5.4 Discussion
5.4.3 Embryological Development
structures and organs at these stages. Following this period, the rate decreased as the young snail neared the maximum prenatal size.
In summary, the temperatures at which snails lay eggs and the relationship between egg development and temperature have important implications for the distribution and
potential spread of an introduced species (Harris and Charleston, 1977). The ascendancy in abundance and distribution of introduced species generally results in a decline of indigenous species (Zukowski and Walker, 2008).
Unsurprisingly, P. acuta proved to have a higher fecundity, a shorter incubation period and wider temperature tolerances than the other species tested. These are characteristics that enhance its success as an invasive. On the basis of this argument, its high
adaptability to changing environmental factors such as temperature, is in agreement with the fact that P. acuta is more widespread in South Africa than L. natalensis (Hamilton- Attwell et al., 1970; De Kock et al., 1989; Brackenbury and Appleton, 1993; Appleton and Brackenbury, 1998; Appleton, 2003; De Kock and Wolmarans, 2007).
This has important implications for R. rubiginosa, since this snail displayed similar characteristics to P. acuta, and exhibited greater adaptability and survival over a wider temperature range than L. natalensis. Several authors (Dondero and Lim, 1976; Mienis 1986) have also commented that it is easy to breed R. rubiginosa in aquaria and this was found to be the case in the present study as well. The indigenous L. natalensis is not as easy to breed and this raises the question, “If R. rubiginosa spreads in South Africa, will it become invasive, probably at the expense of L. natalensis?”
Radix rubiginosa appears to have a number of reproductive advantages over L.
natalensis. It produces larger egg capsules with more eggs per capsule. The eggs of R.
rubiginosa are larger, allowing the developing embryo more space and nourishment, increasing its adaptability and tolerance of environmental conditions after emerging. The hatching success of R. rubiginosa are also high and only marginally lower than the already invasive P. acuta. Also, R. rubiginosa has a much higher hatching success than
L. natalensis at all temperatures tested. The lower frequency of egg abnormalities in both R. rubiginosa and P. acuta further implies that a greater proportion of the eggs produced by these introduced species are likely to hatch.
In addition a shorter incubation period, larger size at hatching, faster rates of growth and development as well as the potential for rapid development over an extended range of temperatures are all advantageous to R. rubiginosa. Therefore the competitive superiority of R. rubiginosa over the indigenous L. natalensis with respect to their reproductive potential presents a situation that allows for rapid spread of the former and possibly the extirpation of the latter.
synchronous development.
Embryonic Stage 20oC 25oC 30oC
Cumulative Incubation
Period
Mean Size (mm)
GGR (mm/day)
Cumulative Incubation
Period
Mean Size (mm)
GGR (mm/day)
Cumulative Incubation
Period
Mean Size (mm)
GGR (mm/day)
First Cleavage 4 – 6 hours 0.122 (±0.016) 3 – 4 hours 0.122 (±0.016) 2 – 3 hours 0.122 (±0.016)
Second Cleavage 6 – 7 hours 0.122 (±0.016) 4 – 5 hours 0.122 (±0.016) 3 – 4 hours 0.122 (±0.016)
Third Cleavage 8 -10 hours 0.122 (±0.016) 5 – 6 hours 0.122 (±0.016) 4 – 5 hours 0.122 (±0.016)
Fourth Cleavage 10 – 11 hours 0.122 (±0.016) 7 – 8 hours 0.122 (±0.016) 6 – 7 hours 0.122 (±0.016) Fifth Cleavage 12 – 14 hours 0.122 (±0.016) 9 – 12 hours 0.122 (±0.016) 7 – 8 hours 0.122 (±0.016) Sixth Cleavage 18 – 22 hours 0.122 (±0.016) 16 – 20 hours 0.122 (±0.016) 11 – 14 hours 0.122 (±0.016)
Blastula 23 – 30 hours 0.127 (±0.006) 0.039 21 – 24 hours 0.126 (±0.007) 0.037 16 – 19 hours 0.125 (±0.008) 0.040 Gastrula 31 – 48 hours 0.139 (±0.006) 0.274 24 – 48 hours 0.135 (±0.015) 0.570 20 – 36 hours 0.131 (±0.010) 0.264 Early Trochophore 3 – 4.5 days 0.221 (±0.014) 0.273 2 – 3 days 0.169 (±0.009) 0.224 1.5 – 2 days 0.165 (±0.008) 0.345 Late Trochophore 4.5 – 6 days 0.347 (±0.031) 0.299 3 – 4 days 0.325 (±0.018) 0.653 2 – 3 days 0.316 (±0.020) 1.304 Early Veliger 6 – 8 days 0.593 (±0.031) 0.358 4 – 5 days 0.559 (±0.038) 0.541 3 – 4 days 0.540 (±0.032) 0.536 Late Veliger 8 – 11 days 0.735 (±0.033) 0.107 6 – 7 days 0.716 (±0.050) 0.124 4 – 5 days 0.659 (±0.025) 0.200
Hatching 12 days 0.875 (±0.041) 0.044 8 days 0.845 (±0.026) 0.083 6 days 0.838 (±0.014) 0.120
an absence of synchronous development.
Embryonic Stage 20oC 25oC 30oC
Cumulative Incubation
Period
Mean Size (mm)
GGR (mm/day)
Cumulative Incubation
Period
Mean Size (mm)
GGR (mm/day)
Cumulative Incubation
Period
Mean Size (mm)
GGR (mm/day)
First Cleavage 4 – 6 hours 0.116 (±0.007) 3 – 4 hours 0.116 (±0.007) 2 – 3 hours 0.116 (±0.007)
Second Cleavage 7 – 8 hours 0.116 (±0.007) 5 – 6 hours 0.116 (±0.007) 4 – 5 hours 0.116 (±0.007)
Third Cleavage 9 – 10 hours 0.116 (±0.007) 6 – 7 hours 0.116 (±0.007) 5 – 6 hours 0.116 (±0.007)
Fourth Cleavage 10 – 13 hours 0.116 (±0.007) 8 – 10 hours 0.116 (±0.007) 6 – 8 hours 0.116 (±0.007) Fifth Cleavage 14 – 18 hours 0.116 (±0.007) 12 – 14 hours 0.116 (±0.007) 10 – 12 hours 0.116 (±0.007) Sixth Cleavage 26 – 34 hours 0.116 (±0.007) 20 – 28 hours 0.116 (±0.007) 16 – 19 hours 0.116 (±0.007)
Blastula 38 – 60 hours 0.125 (±0.008) 0.049 30 – 48 hours 0.123 (±0.006) 0.049 21 – 36 hours 0.120 (±0.007) 0.039 Gastrula 2.5 – 3.5 days 0.134 (±0.009) 0.073 2 – 3 days 0.131 (±0.009) 0.084 1.5 – 2 days 0.128 (±0.009) 0.103 Early Trochophore 3.5 – 5 days 0.175 (±0.014) 0.269 3 – 4 days 0.167 (±0.017) 0.242 2 – 3 days 0.162 (±0.009) 0.471 Late Trochophore 5 – 7 days 0.324 (±0.029) 0.410 4 – 6 days 0.315 (±0.022) 0.632 3 – 5 days 0.313 (±0.027) 0.660 Early Veliger 7 – 10 days 0.495 (±0.030) 0.212 6 – 8 days 0.480 (±0.021) 0.211 5 – 6 days 0.455 (±0.049) 0.187 Late Veliger 10 – 13 days 0.671 (±0.022) 0.101 8 – 11 days 0.641 (±0.055) 0.144 6 – 8 days 0.617 (±0.054) 0.304
Hatching 14 days 0.816 (±0.036) 0.049 12 days 0.783 (±0.024) 0.050 9 days 0.744 (±0.070) 0.063
due to an absence of synchronous development.
Embryonic Stage 20oC 25oC 30oC
Cumulative Incubation
Period
Mean Size (mm)
GGR (mm/day)
Cumulative Incubation
Period
Mean Size (mm)
GGR (mm/day)
Cumulative Incubation
Period
Mean Size (mm)
GGR (mm/day)
First Cleavage 4 – 6 hours 0.119 (±0.006) 3 – 4 hours 0.119 (±0.006) 2 – 3 hours 0.119 (±0.006)
Second Cleavage 7 – 8 hours 0.119 (±0.006) 5 – 6 hours 0.119 (±0.006) 3 – 4 hours 0.119 (±0.006)
Third Cleavage 10 – 11 hours 0.119 (±0.006) 6 – 7 hours 0.119 (±0.006) 4 – 6 hours 0.119 (±0.006) Fourth Cleavage 12 – 14 hours 0.119 (±0.006) 8 – 10 hours 0.119 (±0.006) 6 – 8 hours 0.119 (±0.006) Fifth Cleavage 14 – 20 hours 0.119 (±0.006) 12 – 14 hours 0.119 (±0.006) 10 – 12 hours 0.119 (±0.006) Sixth Cleavage 28 – 48 hours 0.119 (±0.006) 20 – 28 hours 0.119 (±0.006) 15 – 20 hours 0.119 (±0.006)
Blastula 2 – 3 days 0.126 (±0.009) 0.030 1.25 – 2 days 0.125 (±0.009) 0.040 1 – 1.75 days 0.121 (±0.007) 0.017 Gastrula 3 – 4 days 0.135 (±0.015) 0.071 2 – 3 days 0.134 (±0.011) 0.096 2 – 2.5 days 0.129 (±0.009) 0.069 Early Trochophore 4 – 6 days 0.178 (±0.006) 0.274 3 – 4 days 0.175 (±0.005) 0.269 2.5 – 3.5 days 0.169 (±0.008) 0.539 Late Trochophore 6 – 8 days 0.336 (±0.023) 0.318 4 – 6 days 0.319 (±0.018) 0.600 3.5 – 5.5 days 0.309 (±0.014) 0.601 Early Veliger 8 – 11 days 0.512 (±0.025) 0.211 6 – 8 days 0.484 (±0.039) 0.208 5.5 – 7 days 0.463 (±0.037) 0.202 Late Veliger 11 – 14 days 0.705 (±0.030) 0.107 8 – 11 days 0.677 (±0.050) 0.168 7 – 9 days 0.649 (±0.039) 0.225
Hatching 15 days 0.845 (±0.023) 0.045 12 days 0.821 (±0.027) 0.048 10 days 0.793 (±0.029) 0.067
synchronous development.
Embryonic Stage 20oC 25oC 30oC
Cumulative Incubation
Period
Mean Size (mm)
GGR (mm/day)
Cumulative Incubation
Period
Mean Size (mm)
GGR (mm/day)
Cumulative Incubation
Period
Mean Size (mm)
GGR (mm/day)
First Cleavage 3 – 4 hours 0.112 (±0.008) 2 – 3 hours 0.112 (±0.008) 2 – 3 hours 0.112 (±0.008)
Second Cleavage 5 – 6 hours 0.112 (±0.008) 4 – 5 hours 0.112 (±0.008) 3 – 4 hours 0.112 (±0.008)
Third Cleavage 6 – 8 hours 0.112 (±0.008) 5 – 6 hours 0.112 (±0.008) 4 – 5 hours 0.112 (±0.008)
Fourth Cleavage 9 – 11 hours 0.112 (±0.008) 7 – 9 hours 0.112 (±0.008) 5 – 6 hours 0.112 (±0.008) Fifth Cleavage 16 – 18 hours 0.112 (±0.008) 11 – 14 hours 0.112 (±0.008) 7 – 8 hours 0.112 (±0.008) Sixth Cleavage 21 – 26 hours 0.112 (±0.008) 18 – 23 hours 0.112 (±0.008) 12 – 14 hours 0.112 (±0.008)
Blastula 28 – 35 hours 0.118 (±0.012) 0.045 24 – 28 hours 0.117 (±0.010) 0.053 15 – 19 hours 0.116 (±0.007) 0.061 Gastrula 36 – 48 hours 0.129 (±0.011) 0.277 29 – 36 hours 0.125 (±0.012) 0.317 20 – 24 hours 0.125 (±0.100) 0.353 Early Trochophore 2 – 3 days 0.174 (±0.009) 0.594 1.5 – 2 days 0.163 (±0.012) 0.895 1 – 1.5 days 0.154 (±0.014) 1.241 Late Trochophore 3 – 5 days 0.307 (±0.017) 0.567 2 – 3 days 0.291 (±0.012) 1.161 1.5 – 2.5 days 0.283 (±0.015) 1.219 Early Veliger 5 – 6 days 0.455 (±0.027) 0.197 3 – 4 days 0.417 (±0.009) 0.362 2.5 – 3.5 days 0.403 (±0.013) 0.353 Late Veliger 6 – 8 days 0.581 (±0.035) 0.246 4 – 5 days 0.556 (±0.039) 0.287 3.5 – 4.5 days 0.537 (±0.038) 0.287
Hatching 9 days 0.726 (±0.022) 0.074 6 days 0.719 (±0.016) 0.128 5 days 0.710 (±0.017) 0.186
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6
Growth and Life History Parameters of Radix rubiginosa, Lymnaea natalensis and Physa acuta
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