The influence of temperature on the life-cycle and distribution of Biomphalaria pfeifferi (Krauss, 1948) in South-Eastern Africa

Abstract

An annual life-cycle of three over-lapping generations was found for Biomphalaria pfeifferi in a perennial stream on the eastern Transvaal escarpment. The 2nd of these generations appeared during the hottest months of the year—late January and February. This generation sequence was confirmed in a completely different environment, a permanent pond in a climatically different area, the coastal peneplain of north-eastern Zululand though the density of the 2nd generation here was much reduced. The species is moreover absent from certain types of shallow, permanent waterbody (pans) on this plain. In these habitats the thermal regime rose above those in other types during spring and early summer—the maturation period of the 1st generation. A negative partial-correlation, significant at a 1% level, was found between decreasing fecundity of the 1st generation and increasing periods of above-optimal temperatures (> 27°C) during its maturation period. This temperature effect accounts for the species' absence from pans in the area and the critical duration of these above-optimal temperatures appears to lie between mean weekly levels of 120 and 179 deg.h. > 27°C. The discontinuous distribution of B. pfeifferi in the Lake Sibaya area of the Zululand coastal plain corresponds to an ecological succession amongst waterbodies associated with the lake's fluctuating water level. The separation of sheltered bays from the lake was followed by further lowering of their levels and a spread of emergent vegetation. This is believed to have contributed to an increasingly high thermal regime, especially during spring when temperatures rise rapidly. The pans from which B. pfeifferi is absent constitute the present end point of this succession.