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Fishery Bulletin 119(1) 
scenarios (Miller et al., 2009; Talmage and Gobler, 2009; 
White et al., 2013; Waldbusser et al., 2015). In studies of 
other clam species, increasing pCO, above current levels 
resulted in linear decreases in growth and shell size of 
softshell clam (Mya arenaria) (Green et al., 2009, 2013; 
Clements and Hunt, 2014), northern quahog (Talmage 
and Gobler, 2009; Gobler and Talmage, 2014), eastern 
oyster (Talmage and Gobler, 2009), and Pacific oyster 
(C. gigas) (Waldbusser and Salisbury, 2014; Waldbusser 
et al., 2015). However, no effects at pCO, concentrations of 
the RCP 8.5 scenario have been reported for the Olympia 
oyster (Ostrea lurida) (Waldbusser et al., 2016), Antarc- 
tic bivalve (Laternula elliptica) (Bylenga et al., 2015), or 
blue mussel (Gazeau et al., 2013). A hormetic response to 
OA exposure in bivalves, although not often reported, was 
recently reported for juvenile Atlantic surfclam (Pousse 
et al. 2020) and has been noted in other marine species, 
including copepod (Li and Gao, 2012; Thor and Oliva, 
2015) and finfish (Miller et al., 2013, 2016) species. 
Other bivalve species may experience a hormetic 
response, but it may be masked by the pCO, levels cho- 
sen for other experiments, choices that can result in only 
a decrease being observed. For example, in a repeat study 
conducted in 2014 with eastern oyster by using a finer 
exposure resolution than that in our study, a maximal 
hormetic response at a pCO, of 380 patm was found for 
growth and percentage of larvae that completed meta- 
morphosis (Gobler and Talmage, 2014). Unlike larvae of 
estuarine bivalve species, the larval Atlantic surfclam in 
our experiment were able to tolerate moderate OA levels. 
Future research should include a finer exposure resolu- 
tion to determine the width of hormetic response in larvae 
of Atlantic surfclam. 
In previous studies conducted with estuarine bivalves, 
larvae of most species were more sensitive to increased 
pCO, than juveniles or adults. For example, for eastern 
oyster exposed to conditions of the RCP 6.0 scenario, lar- 
vae had reduced rates of growth rates and survival and 
a lower percentage of larvae completed metamorphosis 
(Talmage and Gobler, 2009), and for juveniles no sig- 
nificant difference was observed in growth at the pCO, 
level of the RCP 6.0 scenario (Dickinson et al., 2012) or 
levels greater than that of the RCP 8.5 scenario (1700 
patm; Young and Gobler, 2018). For adult eastern oys- 
ter in other studies, no effects on growth or gaping were 
observed at pCO, levels as high as 8000 patm (Clements 
et al., 2017, 2018). A similar trend has been observed for 
blue mussel with larvae being more sensitive than adults 
(Thomsen et al., 2017). 
Even though the results of this study were surprising, 
with increased growth at pCO, levels predicted for the 
RCP 6.0 scenario, growth decreased at levels predicted 
for the RCP 8.5 scenario. Results of an experiment with 
juveniles of this subspecies indicate a similar hormetic 
response to that of larvae in our study (Pousse et al., 2020). 
Pousse et al. (2020) found metabolic depression in juvenile 
Atlantic surfclam exposed to pCO, levels of 1350 patm, 
making them more susceptible than Pacific oyster (Lannig 
et al., 2010) and blue mussel (Thomsen and Melzner, 
2010). The larval Atlantic surfclam in our study were not 
exposed to the higher levels of pCO, used on juveniles in 
the Pousse et al. (2020) study, but it would be interest- 
ing to determine when metabolic depression occurred in 
larvae considering the similar trends observed between 
these 2 studies. We cannot explain why larval and juve- 
nile Atlantic surfclam may behave similarly or why larval 
Atlantic surfclam appear to tolerate higher levels of pCO, 
than larvae of some other estuarine species like the east- 
ern oyster; however, a possible reason may relate to how 
Atlantic surfclam allocate energy for growth and develop- 
ment. Bioenergetic studies, including those that include 
dynamic energy budget modeling, would provide insight 
into whether the small changes in growth observed in our 
study and in the study of Pousse et al. (2020) would have a 
long-term effect over the time period required for Atlantic 
surfclam to reach harvest size and would help to clarify 
variation in responses between bivalve species. 
Parsons (2001) suggests that a hormetic response may 
reflect evolutionary adaptation of metabolic systems 
to environmental variables and may link to Darwinian 
fitness; however, there is little quantitative evidence 
currently available to confirm this hypothesis. Recent 
research on the response of bivalve larvae to OA has 
focused on population-level responses and the potential 
for evolutionary adaptation. Increased growth of lar- 
val Atlantic surfclam at elevated pCO, concentrations 
during our study may reflect adaptation of adults in the 
source population. The wild adults used in this study 
were exposed to conditions during the summer in LIS, 
where in situ sediment levels of pCO, can periodically 
range from 689 to 1828 patm (Perry et al., 2015; Meseck 
et al., 2018; Snyder et al., 2019). Researchers have found 
that survival of larvae varied significantly between pop- 
ulations of the Sydney rock oyster (Saccostrea glomerata) 
(Parker et al., 2011) and Chilean mussel (M. chilensis), on 
the basis of the history of adult exposure to OA (Duarte 
et al., 2015). These findings indicate potential physiolog- 
ical and metabolic adaptations of shellfish populations to 
OA conditions. 
It was beyond the scope of this study to determine the 
ability of Atlantic surfclam to adapt and evolve to shift- 
ing environmental conditions; however, future evolution- 
ary adaptation studies should include both subspecies of 
Atlantic surfclam. There is an evolutionary divergence of 
13.9% between the northern and southern subspecies, indi- 
cating long-term reproductive isolation of these subspecies 
(Hare and Weinberg, 2005; Hare et al., 2010) that may have 
allowed each to develop differential tolerances to OA condi- 
tions. A comparison between specimens of the 2 subspecies 
from the same location, exposed to increasing pCO., might 
resolve whether there is an evolutionary divergence in OA 
tolerance between the subspecies, and the results of such 
an investigation could support more refined management 
of populations of Atlantic surfclam in New England. 
Slower growth rates and smaller shell height in lar- 
vae from the high- and low-pCO, treatments indicate 
that suboptimal conditions for larval Atlantic surfclam 
may stimulate a shift in allocation of energetic resources 
