The rocky intertidal zone isn't exactly prime real estate. As temperatures can jump by 30oC or more within hours or metres, animals rely on all sorts of tricks to escape serious harm. Some, like barnacles, hunker down and hope for the best as the mercury rises. Limpets, a form of sea snail, do something different: they move. Spencer Virgin and David Schiel from the University of Canterbury, New Zealand, hypothesised that limpets’ mobility contributed to their success in the intertidal zone. They have recently published, in Functional Ecology, their new discovery that limpets move and reorient towards mild microhabitats, avoiding the worst that their rocky habitat has to offer.
Virgin visited the Kaikōura coast of New Zealand, a choice location for the endemic denticulate limpet (Cellana denticulate). He left some limpets as he found them and reoriented others by rotating the boulders to which they were attached by 180 deg; the limpets were now facing in the opposite direction to the prevailing sun. Over the next year, he returned every few weeks to observe where the limpets were located, including noting the temperature and slope of the rock that they clung to and what direction they faced. He also estimated how much food was available for the limpets from the amount of chlorophyll α, a key ingredient of the limpet delicacy of algal biofilms, which they scrape off rocks.
The hottest months were November to February, as expected in the southern hemisphere, but exactly how hot each boulder was depended on its orientation: flat, north- or west-facing surfaces, which received the most direct sunlight, were warmer than steep, south- or east-facing surfaces. However, the chlorophyll α levels followed the opposite trend, peaking in the winter and on south- and east-facing surfaces. Both temperature and the amount of food available influenced where the limpets lived. As the days grew hotter, the limpets gravitated towards cool, algae-rich areas, such as steep slopes facing south or east and the individuals that had been put into unfavourable positions when their boulders were flipped moved more over the year, indicating that limpets actively sought locations that minimized the risk of overheating.
The intertidal zone is also challenging for electronics, and the strong waves foiled the duo's plans to monitor limpet heart rates over the year. However, Virgin and Schiel did eek out 3 days of data from an alternative, more sheltered field site. There, they found that the sunnier north-facing limpets had the highest heart rates and shadier south-facing limpets had the lowest (east- and west-facing limpets fell in the middle), confirming that limpets’ seasonal migrations had real physiological consequences. However, the heart rate differences only emerged on extra hot and sunny days, hinting that orientation mattered most during the height of the summer.
Unsatisfied with their field heart rate data, the researchers brought limpets into the lab, stuck them under a heat lamp and monitored their heart rates. As expected for a snail, heart rate initially rose in step with temperature, as the limpets successfully accommodated higher metabolic rates caused by warming. However, the increase in heart rate started lagging behind the increase in temperature at around 35°C, an early sign of trouble, and the animals’ heart rates stopped increasing entirely around 37°C, indicating that the limpets had hit the highest temperature at which they could live. Reviewing their field observations, Virgin and Schiel realized that the wild limpets came perilously close to the highest temperature they could survive multiple times during the summer and that the refuge provided by hiding out on steep slopes out of the sun could make all the difference for survival.
Though their brains are just a few nerve cells, denticulate limpets in New Zealand know exactly what they want in a home: a southerly or easterly seaside view, on a steep slope carpeted in algae – and they'll search high and low for it, albeit at a snail's pace.
