Meseck et al.: Effects of ocean acidification on larval Spisu/a solidissima from Long Island Sound 73 
away from allometric growth toward maintenance of 
metabolic or physiological homeostasis. Ocean acidifi- 
cation in marine environments can cause invertebrates 
to slow metabolism and may result in reduced growth 
and smaller body size (Pértner et al., 2005; Gobler and 
Talmage, 2014). The hormetic response we observed 
in growth and time to metamorphosis of Atlantic surf- 
clam may result from energetic changes reflected in 
lipid metabolism. Although there was no significant 
difference between the lipid levels in specimens among 
treatments, there was a consistent trend in the concen- 
trations of phospholipids, sterols, and triacylglycerols 
and in the ratio of sterols to phospholipids, with the same 
hormetic response as that observed for growth. The trend 
we observed in these lipid levels among treatments may 
indicate an adaptive response of membranes to different 
environmental conditions, including temperatures and 
pressures (Crockett, 1998; Pernet et al., 2006, 2007), a 
process known as homeoviscous adaptation (HVA). 
Homeoviscous adaptation in other marine organisms 
has been reported previously as an adaptive response 
to OA (Turk et al., 2007; Bennett et al., 2018). Slight 
decreases in phospholipid, sterol, and triacylglycerol lev- 
els and in the sterol-to-phospholipid ratio at low and high 
pCO, levels, like those observed in our study, may provide 
a metabolically less expensive and energy-conserving 
mechanism to reduce expenditure of adenosine triphos- 
phate (Crockett, 1998) that facilitates use of different ion 
exchange pathways (Kusumi et al., 1986). Pousse et al. 
(2020) found an HVA response in respiration rates and 
food selection efficiency of juvenile Atlantic surfclam. Fur- 
ther research on how HVA pathways are used in larvae 
under OA conditions should be pursued with the addi- 
tional measurements of respiration rate, feeding rate, and 
scope for growth to help determine if OA can facilitate dif- 
ferent metabolic pathways. 
Movement of populations of Atlantic surfclam north- 
ward and into deeper water can be attributed to warm- 
ing temperatures (Weinberg, 2005; Munroe et al., 2016). 
Limited pCO, data, combined with modeling results, 
indicate that decreases in pH and Q,,agonite occur in the 
areas to which Atlantic surfclam are moving (Wang, 
2016; Saba et al., 2019; Friedland et al., 2020; Siedlecki 
et al., 2021). This study focused on only the pCO, levels 
predicted for the RCP 6.0 and RCP 8.5 scenarios; we did 
not look at the role of increasing temperature. Under the 
RCP 6.0 and RCP 8.5 scenarios, increases in tempera- 
ture are expected to occur concurrently with increased 
pCO, levels. Future research should include examina- 
tion of the response of Atlantic surfclam to OA under dif- 
ferent temperatures to determine if the combined effects 
of temperature and OA changes their tolerance to pCO, 
concentrations. 
Atlantic surfclam can live up to 35 years and become 
harvestable within 5-7 years, and this study focused on 
a short portion of their lifespan. More information about 
the pCO, levels experienced by larval, juvenile, and adult 
Atlantic surfclam during their lifetime is needed to better 
define the range of pCO, concentrations on which future 
studies should focus. Further research should empha- 
size the bioenergetic pathways governing larval response 
to OA and address adaption and evolution by including 
genetic analysis. Finally, incorporating increased pCO, 
levels into future research would provide information ben- 
eficial to fisheries management efforts. 
Acknowledgments 
The authors thank R. Goldberg, D. Perry, and J. Rose for 
technical assistance, J. Bloom from Copps Island Oysters 
and D. Carey of the Connecticut Department of Agricul- 
ture, Bureau of Aquaculture, for collection of brood stock 
of Atlantic surfclam, and the Northeast Fisheries Science 
Center for funding. 
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