Owl limpets are tenacious little creatures. These territorial marine snails tend a `garden' of algal scum on intertidal rock surfaces, bulldozing off any invaders. Using their foot muscle as a suction cup, limpets hunker down near their garden and resist being prised off by crashing waves or hungry crabs. But there's a downside to being stuck tightly to your home: when the rock heats up, the limpet's large foot absorbs the heat. Because limpets have excellent thermal contact with the rock face, they are perfect model organisms for investigating the role of temperature in intertidal ecology, say Mark Denny, Luke Miller and Christopher Harley of the Hopkins Marine Station of Stanford University.
`These little guys are always in equilibrium with their thermal environment,' Denny explains. This strong link with their physical surroundings means that limpets may help us understand the biological consequences of global climate change. To create a tool to examine the effects of temperature changes, Denny and Harley set out to construct a heat-budget model that predicts limpet body temperature(p. 2409 Hot limpets...). `We translated all the environmental factors that influence rock temperature into one meaningful biological output – limpet body temperature,' Denny explains. The model's strength lies in the fact that it relies solely on easily measured physical and meteorological inputs, such as solar radiation, air temperature and the rock's thermal conductivity.
To test their model, Denny and Harley measured the temperature of artificial limpets and live limpets placed on rocks at the Hopkins Marine Station. They created artificial limpets by filling a limpet shell with wax,casting the wax `body' in silver to ensure good thermal conductivity, and placing it back in the shell. To measure the temperature of live limpets, they sandwiched hair-thin thermocouples between a limpet's foot and the rock. Denny and Harley found that their heat-budget model predicted the daily maximal body temperatures of artificial and live limpets to within a fraction of a degree,suggesting that their model can be used to explore limpet thermal biology.
To demonstrate how their model might be applied, Denny, Miller and Harley used it to investigate whether temperature sets an upper vertical limit on where limpets can live on intertidal rocks(p. 2420 Thermal stress...). They constructed a thermal history for limpets using an extensive record of ecological data taken at the marine station, including air temperature, solar radiation, wind speed, humidity and wave and tide height measured every ten minutes over a five-year period. Calculating limpet body temperatures at eight shore elevations for nine different rock surface orientations in both exposed and protected conditions, they explored the temperature history of 144 `sites'. Denny points out that `it would be hard to collect field measurements at such a large number of sites'. The maximum body temperature that the model predicted for any site was 37.5°C, which kills a substantial number of limpets, but not all of them. This suggests that thermal stress does not set an absolute upper limit on limpets' homes;instead, behaviour or ecological interactions might.
Denny and Harley foresee many practical applications of their model for physiologists. `We can now give a physiologist a five-year thermal history of limpets so they can investigate what this means in terms of thermal stress for these animals,' Denny says. Ecologists can use the model to examine the effects of global warming – it can tell us what a hike in air temperature will do to limpets' body temperatures. `Our model opens up a host of questions for physiologists to explore,' Denny enthuses.