Some adaptational differences in the limpets Patella caerulea and Patella lusitanica in relation to zonation

Abstract

The present work incorporates an experimental study of the resistance shown by the limpets Pat ell a caerulea (L.) and Pat ell a lusitanica (Gmelin) to the environmental stresses which the limpets may experience on the shore. Adaptional differences between the two limpets have been sought in terms of their resistance to these stresses and their respiratory abilities in air and in water. The work has been prompted by the paucity of observations relevant to the zonation of Pat ell a species and Mediterranean limpets in particular. The various limpets are considered briefly in Chapter 1 and the aims of the investigation are outlined. A brief review of the literature on zonation with special reference to limpets and an account of the factors influencing zonation is given in Chapter 2. Chapters 3 to 7 describe experimental observations. These were made on short-term acclimated limpets (except those in Chapter 7). Resistance to high temperatures in air and in water (Chapter 3) and resistance to high and low salinity media (Chapter 5) were investigated. Desiccation effects were studied at a single temperature and relative humidity (Chapter 4). In all the experiments carried out the time to 503 mortality was determined. In general, Patella caerulea and Patella lusitanica were found to show a pattern of resistance to the experimental stresses which may be correlated with their position on the shore. Patella lusitanica was found to be more resistant to high air and water temperatures and to desiccation conditions than Patella caerulea. The instantaneous lethal temperature was found to be 43.3°C. for Patella lusitanica and 41.2°C. for Patella caerulea in water, and 54.7°C. for Patella lusitanica and 50.0°C. for Patella caerulea in air. The time to 503 mortality under desiccation conditions was 36 hours in Patella caerulea and 64 hours in Patella lusitanica irrespective of whether the shell could be utilised or not in resisting desiccation. Both limpets, however, lost slightly more water when the shell could not be utilised. The water loss at 503 mortality was about 30 to 363 in Patella caerulea and 37 to 433 in Patella lusitanica, the lower values pertaining to limpets which could not utilise their shell. In association with its higher resistance to desiccation Patella lusitanica was found co-have a higher percentage of body water than Patella caerulea (Chapter 7). The two limpets were found to have essentially the same resistance to high salinities but Pat ell a caerulea appeared to be more tolerant to low salinities than Patella lusitanica, with ability to survive indefinitely at 803 salinity, whereas Patella lusitanica was observed to show 503 mortality in 9 days at this salinity. These findings are discussed in relation to the particular habitat of each limpet (Chapter 5). The oxygen consumption of the two limpets at temperatures between 10° and 30°C. was investigated in individuals previously acclimated to a temperature of 18°C. The results were interpreted by means of logarithmic rate versus temperature curves. The rate of oxygen consumption of both limpets was observed to increase with temperature in air and in water except that in Pat ell a caerulea a decrease was observed above 25°C. in water. At temperatures between 10° and 25°C. the rate of oxygen consumption of Patella caerulea in water was on the average twice as high as that in air, whilst between 10° and 30°C. the rate of oxygen consumption of Patella lusitanica in air was on the average three times as high as that in water. The higher rate of oxygen consumption in water than in air in Patella caerulea and the higher rate in air than in water in Patella lusitanica have been discussed in relation to the regime of exposure and submergence which the limpets experience at their habitat, and in terms of compensation for the effect of temperature on metabolism (Chapter 6). The relation of shell form to habitat in Patella caerulea and Patella lusitanica was also investigated. Patella caerulea shells were found to be significantly longer, broader and lower in height than Patella lusitanica shells. The regression of the mean of length and breadth on shell height was found to characterise two populations with a small degree of overlap as observed in the distribution of the two limpets on the shore (Chapter 7). The zonation of Patella caerulea and Patella lusitanica in the Mediterranean is finally defined in Chapter 8, and the adaptational differences observed in the two limpets are shown to correlate with the position of Patella lusitanica on relatively vertical surfaces in the upper part of the narrow eulittoral zone and that of Patella caerulea on relatively horizontal surfaces in the lower part of the same zone.