[On the emigration of gastropodes from the sea: Studies on Alderia modesta and Ovatella myosotis]

Abstract

Up to now little attention has been paid to the phylogenetic emigration of gastropods from the sea: almost nothing is known about the physiological changes which enable snails to live in terrestrial habitats. To help solve this question, experiments were carried out on Alderia modesta, an opisthobranch slug of the lower supralittoral, and Ovatella myositis, a primitive pulmonate snail of the upper supralittoral. In each case two subspecies were studied.North Sea Alderias differ from their Baltic counterparts in size and salinitytolerance. The difference in size between Ovatella of the Mediterranean and of the Baltic is very slight. There is a marked difference, however, in the salinity-tolerance. Alderia modesta survives in a comparatively narrow range of different salinities. The optimum is between 10‰ and 20‰ for Baltic specimens and between 15‰ and 35‰ for those of the North Sea. The results are the same in and out of water. Ovatella can live on a freshwater substratum as well as on 55‰. Specimens of the Mediterranean can even become adapted to 90‰. Flooded with water of differrent salinities the tolerated range becomes markedly smaller. Eggs are produced between 5‰ and 50‰ (Baltic specimens) or 5‰ and 65‰ (Mediterranean specimens). All eggs develop without being damaged between 5‰ and 40‰ and 10‰ and 45‰ respectively. The optimal salinity-concentration for growth and egg-production is 10‰. Salinity-concentrations below and above these marks disturb the development. In one spawn-mass some eggs cleave normally, others become deformed, and again others do not cleave at all. This heterogeneous reaction points to the existence of something like a physiological polymorphism in regard to salinity-tolerance. The salinity-concentration of the blood of Ovatella was measured after a long-term acclimatisation. Ovatella is poikilosmotic and slightly hypertonic (3-4‰) throughout almost the whole range of salinities it tolerates. On extremely low concentrations it becomes more hypertonie; on extremely high concentrations it becomes nearly isotonic.