Keeping warm in the cold: On the thermal benefits of aggregation behaviour in an intertidal ectotherm

Temperature is a primary determinant for species geographic ranges. In the context of global warming, most attention focuses upon the potential effects of heat stress on the future distribution of ectothermic species. Much less attention has, however, been given to cold thermal stress although it also sets species thermal window limits, hence distribution ranges. This study was conducted in winter on a South-Australian rocky shore in order to investigate the potential thermal benefits of the aggregation behavior observed in the dominant gastropod Nerita atramentosa. Thermal imaging was used to measure the body temperatures of 3681 aggregated individuals and 226 solitary individuals, and surrounding substratum temperature. N. atramentosa aggregates and solitary individuals were significantly warmer than their surrounding substratum. The temperature deviation between aggregates and substratum was, however, ca. 2 °C warmer than the one observed between solitary individuals and substratum. This result is critical since a body temperature increase of only a few degrees might enhance individual performance, hence organismal fitness, and could potentially drive changes in interspecific relationships. Besides, the potential higher thermal inertia of aggregates might increase the snail adaptive ability to abrupt environmental changes. We further investigate the potential thermal heterogeneity within an aggregate in order to identify any thermally advantageous position. Patch centers are significantly warmer than their edges, hence snails experience greater thermal advantages in the aggregate center. Finally, we examined the potential effect of aggregate size on snail temperature and thermal spatial heterogeneity. We identified an aggregate size threshold (216 individuals) beyond which all snails had equal thermal benefits, regardless of their spatial positions within an aggregate. While the determinism of this aggregate size threshold requires further investigations, the present work uniquely identified the thermal benefits of aggregation behavior for intertidal ectotherms under cold weather conditions. The implications of the present finding are discussed in the general framework of the ability of ectothermic populations to face environmental changes.