Growth and longevity in surfclams east of Nantucket: Range expansion in response to the post-2000 warming of the North Atlantic

The Atlantic surfclam, Spisula solidissima, is distinguished by a well-documented shift in range that accelerated in the 2000s as the northwest Atlantic warmed. Here the extension of the Atlantic surfclam into heretofore Acadian Province waters off the island of Nantucket is documented and compared to the distribution of surfclam shell as an indicator of recent colonization, to the timing of range expansion, and to the physiological implications of a range extension into deeper water. The primary demographic difference observed is the dichotomous distribution of sizes. Smaller surfclams averaged higher in abundance at the deeper offshore sites, whereat the number of large animals was distinctly fewer; thus, the size-frequency distributions at deeper sites were shifted towards the smaller sizes, a finding consistent with the expectation of recruitment into deeper water during a period of range expansion. In confirmation, deeper-water stations where surfclams were aged yielded surfclams no older than 13 yr, whereas shallow-water stations had a mature age frequency with some surfclams exceeding 20 yr. Further support for the more recent occupation of deeper-water sites comes from the distribution of surfclam shell, that was found in limited quantities at stations where recent colonization is inferred and in greater quantities in shallower water where longer-term occupation is surmised. For the shallower-water sites with a mature demographic, growth rates were comparable or higher than observed elsewhere in the stock and surfclam maximum sizes were larger than elsewhere in the geographic range. In contrast, surfclams colonizing deeper water post-2000 grew at a slower rate likely due to a lower average temperature near the deep-water range boundary. The penalty for colonization pushing the range boundary into deeper, cooler water lasted no more than 4–5 years, however, after which growth rates increased to rates typical of surfclams in shallower water. Thus, surfclams responded quickly to a period of rapid climate change in contrast to expectation from their known longevity