Faecal egestion and ammonia excretion in the Antarctic limpet Nacella continna (Strebel, 1908)
Faecal egestion and ammonia excretion were measured in the Antarctic limpet Nacella concinna (Strebel, 1908) at Signy Island during the austral summer of 1986/87. Limpets were sampled from nearby shallow waters and faecal egestion measured over the following 4 days. The mean faecal egestion rate (dry mass) for a limpet of 250 mg dry mass was 0.91 mg · h−1, comparable with rates measured in grazing molluscs from warmer waters. The mass exponent (the slope of the regression of log rate on log mass) was 0.94 for faecal egestion measured as dry mass, and 0.85 for faecal organic content; faecal ash content increased with limpet size. Assuming absorption efficiencies similar to those reported for other grazing gastropods these data indicate typical energy intakes in free-living limpets of 2–3% body mass · day−1 during summer. The exponent relating energy intake (absorption) to limpet dry mass was 0.85. Ammonia excretion was measured on freshly sampled limpets which had been allowed to recover from handling stress for ≈ 12 h. The mean ammonia excretion rate for a limpet of 250 mg dry mass was 162 ng-at N · h−1; the mass exponent was 0.82. Comparison with published data for oxygen consumption suggests an O:N atomic ratio of 30–50. The ammonia production rate of the shallow water population of N. concinna at Signy in summer was estimated to be 210 μmol · m−2 · day−1. Laboratory experiments in UK indicated that urea excretion was important in feeding limpets whereas in starved limpets excretion was almost exclusively ammonia.