Thermoregulatory Behavior in the Tropical Periwinkle Planaxis sulcatus

The tropical periwinkle Planaxis sulcatus (Born, 1778) forms randomly distributed, pyramid-shaped clusters on an intertidal mudflat in eastern Bahrain during the summer. Periwinkle pyramids are observed only on the lower section of the mudflat, where the maximum temperature of the surficial mud is less than about 40 degrees C, while the upper portion of the mudflat, which experiences temperatures > 52 degrees C, is barren of gastropods. The periwinkle perched at the apex of the pyramid experiences a temperature that as much as 4 degrees lower than that of the surficial mud. Because we did not observe pyramid-building behavior among Gulf periwinkles during winter, we conclude that pyramid-building behavior in P. sulcatus is a thermoregulatory adaptation.