Ecology and Vulnerability

Bay Scallop

Marine bivalves are particularly vulnerable to ocean acidification due to their carbonate shells, with much of the literature focused on this aspect of climate change either alone or in combination with other stressors like elevated temperatures or dissolved oxygen. These stressors, both individually and in synergy, have documented physiological, morphological, and survival. Much of the climate change research for Atlantic Sea and Bay Scallops has focused on the effects of ocean acidification and increasing water temperatures, with little known about the impacts of these other large-scale marine habitat changes on the species. Some of the research was from the fisheries perspective with focused questions relevant to stock management, such as the selective breeding of Bay Scallops for thermal tolerance (e.g., Yang et al., 2015; Zhu et al., 2021). Needed research to fill data gaps for these marine bivalves include population-level effects of ocean acidification (Rheuban et al., 2018), as well as the interactive effects of ocean acidification and warmer waters on physiology with benthic habitat disturbance and losses both in estuaries for Bay Scallops and offshore for Atlantic Sea Scallops. 

In addition, a better understanding of the effects of shifting or changing currents on spawning, recruitment, and
settlement and the effects of increased frequency and intensity of coastal storms and extreme
weather events on Bay Scallop populations and habitat is needed.

Changes to Morphology or Physiology

Bay Scallops are particularly sensitive to ocean acidification during the larval life stage, with legacy consequences for their later life stages. Exposure to ocean acidification has the highest impact during the earliest larval stage, with negative impacts on growth and survival observed in laboratory experiments in as little as one day or less (Gobler and Talmage, 2013; White et al., 2013 & 2014). Exposure to elevated CO2 levels during fertilization decreased survival rates of larval Bay Scallops in an experiment conducted by White et al. (2014), with results indicating that the first two hours are a critical exposure window for ocean acidification. Gobler and Talmage (2013) found that high CO2 concentrations resulted in cascading negative impacts on Bay Scallop physiology, partly due to lower calcification rates for their shells. Survival rates declined significantly with exposure to elevated CO2 levels during the first four days of larval development. However, a ten-day exposure later in the larval development period did not reduce survival, and individuals who survived exposure to high CO2 levels during larval development had higher juvenile growth rates when exposed to normal CO2 levels when compared to individuals who were not exposed as larvae to high CO2 levels (Gobler and Talmage 2013). Scallops that were exposed to even moderate CO2 levels during larval development were smaller than those grown with normal CO2 levels (Gobler and Talmage, 2013; White et al., 2013 & 2014), with legacy effects observed at ten months of age (Gobler and Talmage, 2013). Increased juvenile growth rates did not overcome these size differences, suggesting potential population-level effects from ocean acidification (Gobler and Talmage 2013). White et al. (2014) also found shell deformities in scallops exposed in the laboratory to continuous high CO2 levels when compared to those only exposed after two hours postfertilization. Experiments by Young and Gobler (2018) observed reduced growth rates in shell and/or tissue growth of Bay Scallops and three other estuarine bivalves with exposure to ocean acidification but found that the presence of the macroalgae sea lettuce (Ulva spp.) reduced the degree of impact from the ocean acidification. They also documented higher calcium carbonate saturation states when Ulva spp. was present with and without ocean acidification conditions, suggesting that photosynthesis and/or nitrate assimilation by the macroalgae increased alkalinity levels. Young and Gobler (2018) suggested that large collections of macroalgae, either naturally occurring or aquaculture collections, could provide refuges for bivalves to mitigate the negative effects of ocean acidification. Xing et al. (2016) tested the thermal tolerance of Bay Scallops at different life stages, finding evidence that indicated that younger and smaller scallops have higher thermal tolerances than older and larger scallops and that spawning behavior by adults may decrease thermal tolerance. Some populations of Bay Scallops in China have been observed to have higher tolerance to thermal stress than others, spurring research efforts into underlying genetic markers (Yang et al., 2013; Yang et al., 2014; Yang et al., 2015; Zhu et al., 2021; Liu et al., 2022). The A. i. irradians subspecies exhibited more sensitivity to thermal stress than the A. i. concentricus subspecies, the latter of which appeared to have more thermal adaptation (Yang et al., 2014; Yang et al., 2015; Zhu et al., 2021; Liu et al., 2022). In estuaries, Bay Scallops are more sensitive to climate change stressors such as intermittent hypoxia compared to Eastern Oysters (Crassostrea virginica), Hard Clams (Mercenaria mercenaria), and Blue Mussels (Mytilis edulis), although those bivalves also exhibit negative physiological effects to varying degrees (Clark and Gobler, 2016; Ivanina et al., 2016; Ivanina and Sokolova, 2016; Young and Gobler, 2018). Bay Scallops also showed evidence of oxidative stress and cellular damage but did not lead to a severe deficiency in energy levels. Eutrophic estuaries commonly experience seasonal patterns of diurnal fluctuations in dissolved oxygen and pH levels (Clark and Gobler, 2016; Gobler et al., 2017; Yang et al., 2021a). Yang et al. (2021a) found that dissolved oxygen is significantly depleted during summer in both surface and bottom waters due to temperature, microbial respiration, and photosynthesis drivers. Seasonal temperature was the most important driver for depleting dissolved oxygen. Photosynthesis could offset some but not all temperature-induced deoxygenation in surface waters. In bottom waters, biological respiration plays more of a role with temperature drivers. Yang et al. (2021a) concluded that seasonal hypoxia and acidification in the coastal waters of the Bay Scallop habitat are highly associated with the stratification of the water column due to temperature with complex environmental processes. When Bay Scallops are exposed to continuous acidification or hypoxia conditions, larval survival, growth, and development declined. Combined exposure to acidification and hypoxia resulted in negative additive effects (Clark and Gobler 2016). However, juveniles were more tolerant to acidification exposure than larvae (Gobler et al., 2017). If exposure was diurnal, the return to normal conditions was not long enough to reverse the physiological stress, and/or the severity level of the hypoxia and acidification was too intense to overcome fully (Clark and Gobler 2016). When acidification and hypoxia in combination had diurnal variation, larval Bay Scallop survival was significantly lower than when levels were held constant (Gobler et al., 2017). These results have led to the conclusion that these interactive effects lead to higher vulnerability and can be a significant threat to Bay Scallop larvae with potential population-level impacts as these conditions intensify in the future (Clark and Gobler, 2016; Gobler et al., 2017). Tomasetti et al. (2023) assessed the vulnerability of Northern Bay Scallops (A. irradians irradians) in the Northeast to hypoxic and thermal stress in combination. Most of the geographic range of Northern Bay Scallop experienced significant warming during the summer from 2003 to 2020. Tomasetti et al. (2023) documented the mortality of a local population of Northern Bay Scallops in New York during an eight-day heatwave in their estuary that coincided with a severe diel-cycling hypoxia period. At the same time, a population in Massachusetts with similar dissolved oxygen levels but lower mean daily temperatures did not suffer a die-off. Subsequent laboratory experiments sought to recreate these observed conditions and found an increase in the likelihood of mortality by 120 times with the combined effects of high temperatures and hypoxia (Tomasetti et al., 2023). Field experiments in New York and Massachusetts found that higher daily temperatures and low dissolved oxygen levels negatively impacted Bay Scallops' oxygen consumption rates and aerobic activity. Overall, these results provide strong evidence that the combined effects of hypoxic and thermal stress on Bay Scallops negatively impact their physiology and survival with potential population-level effects. Gobler et al. (2014) tested the combined effects of low oxygen and ocean acidification on the early life stages of Bay Scallop and Hard Clam. Scallop larval survival was reduced by more than 50% with ocean acidification alone, and low oxygen reduced growth and metamorphosis by more than 50% alone. The combined effects of both ocean acidification and low oxygen were additive and negative, leading Gobler et al. (2014) to recommend that climate change stressors to Bay Scallops (and Hard Clams) should be considered in combination and not individually to predict species responses. Stevens and Gobler (2017) subsequently tested the individual and interactive effects of ocean acidification, thermal stress, and hypoxia on four marine bivalves – Bay Scallop, Oyster, Hard Clam, and Blue Mussel (Mytilus edulis). Higher temperatures resulted in the most consistent physiological responses of the bivalves, but the responses were both negative and positive. Bay Scallops were the most sensitive of the four species, with reduced survival, shell growth, and/or tissue weight when exposed to either low dissolved oxygen or elevated acidity. Hypoxia exposure resulted in significantly increased rates of respiration in Bay Scallops. The interactive effects of low oxygen and acidification often were antagonistic and resulted in higher growth rates than they would individually, suggesting a potential anaerobic metabolic response. The interactive effects of acidification and higher temperatures were antagonistic and synergistic (Stevens and Gobler, 2017).

Changes in Population

Griffith and Gobler (2017) found that the negative impacts of ocean acidification on Bay Scallops are transferred to their offspring, with offspring significantly more vulnerable to not only ocean acidification effects but to other stressors like thermal stress, exposure to harmful algae, and limitations in food resources as well. They conclude that Bay Scallops are not likely to acclimate to ocean acidification in the short term and that with continued ocean acidification that the negative impacts to scallop populations will compound and become more severe over time (Griffith and Gobler 2017). Grear et al. (2020) developed population models for Bay Scallops and Hard Clams to evaluate the population-level impacts of acidification on the bivalves. The cumulative risk of extinction was higher for Bay Scallops than for Hard Clams. The five-year risk of Bay Scallop extinction ranged from 56% to greater than 99% with increasing acidification levels (Grear et al., 2020). 

Indirect Effects

Griffith et al. (2019) documented how the presence of two algae known to cause harmful algal blooms, Cochlodinium polykrikoides and Aureococcus anophagefferens, interacted with ocean acidification and higher temperatures negatively impacted the larval physiology of Bay Scallops in their synthesis of protein, shell growth, cellular metabolism, and membrane transport. With ten days of exposure, each harmful algae decreased larval survival at levels significantly higher than the individual stressors (Griffith et al., 2019). Population declines due to the loss of juvenile estuarine habitat for Bay Scallops are amplified by the impacts of ocean acidification, which decrease larval survival and shell size (White et al., 2014). The impacts of ocean acidification may be amplified by intensive shellfish aquaculture (Yang et al., 2021b). Yang et al. (2021b) found that scallop calcification and mixing were the main drivers of seasonal variation in alkalinity levels. The key environmental drivers for levels of dissolved inorganic carbon were mixing, exchange between the air and water, and microbial activity, such as microbial respiration processes and photosynthesis. The combined effects of temperature, air-sea exchange, scallop metabolic activities, microbial activitycontrolled pH, pCO2, and aragonite saturation state levels. They found that large-scale scallop farming significantly increases the ratio of dissolved inorganic carbon to alkalinity, reducing the total alkalinity concentration and, thus, the buffering capacity of the carbonate system. The growth of Bay Scallop shells in intensive aquaculture removes carbonate from the seawater, likely accelerating ocean acidification at the local level (Yang et al., 2021b).