-rich substrates and to minimize the accumulation of catabolic byproducts could partly explain the ability of the species to survive prolonged subzero conditions. [ABSTRACT FROM AUTHOR]

This paper provides the first continuous assessment of behavioural quiescence in Eastern oysters ( Crassostrea virginica ) overwintered at their northernmost (48°N) distribution range. The paper also describes how quiescent oysters respond to sediment burial, which may occur inadvertently when cultured stocks are lowered onto soft-sediment seabeds. Long term monitoring of valve opening indicated that quiescence persisted for 164.9 ± 1.8 days (mean ± SE, n = 14 oysters) in 2014–2015. Quiescence was initiated in mid-November 2014 when the water temperature fell to 2.7 ± 0.2 °C and ended in early May 2015 when the water temperature rose to 4.4 ± 0.4 °C. The low mortality rate (3.4 ± 1.9%) of the control groups indicated that this species is well-adapted to such overwintering conditions. When buried in late October, oysters initially exhibited low-amplitude and sporadic shell movements, possibly in an attempt to clear the sediment covering their valve margins. Following a period of quiescence, behavioural morbidity ensued, with lethargic opening of valves beginning 153.3 ± 9.9 days after burial (n = 16 oysters). Death occurred 186.5 ± 3.6 days following burial (n = 16 oysters). Removing buried quiescent oysters from the sediment in late winter accelerated the depletion of energy reserves, particularly lipids and proteins, thereby advancing death. In conclusion, detrimental health effects associated with the burial of oysters in autumn cannot be mitigated through winter or spring desiltation. Thus, reducing the risk of burial by avoiding soft estuarine sediment whenever possible remains the best husbandry practice. [ABSTRACT FROM AUTHOR]

In the Gulf of St. Lawrence estuaries, mesh bags containing cultured oysters (Crassostrea virginica) are lowered onto the bottom in autumn prior to the formation of a thick ice cover. The oysters remain quiescent and unattended in near-freezing waters for four to five months, during which time they are susceptible to accidental burial by sedimenting particles. The objectives of this study were (1) to gain insight into the mechanism cueing the spring awakening of oysters, (2) to determine the approximate burial depth that oysters can withstand, and (3) to estimate the time it takes for mortality to occur under conditions of excessive siltation. Results indicate that water temperature is the primary factor controlling the timing of awakening, with the majority of oysters suddenly opening their valves when temperatures increased to 2.61±0.66°C. Supplementing the diet to mimic spring bloom conditions had no modulating influence on awakening behaviour. Oysters buried under 20mm of sediment initially exhibited erratic valve movements and sometimes remained closed for days. Within 2weeks, however, they did succeed in expulsing the overlying silt around their valve margins and in resuming normal valve movements, including circadian rhythmicity. By comparison, burial under 40 or 60mm of silt invariably led to death within 11.7±1.3days. It is concluded that oysters should be re-suspended as soon as the ice cover breaks apart and moves offshore in the spring. [ABSTRACT FROM AUTHOR]

To test the hypothesis that oysters, Crassostrea virginica, from the northernmost part of the species range in the Gulf of St. Lawrence (48°N) open their valves at lower temperatures than those reported for more southern oysters, Hall element sensors were used to monitor their gaping behaviour. These observations were made in a flow-through system and the temperature, salinity and relative fluorescence of unfiltered seawater were monitored. Photoperiod was controlled (15 h dark:9 h light) and light levels were measured but not closely controlled. Gaping behaviour was followed from February to June 2010 (113 days) and from April to May 2011 (34 days) and was classified into three successive phases: quiescent, awakening and active. Although valves were either closed or slightly open during the quiescent phase (maximum gape angle = 0.49°, SE = 0.04), they abruptly opened to maximum angles of about 5.88° (SE = 0.29) during the awakening phase. Moreover, there was noticeable synchrony amongst individuals, since approximately one-half of the monitored population awoke within a 6.6-h period in both study years. Correlative analyses identified temperature as a factor influencing valve movement, and oysters awakened when temperatures were 0.2-4.0 °C (mean = 2.2, SE = 0.2). Oysters exerted their maximal gape angle as soon as temperatures reached 2.8-6.6 °C (mean = 4.8, SE = 0.2). During the active phase, valves remained open 68.6 % (2010) and 79.7 % (2011) of the time. An unexpected result was the observation of a diurnal rhythm in valve openness whereby the openness was greatest near the end of the afternoon and least in the early morning. [ABSTRACT FROM AUTHOR]

In Prince Edward Island, the spring oyster fishery occurs from early May to mid-July. During this time, oysters may be harvested and held for multiple days by harvesters before being brought to oyster processors, who then relay them in seawater at leased sites. Over the past decade, however, processors have reported high mortalities of relayed catch during July. We investigated duration of air exposure as a potential driver for the high July mortalities reported by oyster processors. Wild oysters were removed from fishing beds in June and July 2017 and handled to mimic techniques used by harvesters. After removal, the oysters were exposed to atmospheric conditions (air) for 0, 24, 48, 72, or 96 h and mortality was quantified. The oysters were then placed at a holding site for 60 days and mortality was quantified on days 7, 14, 30 and 60, and condition index was measured on remaining oysters from each air exposure treatment. Increased mortality was associated with increased air exposure duration, and this effect was most pronounced during July. Oysters exposed to air for 72–96 h in June experienced mortality rates of 4–11%, while the same air exposure duration in July yielded mortality rates four times higher than in June (16–44%). Most of the associated mortality occurred during air exposure and within 7 days of placing oysters at the holding site. Condition index was also lower in July; however there was no evidence that oysters had spawned. Higher mortalities and lower condition indices in July were associated with higher average air and seawater temperatures (both ∼3 °C higher than June). These results ultimately suggest that increased air exposure can result in substantial mortality of PEI oysters fished in July, and corroborate the reports of high July mortalities from oyster processors. [ABSTRACT FROM AUTHOR]

While high levels of siltation are known to be deleterious to eastern oysters (Crassostrea virginica), the collective effect of suspended and bedded sediment is understudied from the perspective of oyster farming and bed restoration. In this study, we used laboratory experiments to explore spat settlement rates on a wild bed proxy substrate (i.e., empty shells on the bottom of experimental tanks) in conditions simulating a siltation event and the presence of suspended spat collectors. Using high-frequency valvometry, we also described the behavioural effects of acute sediment burial on wild adult oysters in situ. The vast majority of larvae settled on bottom substrate as opposed to suspended collectors. Sediment negatively affected overall oyster spat settlement on bottom shell, as spat densities were ≈3 × lower when sediment was present. This negative effect was largely attributed to severely depressed spat densities on the upper side (top) of bottom shells. Settlement on the underside of bottom shell was less affected. Wild adult oyster behaviour was negatively affected by acute burial, which ultimately resulted in death. We suggest that the reduction in settlement in the presence of siltation is likely due to the combined effects of suspended sediment on cue detection and bedded sediment on substrate availability. Given that oysters are ecosystem engineers, the negative effects of siltation on both larval and adult oysters can ultimately result in cascading effects to the surrounding biological community. • Tested effect of silt on oyster larval settlement to artificial collectors and natural shell substrate. • Compared settlement on upper and under side of benthic shell substrate. • Examined adult oyster health during an acute siltation event. • Settlement higher on benthic shell than suspended substrate; silt negatively affected settlement and adult health. • Negative effects of siltation were less severe on underside of benthic shell. [ABSTRACT FROM AUTHOR]

• Investigated efficacy of "bouncing" technique to increase shell thickness and strength for on-bottom cultured oysters. • Bouncing increased shell thickness and strength for oysters >45 mm shell height compared to control oysters. • Bouncing decreased shell thickness and strength for oysters <45 mm shell height compared to control oysters. • Bouncing had no effect on shell shape. • The size at which bounced oysters are released after grow out will dictate usefulness of bouncing technique. Through physical manipulation, oyster growers can modify the morphological features of young oysters in order to improve their robustness and resiliency to predation. In this study, the efficacy of a novel passive culturing technique using floating buckets that "bounce" oysters prior to the benthic grow-out phase for enhancing shell thickness and strength, while not compromising oyster shape, was investigated. Using a field experiment, we quantified and compared the shell thickness (mm), compressive strength (Newtons), and shape (fan ratios and cup ratios) of juvenile oysters subjected to this bouncing technique with oysters grown in floating bags, the current industry standard. Results indicated that bouncing increased shell thickness and strength for oysters >45 mm shell height compared to control oysters, but also decreased shell thickness and strength for oysters <45 mm shell height compared to control oysters. There was no difference in shell shape between bounced and control oysters, with both groups having similar fan and cup ratios. Ultimately, our results suggest that the size at which bounced oysters are released to the benthos for grow out will dictate the usefulness of bouncing technique for reducing predation-related mortality. [ABSTRACT FROM AUTHOR]