Meseck et al.: Effects of ocean acidification on larval Spisu/a solidissima from Long Island Sound 67 
United States, shellfish species were categorized as highly 
susceptible to changing climate conditions (Hare et al., 
2016). A conclusion of most of the research on effects of 
OA on marine mollusks has been that larval shellfish are 
more sensitive to OA than those in juvenile and adult 
stages (Gledhill et al., 2015; Siedlecki et al., 2021). In lar- 
val experiments with bay scallop (Argopecten irradians) 
(Talmage and Gobler, 2009; White et al., 2013), eastern 
oyster (Crassostrea virginica) (Miller et al., 2009; Talmage 
and Gobler, 2009; Gobler and Talmage, 2014), and northern 
quahog (Mercenaria mercenaria) (Green et al., 2004, 2009; 
Talmage and Gobler, 2009), reduced rates of survival and 
growth have been observed when larvae were exposed to 
acidification levels predicted to occur by 2100. The findings 
of these studies indicate that an Q,,,onite Of ~1.5 results 
in physiological responses in marine bivalve species. These 
studies focused on bivalve species that reside in estuarine 
water, where changes in temperature, salinity, and partial 
pressure of CO, (pCO,) can occur at daily rates (Feely et al., 
2010; Dickinson et al., 2013; Duarte et al., 2013), and their 
results may not be applicable to coastal bivalve species. 
A synthesis of available reports on ecological conse- 
quences of ocean and coastal acidification for species in 
the Northeast U.S. continental shelf large marine eco- 
system has identified that to date little is known on how 
coastal bivalve species will respond to OA (Hare et al., 
2016). Recently, juvenile Atlantic surfclam (Spisula solid- 
issima) were found to have modified physiological pro- 
cesses at CO, levels of RCP 8.5 scenario during a 12-week 
exposure, with them being more sensitive than previously 
studied estuarine bivalves (i.e., oyster species and the blue 
mussel, Mytilus edulis) to increasing pCO, levels (Pousse 
et al., 2020). The results of that study highlight the need 
to understand how larval Atlantic surfclam will respond 
to OA conditions. 
Atlantic surfclam are planktonic larvae, transition- 
ing to the pediveliger stage with a foot and “swimming- 
crawling” behavior (Fay et al., 1983; Cargnelli et al., 
1999). Larval Atlantic surfclam are concentrated near the 
thermocline and are transported horizontally by currents 
(Zhang et al., 2015, 2016; Chen et al., 2019). Along the 
coast of the northeastern United States, surface waters of 
the Atlantic Ocean and Gulf of Maine are characterized 
by high variability, both spatially (with increased acidifi- 
cation northward) and seasonally (with the lowest values 
during winter and the highest values in summer) (Biao 
et al., 2004; Wang et al., 2013; Xu et al., 2017; Goldsmith 
et al., 2019; Friedland et al., 2020). Records of carbonate 
chemistry of the subsurface water column in the Gulf of 
Maine have included ,,,,onite levels that were lowest in 
the spring, ranging from 0.9 at the seafloor to 2.2 at the 
surface, and highest during the summer, ranging from 
1.4 at the bottom to 2.6 at the surface, with a seasonal 
range of 1.0—2.0 (Wanninkhof et al., 2015; Wang, 2016). 
Wang et al. (2017) observed a decrease of 0.01 in 2, agonite 
for every 1 pmol/kg increase in dissolved inorganic car- 
bon (DIC), similar to what has been observed in the open 
ocean (Bates et al., 2014). Recently, an oceanographic 
glider, an autonomous underwater vehicle, was used off 
the coast of New Jersey to measure subsurface pH levels, 
which ranged from 7.91 to 8.20, and to measure Q.,agonite> 
which ranged from 1.5 to 2.2 with higher readings at the 
surface (Saba et al., 2019). 
Although there is evidence of increasing pCO, concen- 
trations in waters where larval Atlantic surfclam are 
concentrated during development, data are sparse at 
the thermocline, and the processes and drivers of these 
changes are not understood fully (Boehme et al., 1998; 
Wang et al., 2017). The limited data that is available, com- 
bined with model projections for the entire region, indicate 
that some areas in the region where Atlantic surfclam con- 
centrate will reach global levels of pCO, predicted for 2100 
as early as 2030-2050 (Ekstrom et al., 2015; Siedlecki 
et al., 2021). 
Characterizing the effects of OA on Atlantic surfclam 
is complex in part because of the presence of 2 subspe- 
cies along the coast of the northeastern United States 
(Hare and Weinberg, 2005). Both subspecies are referred 
to as Atlantic surfclam; however, there are physiologi- 
cal differences between the 2 subspecies. The more 
northern subspecies, S. s. solidissima, is larger in size 
(length: 150-200 mm) and lives longer (25-30 years) 
than the more southern subspecies, S. s. similis, which 
ranges in length from 76 to 122 mm and lives for 4.0—5.5 
years (Walker and O’Beirn, 1996; Weinberg and Helser, 
1996). The subspecies differ in geographic range, with 
the northern subspecies of Atlantic surfclam found from 
the Gulf of Saint Lawrence in Canada south to Cape 
Hatteras in North Carolina and with the southern sub- 
species thought to be distributed primarily in shallow 
nearshore environments along the coast of Cape Hat- 
teras and in waters of the Gulf of Mexico off the coast of 
the southeastern United States. 
The Connecticut Bureau of Aquaculture identifies 
Atlantic surfclam in Long Island Sound (LIS) as S. solidis- 
sima, but results of DNA analysis indicate that S. s. simi- 
lis, although it is considered the more southern subspecies, 
is located primarily off the coast of Massachusetts; there 
also is a confirmed population of the southern subspecies 
in waters of New York in LIS (Hare et al., 2010). Results of 
a recent survey conducted by the NOAA Northeast Fish- 
eries Science Center indicate that the southern subspecies 
of Atlantic surfclam may be shifting northward because 
of increased water temperatures (NEFSC, 2017). The 
Mid-Atlantic Fishery Management Council in fiscal year 
2019 solicited studies to examine whether there has been 
an expansion of the distribution of the southern subspe- 
cies of Atlantic surfclam into the habitat of the northern 
subspecies, recognizing there are potential implications 
for stock assessments if multiple species that are genet- 
ically different are managed together. 
Until the geographical distribution of both subspecies is 
better understood, we will make no assumption of which 
subspecies was used in our experiment and will refer to 
the brood stock and larvae of Atlantic surfclam in our 
study as S. solidissima. This choice is consistent with cur- 
rent management of the commercial industry for Atlantic 
surfclam as a single fishery, with harvesting concentrated 
