COMPARISONS OF THE BIOLOGY OF THE INTERTIDAL SUB-ANTARCTIC LIMPETS NACELLA-CONCINNA AND KERGUELENELLA-LATERALIS

Two limpet species occur intertidally on subantarctic South Georgia, t he patellid Nacella concinna and the siphonariid Kerguelenella lateral is. N. concinna is confined to the lower shore close to LWS; K. latera lis occurs in middle shore pools, so their distributions do not overla p. N. concinna has a much narrower thermal niche (-12.9 degrees C to 15.6 degrees C) than K. lateralis (-17.8 degrees C to +31.8 degrees C) . Environmental data are presented to show that the upper lethal tempe rature of N. concinna is low enough to prevent the limpet living highe r on the shore. Both limpet species are slow-moving, but K. lateralis shows increasing speed with rising temperature, peaking at 15-20 degre es C. In contrast, N. concinna moves actively down to -1.9 degrees C ( when sea water freezes), but there is a steady decrease in speed of lo comotion above +2 degrees C. Locomotion ceases at 14 degrees C in N. c oncinna (c.f. 30 degrees 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 te nacity. Both species show a positive allometric relationship between f oot 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 dominic anus showed that the gulls did not eat K. lateralis, though they ate g reat quantities of the less accessible N. concinna. Gulls ate N. conci nna as small as 11 mm shell length (within the size range of K. latera lis). Experiments on gulls demonstrated an unwillingness to eat K. lat eralis, probably because the siphonariid extrudes a viscid white mucus when the foot is touched.