Behavioural thermoregulation and food availability drive fine-scale seasonal habitat partitioning in limpets

1. Small-scale spatial variation in temperature plays a key role in limiting the distribution of organisms in thermally heterogeneous environments. In the rocky intertidal zone, intra-day temperature variation at small scales (cm-m) can easily exceed 30 degrees C. 2. To experimentally test the impact of this small-scale temperature heterogeneity on the distribution of an ecologically important limpet species (Cellana denticulata), boulders on an intertidal rocky reef in New Zealand were rotated and small-scale temperature variability and food availability were measured throughout 1 year. Small-scale variability in thermal tolerance, heart rate and heat shock protein expression was also measured to relate in situ limpet distributions with environmental conditions. Temperature variability was measured using limpet bio-mimics and HOBO pendant temperature loggers, while food availability (chlorophyll a concentration) was measured on in situ concrete fibreboard tiles. 3. To measure limpet distributions, every tagged limpet on each of 18 boulders was followed from June 2021 to 2022. During each sampling, the location of each limpet, compass direction and slope of the surface that each limpet inhabited was measured. To test for physiological differences among limpets from each microhabitat, thermal tolerance and heat shock protein expression were measured in collected limpets; in situ heart rates and body temperatures were also measured sporadically on hot days. 4. Maximum predicted body temperatures (36-39 degrees C), heart rates and actual body temperatures were greatest on equatorial surfaces (i.e. surfaces facing the equator) whereas food availability was greatest on poleward-facing surfaces. During summer, limpets moved towards surfaces that minimised their body temperatures and maximised food availability, namely vertical and poleward-facing surfaces and undersides of boulders. 5. Thermal tolerance, measured using Arrhenius breakpoint temperatures (mean +/- SE: 34.7-35.8 +/- 0.4-0.7 degrees C) and flat line temperatures (36.2-37.4 +/- 0.3-0.4 degrees C), was similar among limpets facing different directions. Surprisingly, heat shock protein expression was relatively consistent throughout the year and among directions. 6. Overall, limpet populations are resilient to thermal stress because they effectively use behavioural thermoregulation to exploit cooler microhabitats that also have greater food availability during summer.