EFFECT OF WATER TEMPERATURE ON SURVIVAL, GROWTH AND REPRODUCTIVE PERFORMANCE OF THE MEDICALLY IMPORTANT FRESH WATER SNAIL SPECIES OF LAKE MALAŴI, BULINUS NYASSANUS (SMITH, 1877)

Abstract

Studies were conducted under laboratory conditions at Bunda College, University of Malaŵi to investigate the effect of water temperature (22, 25, 28 and 31oC) on the rate of egg development, hatchability (%), time at first hatch (days), survival (%), growth and reproductive performance of Bulinus nyassanus. Snails were collected from the shores of Lake Malaŵi at Cape Maclear, Chembe Village and then taken to the laboratory where they were fed on a locally formulated feed containing 30% crude protein. Snails were kept in de-chlorinated tap water and the studies were done from April to November, 2006. To estimate the reproductive performance of B. nyassanus, the number of egg-masses, eggs and numbers of eggs per egg-mass deposited per snail at of 3-day intervals were recorded for 33 days. Hatchability and time at first hatch were determined by counting the number of eggs that hatched and number of days before the first hatchlings emerged. Changes in live body weight (g) of B. nyassanus juveniles were recorded fortnightly and used to estimate growth rate at the four study temperatures; while percentage survival was determined by recording the number of dead and live snails. Water temperature, conductivity, dissolved oxygen and pH were monitored throughout the experiment. Data were analyzed using SPSS and in case of significant differences in the main effect, means were separated using Duncan’s Multiple Range Test (DMRT). P-values <0.05 were taken as being significant. Survival of both adult and juvenile B. nyassanus snails (% ± SE) was significantly higher (P<0.05) at 25oC than at the other temperatures. The highest juvenile survival after 84 days was 78.33 ± 2.84% while that of adults was 69.33 ± 6.18% after 33 days. The lowest survival of adults (36.00 ± 6.86%) was at 22oC while that of juveniles (44.00 ± 6.96%) was at 31oC. Eggs/egg-mass deposited were significantly higher (P < 0.05) at 25oC (15.33 ± 0.79) than at 31oC (10.69 ± 1.01). The number of eggs per egg-mass deposited at 3 day-intervals at 22, 25 and 28oC did not differ significantly. Eggs deposited per snail at 3-day intervals were highest (12.7 ± 2.9) at 28oC and lowest (8.5 ± 1.03) at 22oC. This did not significantly differ amongst temperatures, however. Pseudo net reproductive rate (nrr = ∑×=×=111LxMx, where Lx is survival of each 3 day period i.e., x, and Mx is the number of eggs deposited per snail during each x) was significantly lower (58.67 ± 10.51) at 22oC than at the three higher experimental temperatures. Both egg development and juvenile growth increased steadily with temperature with the fastest growth rate at 31oC. Both temperature (oC) and exposure time (days) had a unique significant effect (R2 = 0.477; P < 0.05) on live weight of B. nyassanus juveniles; with temperature having a weaker influence than exposure time. While 25oC was the optimal temperature for the survival of both adults and juvenile B. nyassanus, temperatures ranging between 25 – 28oC were optimal for reproduction. Juvenile growth, egg development and hatchability of B. nyassanus eggs increased with increasing temperature while time at first hatch decreased with temperature. Survival and reproductive performance results agreed with the research hypothesis that their optimum temperature ranges between 25 – 28oC; while juvenile growth, egg development, hatchability and time at first hatch of B. nyassanus eggs falsified the hypothesis.