Limpet feeding rate and the consistency of physiological response to temperature.

Thermal reaction norms are fundamental relationships for geographic comparisons of organism response to temperature. They are shaped by an organism's environmental history and provide insights into both the global patterns of thermal sensitivity and the physiological mechanisms underlying temperature response. In this study we conducted the first measure of the thermal reaction norm for feeding, comparing the radula rasping rate of two tropical and one polar limpet species. The consistency of thermal response was tested through comparisons with limpet duration tenacity. Feeding and duration tenacity of limpets are ecologically important muscular mechanisms that rely on very different aspects of muscle physiology, repeated concentric (shortening) and isometric (fixed length) contraction of muscles, respectively. In these limpets the thermal reaction norms of feeding limpets were best described by a single break point at a maximum temperature with linear declines at higher (Siphonaria atra) or lower temperatures (Nacella concinna and Cellana radiata) rather than a bell-shaped curve. The thermal reaction norms for duration tenacity were similar in the two tropical limpets. However, the rasping rate in Antarctic N. concinna increased linearly with temperature up to a maximum at 12.3 °C (maximal range 8.5-12.3 °C) when feeding stopped. In contrast, duration tenacity in N. concinna was maximal at 1.0 °C (-0.6 to 3.8 °C) and linearly decreased with increasing temperature. The thermal reaction norms of muscular activity were, therefore, inconsistent within and between species, indicating that different mechanisms likely underlie different aspects of species sensitivities to temperature.