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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. 

A survey of the region eastward of Nantucket provided an opportunity to examine the cold temperate–boreal boundary along the high‐energy Great South Channel. Here described are the benthic macroinvertebrate community types encountered, with a focus on the influence of climate change on the range boundaries of the benthic biomass dominants and the potential existence of transient multiple stable states. The survey identified three primary community types. The shallowest sites were occupied by a surfclam‐dominated community, comprising an abundance of large (≥150 mm) surfclams, and a few common attached epibiota primarily attached to exposed surfclam shell. Two communities exist at intermediate depths, one dominated by submarket and small market‐size surfclams (<150 mm) and the other, created by mussel mats and their attendant epibiota, crabs, sea urchins, and other mobile epifauna. Mussels are a foundational species, establishing a hard‐bottom terrain conducive to these other denizens in soft‐bottom habitat. Cobbles were nearly ubiquitous, rocks were routinely recovered, and boulders were encountered occasionally. Slow growing attached epibionts were exceedingly rare and mobile epifauna were not obviously associated with these large sedimentary particles; nor were the surfclam or mussel communities. The frequency of barnacle scars suggests sediment scour under the high‐flow regime characteristic of the surveyed region, which voids the habitat potential of these sedimentary particles. The abundance of surfclam shell indicates that surfclams have inhabited the shoaler depths for an extended time; limited shell at deeper sites supports the inference from the absence of large animals that these sites are relatively newly colonized and represent further evidence of an offshore shift in range brought on by increasing bottom water temperatures. The dichotomous nature of the two primary community types at mid‐depths suggests that these two communities represent multiple stable states brought on by the interaction of an invading cold temperate species with the receding boreal fauna resulting in a transient intermingling of species, which, however, structure the habitat into exclusionary stable states rather than overlapping in a co‐occurrence ecotone.