Depth gradients in shell morphology correlate with thermal limits for activity and ice disturbance in Antarctic limpets

To fully understand how species distributions will respond to changing environments it is essential to understand the mechanisms underlying variation in animal performance and the relative importance of different ecological and environmental factors. A performance measure that has previously been used as an indicator of thermal capacity of the Antarctic limpet (Nacella concinna) to cope with regional warming is the ability to right if removed from the substratum and turned upside down. As part of an on-going study into limpet genetics and phenotypic plasticity, we tested the temperature limits for 50% righting of limpets from 6 and 30 m depth. The 50% threshold for limpets collected from 6 m (4.7 °C) was higher than for those collected from 30 m (0.7 °C). This compares with a previously published limit of 2.2 °C for limpets collected from 12-15 m at the same location. These thermal limits positively correlated with a depth gradient in shell height to length ratio; thickness and strength. Flatter limpets, had a reduced thermal limit for righting than taller limpets which we hypothesise is related to increased energy requirements of flat limpets, which have to turn through a greater angle to right than tall limpets. Of the factors that cause morphological plasticity of gastropod shells, iceberg disturbance is the most likely cause of the sub-tidal gradient in N. concinna shell shape, and therefore the thermal limit for righting of limpets from 6 to 30 m depth, rather than environmental temperature.