Yearly shifts in shell quality for the hermit crab Pagurus longicarpus in coastal Massachusetts

Global warming is expected to result in milder New England winters. In this study we monitored changes in hermit crab shell quality at Nahant, Massachusetts, USA, at intervals during 17 yr between 1997 and 2014 for a single species of hermit crab Pagurus longicarpus occupying the shells of the intertidal periwinkle Littorina littorea. The incidence of hermit crabs occupying damaged shells, shells housing the symbionts Hydractinia, Crepidula plana, and C. convexa, and showing poor shell fit varied significantly among years. Shell quality was especially bad in the summer of 2012, following one of the warmest winters on record: more than 60% of the sampled hermit crabs were found living in damaged shells and more than 50% of the shells bore symbionts. Moreover, an unusually low number (only 15%) of the hermit crabs sampled that summer were found in shells that were undamaged, without symbionts, and of adequate size, compared with 50 to 55% of hermit crabs being found in such 'perfect' shells in 1997 and 2013. We suggest that new, high-quality shells are added to hermit crab populations at our study site largely through the freeze-induced mortality of intertidal periwinkles, and that the poor shell quality documented in summer 2012 reflects reduced periwinkle mortality during the unusually warm preceding winter. If so, then P. longicarpus might serve as a coastal 'canary in the coal mine': documenting yearly shifts in the quality of shells used as shelters by coastal marine hermit crabs in New England may provide a convenient means of monitoring the impact of climate change on intertidal populations.