Siltation negatively affects settlement and gaping behaviour in eastern oysters.

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.

Behavioural responses to predators in Mediterranean mussels (Mytilus galloprovincialis) are unaffected by elevated pCO2.

Ocean acidification is expected to affect marine organisms in the near future. Furthermore, abrupt short-term fluctuations in seawater pCO2 characteristic of near-shore coastal regions and high-density aquaculture sites currently have the potential to influence organismal and community functioning by altering animal behaviour. While anti-predator responses in fishes exposed to elevated pCO2 are well documented, such responses in benthic marine invertebrates are poorly studied. We used high frequency, non-invasive biosensors to test whether or not short term (3-week) exposure to elevated pCO2 could impact behavioural responses to the threat of predation in adult Mediterranean mussels from Galicia on the northwestern coast of Spain. Predator alarm cues (crushed conspecifics) resulted in a prolonged (1 h) reduction in the degree of valve opening (-20%) but had no clear effect on overall valve movement activity, while elevated pCO2 did not affect either response. Our results add to the increasing body of evidence suggesting that the effects of end-of-century pCO2 levels on marine animal behaviour are likely weak. Nonetheless, longer-term exposures spanning multiple generations are needed to better understand how ocean acidification might impact behavioural responses to predation in marine bivalves.

Refining the tethering of American oysters (Crassostrea virginica) to measure the effects of two environmental stressors.

Tethering assays, or the physical restraint of test organisms, has been used in the past to measure selected organisms' response to stressors while removing the observer from the experimental setting. Although informative for monitoring and hypothesis testing, these assays often used microfilaments that have been found to be too invasive or prone to biases given their effects on test organisms' behavior. Here, we describe a new variation of tethering using American oysters (Crassostrea virginica) and illustrate its use in the study of their mortality rates as a result of two stressors: siltation and predation by a non-indigenous species. Our protocol identified a resistant (non-toxic) glue that could be used to attach oysters to stone slabs, thus partially mimicking the natural cementation of the shell to natural substrates. This variation of tethering was harmless and maintained oysters' body position and natural ability to filter feed. Using tethered oysters in separate two-week field cage experiments, we also show how siltation and predation by a non-indigenous species (the European green crab, Carcinus maenas), caused a gradual, easily measurable increase in oyster mortality rates. We argue that this variation of tethering is a cost-effective and advantageous way to monitor or test the effects of these and other stressors on oysters and similar species.

Non-indigenous predators threaten ecosystem engineers: Interactive effects of green crab and oyster size on American oyster mortality.

Non-indigenous green crabs (Carcinus maenas) are emerging as important predators of autogenic engineers like American oysters (Crassostrea virginica) throughout the eastern seaboard of Canada and the United States. To document the spreading distribution of green crabs, we carried out surveys in seven sites of Prince Edward Island during three fall seasons. To assess the potential impact of green crabs on oyster mortality in relation to predator and prey size, we conducted multiple predator-prey manipulations in the field and laboratory. The surveys confirmed an ongoing green crab spread into new productive oyster habitats while rapidly increasing in numbers in areas where crabs had established already. The experiments measured mortality rates on four sizes of oysters exposed to three sizes of crab, and lasted 3-5 days. The outcomes of experiments conducted in Vexar® bags, laboratory tanks and field cages were consistent and were heavily dependent on both crab size and oyster size: while little predation occurred on large oysters, large and medium green crabs preyed heavily on small sizes. Oysters reached a refuge within the 35-55 mm shell length range; below that range, oysters suffered high mortality due to green crab predation and thus require management measures to enhance their survival. These results are most directly applicable to aquaculture operations and restoration initiatives but have implications for oyster sustainability.