No abstract

Desiccation tolerance levels have been estimated for Nucella in relation to specific age‐groups which occupy distinct tidal levels. The values obtained have been shown to follow the natural age progression, and hence the natural zonation levels. The likelihood of critical or lethal fluid‐loss values being reached during six‐hour periods of exposure in absence of both shelter and consideration of natural zonation has been investigated. It is concluded that Nucella attempts to prevent loss of excessive volumes of fluid during emersion by occupying specific tidal levels in accordance with natural fluid‐loss tolerances. The species is further assisted by a natural inclination to seek sheltered positions during rough weather, and the presence of a comparatively large reservoir of fluid held extra‐viscerally in the mantle cavity. Summary The age‐analysis system used throughout the experimental section is explained briefly and natural zonation levels of differently aged dog‐whelks are described for Blackrock individuals. Critical and lethal fluid‐loss values are estimated for each age‐group; the results are seen to follow the natural age progression. Fluid losses occurring after six hour exposure periods in wind and calm conditions in absence of shelter at 9°, 15° and 25°C are illustrated for all age‐groups. The results showed how group 4 and 5 individuals were likely to reach at least critical levels if subjected to wind at 25°C for six hours. However, neither of these age‐groups occupy levels of the shore where this would be likely to occur; their natural zonation levels only receiving three to four hours of exposure under normal conditions. Extra‐visceral fluid volumes are calculated for each age‐group. The values obtained also follow the natural age progression. It is thought likely that extra‐visceral fluid plays an important part in keeping Nucella moist during emersion.