INFLUENCE OF TEMPERATURE ON SURVIVAL AND GROWTH OF TWO FRESHWATER PLANORBID SPECIES, PLANORBARIUS CORNEUS (L.) AND PLANORBIS PLANORBIS (L.)

The survivorships and growth rates ofPlanorbarius corneus andPlanorbis planorbis were measured during the whole life spanof the snails at constant temperatures of 5, 10, 15, 20 and25°C. Life expectancy tables were constructed. The maximumlongevities for P. corneus (231 weeks) and P. planorbis (175weeks) occurred at 15°C and 10°C respectively. The extremetemperatures were particularly unfavourable. Snail growth which was expressed logarithmically was most sensitiveto temperature during the exponential stage. However, if weconsider a given species duringjts entire life span, there wereno significant differences between temperature groups whichwere in order of decreasing suitability: —P. corneus: 25, 20 and 15°C; 10°C; 5°C; —P.planorbis: 20 and 15°C; 25 and 10°C; 5°C. Using the growth constants from the Von Berta-lanffy's modeladjusted to give a gaussian curve, it was calculated that theoptimum temperatures for the growth of P. planorbis and P. corneuswere 19 and 20.5°C respectively. A mathematical model basedon spline functions makes it possible to predict the growthof snails over a wide range of temperatures. (Received 2 June 1993; accepted 18 November 1993)