6.3 Results
6.3.1 Growth
At 25oC (Figure 6.2), R. rubiginosa attained a maximum shell length of 17.40 mm, while L. natalensis from both the UKZN and Greyville Ponds had maximum shell lengths of 13.95 mm and 13.35 mm respectively. The maximum shell length recorded by P. acuta at this temperature was 10.20 mm.
Figure 6.2 also shows that P. acuta had the greatest longevity of 28 weeks followed by R.
rubiginosa (25 weeks) and finally L. natalensis (18 weeks for the UKZN Pond population and 16 weeks for the Greyville Pond population).
Figure 6.2: Growth expressed as an increase in the mean shell length for R. rubiginosa, L. natalensis (UKZN and Greyville Ponds) and P. acuta at 25oC (n = 90).
At 30oC, the pattern for shell growth and longevity was similar to that observed at 20oC and 25oC. Figure 6.3 shows that the maximum shell length attained by R. rubiginosa, L.
natalensis (UKZN Pond), L. natalensis (Greyville Pond) and P. acuta were 16.10 mm, 11.25 mm, 11.65 mm and 9.20 mm respectively.
0 2 4 6 8 10 12 14 16 18 20
0 5 10 15 20 25 30 35 40
Age (weeks)
Shell Length (mm)
Radix rubiginosa Lymnaea natalensis (UKZN Pond) Lymnaea natalensis (Greyville Pond) Physa acuta
At 30oC (Figure 6.3) P. acuta attained the greatest longevity (22 weeks) followed by R.
rubiginosa (21 weeks). The longevity of L. natalensis at 30oC was 12 weeks (UKZN Pond) and 14 weeks (Greyville Pond).
Figure 6.3: Growth expressed as an increase in the mean shell length for R. rubiginosa, L. natalensis (UKZN and Greyville Ponds) and P. acuta at 30oC (n = 90).
Overall, Figures 6.1 - 6.3 show that as temperature increased the maximum mean shell length decreased for all four populations investigated. Table 6.1 presents the von Bertalanffy growth parameters that were estimated using the Ford-Walford method to describe the growth patterns for the four snail populations. The growth coefficient (K) is a measure of the growth rate of a population. The higher the value of K, the more rapid the growth (expressed as an increase in the mean shell length). Figures 6.4 - 6.6 show the von Bertalanffy growth curves on pages 148 - 150.
From Table 6.1 it is apparent that for all populations the highest and lowest K values recorded were at 20oC and 30oC respectively, i.e. the growth was fastest at 20oC but less so at 30oC. At 20oC, R. rubiginosa had the highest K value (0.1316), followed by P.
0 2 4 6 8 10 12 14 16 18
0 5 10 15 20 25 30 35 40
Age (weeks)
Shell Length (mm)
Radix rubiginosa Lymnaea natalensis (UKZN Pond) Lymnaea natalensis (Greyville Pond) Physa acuta
acuta (0.1236), L. natalensis from the UKZN Pond (0.1128) and finally L. natalensis from the Greyville Pond (0.1017). At 25oC and 30oC, P. acuta had the highest K value, followed by R. rubiginosa, the UKZN Pond L. natalensis population and finally the L.
natalensis population from the Greyville Pond.
Importantly, R. rubiginosa exhibited a growth coefficient (K) at 20oC that was 1.06 times larger than that of P. acuta, while those exhibited by P. acuta at 25oC and 30oC were 1.05 and 1.04 times greater than that of R. rubiginosa (Table 6.1). The growth coefficients for both R. rubiginosa and P. acuta were 1.10 - 1.29, 1.19 - 1.35 and 1.11 - 1.36 times larger than those of the two L. natalensis populations at 20oC, 25oC and 30oC respectively.
Table 6.1: Estimated growth parameters of the four snail populations maintained at the three temperatures.
These parameters were calculated using the Ford-Walford method.
Temperature Snail Populations K L∞ t0
20oC R. rubiginosa 0.1316 19.00 -3.5289
L. natalensis (UKZN Pond) 0.1128 18.00 -1.8652
L. natalensis (Greyville Pond) 0.1017 18.50 -1.2566
P. acuta 0.1236 12.00 -1.9005
25oC R. rubiginosa 0.1234 18.75 -2.3598
L. natalensis (UKZN Pond) 0.1034 16.50 -1.1934
L. natalensis (Greyville Pond) 0.0960 17.00 -0.8667
P. acuta 0.1299 10.50 -2.5512
30oC R. rubiginosa 0.1014 18.50 -1.7535
L. natalensis (UKZN Pond) 0.0910 16.25 -0.1989
L. natalensis (Greyville Pond) 0.0779 17.50 -0.4134
P. acuta 0.1059 10.25 -1.7686
K - the growth coefficient; L∞ - the asymptotic length; t0 - the time at which shell length is theoretically zero.
The asymptotic length (L∞) was largest at 20oC and the smallest at 30oC, with the exception of L. natalensis (Greyville Pond) that displayed its smallest L∞ at 25oC (Table 6.1). For each of the three temperatures, R. rubiginosa had the largest L∞, followed by the UKZN Pond and Greyville Pond populations of L. natalensis and finally P. acuta.
The L∞ (Table 6.1) was always larger than the observed maximum shell length (Figures 6.1 - 6.3).
The time at which shell length is theoretically zero (t0) displayed a similar pattern to the growth coefficient (K). From Table 6.1, it is seen that the highest and lowest t0 values were at 20oC and 30oC respectively. For each of the three temperatures R. rubiginosa and P. acuta exhibited the highest t0 values, while the lowest t0 values were recorded for both the L. natalensis populations (Table 6.1).
The estimated growth parameters and the observed age for the four populations were fitted into the von Bertalanffy growth equation, resulting in the growth curves shown in Figures 6.4 - 6.6.
Figure 6.4: The von Bertalanffy growth curve for R. rubiginosa, L. natalensis (UKZN and Greyville
0 2 4 6 8 10 12 14 16 18 20
0 5 10 15 20 25 30 35 40
Age (weeks)
Shell Length (mm)
Radix rubiginosa Lymnaea natalensis (UKZN Pond) Lymnaea natalensis (Greyville Pond) Physa acuta
Using the von Bertalanffy growth curve for 20oC (Figure 6.4), it was predicted that R.
rubiginosa would reach the largest shell length of 18.74 mm followed by both the UKZN Pond (16.78 mm) and Greyville Pond populations (16.14 mm) of L. natalensis and finally P. acuta (11.87 mm). From Figure 6.1, the observed maximum shell lengths attained by R. rubiginosa, L. natalensis (UKZN Pond), L. natalensis (Greyville Pond) and P. acuta were slightly lower at 18.20 mm, 16.25 mm, 15.65 mm and 11.80 mm respectively.
At 25oC the maximum predicted shell lengths (Figure 6.5), for R. rubiginosa, the UKZN Pond and Greyville Pond populations of L. natalensis and P. acuta were 18.11 mm, 14.23 mm, 13.63 mm and 10.30 mm respectively. Figure 6.2 showed the observed maximum shell length attained by R. rubiginosa, L. natalensis (UKZN Pond), L. natalensis
(Greyville Pond) and P. acuta at 25oC were 17.40 mm, 13.95 mm, 13.35 mm and 10.20 mm respectively.
Figure 6.5: The von Bertalanffy growth curve for R. rubiginosa, L. natalensis (UKZN and Greyville Ponds) and P. acuta at 25oC.
0 2 4 6 8 10 12 14 16 18 20
0 5 10 15 20 25 30 35 40
Age (weeks)
Shell Length (mm)
Radix rubiginosa Lymnaea natalensis (UKZN Pond) Lymnaea natalensis (Greyville Pond) Physa acuta
At 30oC, the maximum predicted shell length using the von Bertalanffy growth curve for R. rubiginosa, the UKZN Pond and Greyville Pond populations of L. natalensis and P.
acuta were 16.66 mm, 10.90 mm, 11.81 mm and 9.42 mm respectively. From Figure 6.3, the observed maximum shell lengths attained at this temperature by R. rubiginosa, L.
natalensis (UKZN Pond), L. natalensis (Greyville Pond) and P. acuta were 16.10 mm, 11.25 mm, 11.65 mm and 9.20 mm respectively. For all three temperatures the
maximum predicted shell lengths using the von Bertalanffy growth equation for all four populations (Figures 6.4 - 6.6) were larger than the observed maximum shell lengths (Figures 6.1 - 6.3).
Figure 6.6: The von Bertalanffy growth curve for R. rubiginosa, L. natalensis (UKZN and Greyville Ponds) and P. acuta at 30oC.
0 2 4 6 8 10 12 14 16 18
0 5 10 15 20 25 30 35 40
Age (weeks)
Shell Length (mm)
Radix rubiginosa Lymnaea natalensis (UKZN Pond) Lymnaea natalensis (Greyville Pond) Physa acuta