The Lethal Temperatures of Some Common British Littoral Molluscs

Abstract

The zonation of the more common British littoral molluscs is known from the work of various shore ecologists, but the causal factors which determine the vertical limits of distribution of these species between tide-marks are largely a matter of conjecture. Among what might be considered as the local climatic factors acting on intertidal organisms exposed to the air it appears that temperature, desiccation and possibly light effects are the most critical, i.e. the 'heat-lightdesiccation 'complex. In the case of organisms which inhabit permanent rock pools, the desiccation factor is eliminated, and it is likely that changes in the temperature and salinity of these pools during periods of exposure will be important. Intertidal algae have been the chief subject of investigations along these lines, and relatively few studies have been made on littoral fauna. The effect of heat, desiccation and salinity on a series of six gastropods has been studied experimentally by Broekhuysen (1940) in False Bay, South Africa, while Gowenloch & Hayes (1926) have investigated the action of certain factors of the physical environment on three species of Littorina in Canada. Little work of a similar nature seems to have been done on the British littoral molluscs, even though experimental work of this type could be used with advantage to supplement the data obtained in the field by shore ecologists. From what is already known it appears probable that the effects of temperature acting as an isolated factor (i.e. 'pure temperature effects') are of subsidiary importance to the drying action of supernormal temperatures in controlling the upper limits of distribution of littoral plants and animals. Thus in the case of the South African molluscs it was found that between the temperatures necessary to kill the snails in the laboratory and the temperatures to which they are normally subjected in their natural environment, there exists a sufficiently wide difference to rule out the possibility of heat death among these species on the shore. This principle will be examined with respect to certain British molluscs. Huntsman & Sparkes (1924) demonstrated that for marine organisms in general the degree of resistance to heat is correlated with the temperature ranges at which such species exist in nature. Gowenloch & Hayes (1926) reported that individuals of the same species (Littorina littorea, L. rudis or L. palliata) collected from different tidal levels on the shore exhibit a gradation in thermal death temperatures, i.e. a 'tidal level differentiation' exists in lethal temperatures. Henderson (1929) has shown that, among the lamellibranchs, deep-water forms are less tolerant of heat than those occurring in the lower part of the intertidal zone, that these in turn are less tolerant than upper-shore species, while lamellibranchs characteristically found in warmer waters are the most tolerant of all. Finally, Broekhuysen (1940) found that the series of thermal death points for his six gastropods is identical with the order in which these species are zoned on the shore of False Bay, L. knysnaensis, occurring to well above extreme high water of spring tides, has a lethal temperature of 48.60 C.; Oxystele sinensis, which barely reaches mean sea level, dies at 39.60; while the other four are zoned between these two species and have thermal death points intermediate between these temperatures. He also showed that, in their ability to withstand prolonged high temperatures, the six snails fall into a similar series. Furthermore Broekhuysen tabulates the lethal temperatures of a series of marine molluscs as determined by Vernon (I899), Gowenloch (1926), Gowenloch & Hayes (1926) and Henderson (1929), and concludes that 'generally speaking, the lowest lethal temperatures are found in snails inhabiting the lowest levels, while lethal temperatures increase with increasing height above low water level '. Thus, from all the evidence, it would appear that there exists a fairly definite correlation between the ability of various snails to withstand pure temperature effects and the temperatures to which they are likely to be exposed in nature. In the present paper, the results of experiments on eleven molluscs common on the shores of Cardigan Bay are described. The following species were chosen: Littorina neritoides (L.), L. rudis (Maton), L. littoralis (L.), L. littorea (L.), Gibbula cineraria (L.), G. umbilicalis (da Costa), Patella vulgata L., P. depressa Pennant, P. athletica Bean, Nucella lapillus (L.), Osilinus lineatus (da Costa). The distribution and zonation of these in Cardigan