Synopsis The presence of standing genetic variation will play a role in determining a population's capacity to adapt to environmentally relevant stressors. In the Gulf of Mexico, extreme climatic events and anthropogenic changes to local hydrology will expose productive oyster breeding grounds to stressful low salinity conditions. We identified genetic variation for performance under low salinity (due to the combined effects of low salinity and genetic load) using a single-generation selection experiment on larvae from two populations of the eastern oyster, Crassostrea virginica. We used pool-sequencing to test for allele frequency differences at 152 salinity-associated genes for larval families pre- and post-low salinity exposure. Our results have implications for how evolutionary change occurs during early life history stages at environmentally relevant salinities. Consistent with observations of high genetic load observed in oysters, we demonstrate evidence for purging of deleterious alleles at the larval stage in C. virginica. In addition, we observe increases in allele frequencies at multiple loci, suggesting that natural selection for low salinity performance at the larval stage can act as a filter for genotypes found in adult populations.
more »
« less
Evidence for stage-based larval vulnerability and resilience to acidification in Crassostrea virginica
ABSTRACT Using image analysis of scanning electron micrographs (SEMs), we compared differences in growth of D-stage veligers [i.e. prodissoconch I and II (PI and PII) larvae] of eastern oysters Crassostrea virginica grown in mesohaline water under high- and low-CO2 conditions. We found SEMs to reveal no evidence of dissolution or shell structure deformity for larval shells in either of the CO2 treatments but detected prominent growth lines in the PII regions of larval shells. The number of growth lines closely approximated the duration of the experiment, suggesting that growth lines are generated daily. Mean growth line interval widths were 20% greater for larval shells cultured in low- vs high-CO2 conditions. Crassostrea virginica veliger larvae were shown to tolerate high CO2 levels and aragonite saturation states (Ωarag) < 1.0, but larval growth was slowed substantially under these conditions. Differences in growth line interval width translate into substantial changes in shell area and account for previously observed differences in total shell area between the treatments, as determined by light microscopy and image analysis. Other studies have documented high mortality and malformation of D-stage larvae in bivalves when pre-veliger life stages (i.e. eggs, gastrula and trochophores) were exposed to elevated CO2. Our experiments revealed statistical differences in rates of larval survival, settlement and subsequent early-stage spat mortality for veligers reared in high- and low-CO2 conditions. Although each of these rates was measurably affected by high CO2, the magnitude of these differences was small (range across categories = 0.7–6.3%) suggesting that the impacts may not be catastrophic, as implied by several previous studies. We believe the apparent disparity among experimental results may be best explained by differential vulnerability of pre-veliger stage larvae and veligers, whereby PI and PII larvae have greater physiological capacity to withstand environmental conditions that may be thermodynamically unfavourable to calcification (i.e. Ωarag < 1.0).
more »
« less
- Award ID(s):
- 1659668
- NSF-PAR ID:
- 10248873
- Date Published:
- Journal Name:
- Journal of Molluscan Studies
- Volume:
- 86
- Issue:
- 4
- ISSN:
- 0260-1230
- Page Range / eLocation ID:
- 342 to 351
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Restoration of native oyster ( Crassostrea virginica ) populations in Chesapeake Bay shows great promise after three decades of failed attempts. Population models used to inform oyster restoration had integrated reef habitat quality, demonstrating that reef height determines oyster population persistence and resilience. Larval recruitment drives population dynamics of marine species, yet its impact with reef height and sediment deposition upon reef restoration is unknown. To assess the influence of reef height, sediment deposition and larval supply, we adapted a single-stage population model to incorporate stage structure using a system of four differential equations modeling change in juvenile density (J), and changes in volume of adults (A), oyster shell reef (R), and sediment (S) on an oyster reef. The JARS model was parameterized with empirical data from field experiments. Larval supply included larvae from the natal population and from outside populations. The stage-structured model possessed multiple non-negative equilibria (i.e., alternative stable states). Different initial conditions (e.g., oyster shell reef height) resulted in different final states. The main novel findings were that the critical reef height for population persistence and resilience was jointly dependent on sediment input and larval supply. A critical minimum larval supply was necessary for a reef to persist, even when initial sediment deposition was zero. As larval supply increased, the initial reef height needed for reef persistence was lowered, and oyster reef resilience was enhanced. A restoration oyster reef with higher larval influx could recover from more severe disturbances than a reef with lower larval influx. To prevent local extinction and assure a positive population state, higher levels of larval supply were required at greater sediment concentrations to overcome the negative effects of sediment accumulation on the reef. In addition, reef persistence was negatively related to sediment deposited on a reef prior to larval settlement and recruitment, implying that restoration reefs should be constructed immediately before settlement and recruitment to minimize sediment accumulation on a reef before settlement. These findings are valuable in oyster reef restoration because they can guide reef construction relative to larval supply and sediment deposition on a reef to yield effective and cost-efficient restoration strategies.more » « less
-
Estuaries provide valuable habitat for the eastern oyster (Crassostrea virginica). Although salinity at a given location fluctuates regularly with tides, upbay and downbay salinity differences span a broad estuarine salinity gradient. Higher salinity habitats downbay support faster oyster growth, whereas lower salinities upbay act as a refuge from predation and disease but slows growth. Two experiments were performed to investigate the effect of salinity, postsettlement salinity changes, and shell morphology on juvenile oyster growth. One experiment used wild oyster spat collected from three distinct Delaware Bay salinity zones that were then transplanted into various salinity conditions in the laboratory, where growth was monitored. Transplanting into low salinity led to decreased growth compared with transplanting to higher salinity, and growth of oyster spat was overall highest for spat from the lowest salinity source. Growth did not differ among shell morphologies. A second experiment used hatchery reared larvae set in one of four different salinity conditions. Those spat were maintained in settlement salinities 22, 16, 10, and 6 for 2–3 wk postsettlement, then measured before fully factorial transfer into new salinity conditions with measurement 3 wk later. Lower final salinity treatments were associated with lower growth, lower initial salinity treatments were associated with faster final treatment growth, and final growth depended on the interaction between initial and final salinity. Therefore, in addition to the effects of acute salinity changes on growth, early postsettlement hyposalinity stress can generate compensatory juvenile oyster growth. As increased freshwater events due to climate change are expected in the Delaware Bay and regionally in the Northeast, these results indicate that nonlinear early life stress responses are important to quantify to better understand oyster stock resilience and plan management.more » « less
-
more » « less
Abstract Larvae of marine calcifying organisms are particularly vulnerable to the adverse effects of elevated
p CO2on shell formation because of their rapid calcification rates, reduced capacity to isolate calcifying fluid from seawater, and use of more soluble polymorphs of calcium carbonate. However, parental exposure to elevatedp CO2could benefit larval shell formation through transgenerational plastic responses. We examined the capacity of intergenerational exposure to mitigate the adverse effects of elevatedp CO2on Eastern oyster (Crassostrea virginica ) early larval shell growth, shell morphology, and survival. Adult oysters were exposed to control (572 ppmp CO2) or elevatedp CO2(2827 ppmp CO2) conditions for 30 d during reproductive conditioning. Offspring from each parental treatment were produced using a partial North Carolina II cross design and grown under control and elevatedp CO2conditions for 3 d. We found evidence of transgenerational plasticity in early larval shell growth and morphology, but not in survival, in response to the parentalp CO2exposure. Larvae from parents exposed to elevatedp CO2exhibited faster shell growth rates than larvae from control parents, with this effect being significantly larger when larvae were grown under elevatedp CO2compared to control conditions. Parental exposure to elevatedp CO2, however, was insufficient to completely counteract the adverse effects of the prescribed elevatedp CO2on early larval shell formation and survival. Nevertheless, these results suggest that oysters have some capacity to acclimate intergenerationally to ocean acidification. -
Eastern oysters (Crassostrea virginica) are sessile, relying on a larval phase to disperse in estuaries. Oyster larval swimming behavior can alter dispersal trajectories and patterns of population connectivity. Experiments were conducted to test how both (1) acclimation time to new environmental conditions and (2) larval swimming behavior change with salinity and larval age. Acclimation time to changes in salinity was longest in lower salinity (6 ppt) and decreased with age. To test changes in behavior with salinity, larvae were placed into four salinities (6, 10, 16, and 22 ppt) where swimming was recorded. To test changes in behavior with age, larvae aged 6, 12, and 15 days were recorded. In both experiments, swimming paths were mapped in two dimensions, behavior of each path was categorized, and speed, direction, and acceleration were calculated. The frequency of upward, neutral, and downward swimming behaviors did not differ across salinity treatments but did vary with age, whereas the frequency of behavior types varied with both salinity and ontogeny. As an example, diving was observed more frequently in low salinity, and more downward helices were observed in moderate salinity, while younger larvae swam upward with more frequency than older larvae. Surprisingly, diving was observed in 10%–15% of all larvae across all ages. Given the consequence of larval behavior to marine invertebrate dispersal, changes in swimming over larval age and in response to environmental changes have important implications to marine population stability and structure.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/mollus/eyaa022
