Comparisons of the biology of the intertidal Subantarctic limpets Nacella concinna and Kerguelenella lateralis
Two limpet species occur intertidally on subantarctic South Georgia, the patellid Nacella concinna and the siphonarlid Kerguelenella lateralis. N. concinna is confined to the lower shore close to LWS; K. lateralis occurs in middle shore pools, so their distributions do not overlap. N. concinna has a much narrower thermal niche (−12.9°C to +15.6°C) than K. lateralis (−17.8°C to +31.8°C). Environmental data are presented to show that the upper lethal temperature of N. concinna is low enough to prevent the limpet living higher on the shore. Both limpet species are slow-moving, but K. lateralis shows increasing speed with rising temperature, peaking at 15–20°C. In contrast, N. concinna moves actively down to −1.9°C (when sea water freezes), but there is a steady decrease in speed of locomotion above +2°C. Locomotion ceases at 14°C in N. concinna (c.f. 30°C in K. lateralis). Both species exhibit very low tenacities, but in N. concinna tenacity decreases with increasing shell length. In K. lateralis there is no effect of temperature on tenacity. Both species show a positive allometric relationship between foot area and shell length. N. concinna feeds upon microbial films and microepiflora, but K. lateralis eats colonial diatoms and Enteromorpha bulbosa. Observations on shell middens of the kelp gull Larus dominicanus showed that the gulls did not eat K. lateralis, though they ate great quantities of the less accessible N. concinna. Gulls ate N. concinna as small as 11 mm shell length (within the size range of K. lateralis). Experiments on gulls demonstrated an unwillingness to eat K. lateralis, probably because the siphonariid extrudes a viscid white mucus when the foot is touched.