Physiological responses to heat stress on a tropical shore: the benefits of mushrooming behaviour in the limpet Cellana grata

Sub-lethal responses to heat stress were investigated in the limpet Cellana grata. During summer low tides, foot temperatures were hotter than rock temperatures, but positively correlated with, heart rate, air and rock temperatures. Hotter limpets showed mushrooming behaviour, raising their shell from the rock. Over 30 % of monitored limpets were not relocated during the subsequent daytime low tide. Missing animals were mostly situated on horizontal surfaces, were smaller than those recaptured, and had higher body temperatures, mushrooming heights and heart rates. A laboratory protocol was designed to resemble on-shore thermal stress conditions. Firstly, small and large limpets were held on a hot plate for 60 min and either constrained or allowed to mushroom. Secondly, unconstrained, large animals were held on the hot plate for 120 min. Unconstrained limpets were able to mushroom and had lower foot temperatures but higher heart rates than those constrained, suggesting mushrooming is an active response. Small animals had higher heart rates than large individuals. Mantle water could not be collected from most small, mushrooming limpets but was from constrained animals, and was more concentrated in small limpets. Small and constrained limpets had more concentrated haemolymph than large or mushrooming animals. Mantle and haemolymph osmolalities were positively related, except at high mantle water osmolalities. Smaller animals lost relatively more water, and constrained limpets more than those allowed to mushroom. Large limpets on the hot plate for 120 min showed similar mushrooming heights and heart rates but had hotter foot temperatures, higher haemolymph concentrations, lost all their mantle water and nearly twice as much water than those held for 60 min. Mushrooming behaviour appears to be a short-term, high-risk strategy that allows limpets temporary relief from stressful conditions and may increase their chance of survival until the next tidal immersion.