In this study, however, L. natalensis from the UKZN Pond had a central tooth that was asymmetrically bicuspid, with the smaller cusp located on the left side towards the base of the main cusp. This smaller cusp diminished in size and disappeared after the 10th – 15th transverse row, towards the rear of the radula. This could explain the unicuspid characteristic described by Hubendick (1951) and de Azevedo et al. (1961). According to Pretorius and van Eeden (1969), the diminished size and disappearance of the
accessory cusp could be attributed to detrition. This was found to be the case in this study, as the accessory cusp was present on teeth that had not yet been used for grazing.
The transition from the tricuspid lateral to multicuspid marginal teeth was abrupt with the 9th and 10th pairs of laterals being the transitional or intermediate teeth (Figure 3.12). In the 9th pair, the ectocone split into two smaller, acute-shaped denticles (see arrow, Figure 3.12). This unique characteristic was noted only by de Azevedo et al. (1961):
“Intermediate teeth with four cusps, irregular, pointed; the internal two larger and joined halfway, and the external two smaller and isolated.”
Mantle pigmentation patterns have been used as a useful diagnostic character in the descriptions of many lymnaeid species. Jackiewicz (1993) reported that these patterns on the mantle showed great diversity, being similar in some species only. From a systematic standpoint it is pertinent to know to what extent these patterns are stable or to what extent they vary within a species. The pigmentation pattern was consistent among all
individuals sampled from the UKZN Pond. The intensity of the pigmentation on the mantle varied however among individuals, but the distribution pattern of the spots was distinct and similar to those described by other authors (Hubendick, 1951; de Azevedo et al., 1961; Pretorius and van Eeden, 1969).
There were no significant differences between the reproductive anatomy of L. natalensis from the UKZN Pond and representative of the species described by other authors (Hubendick, 1951; de Azevedo et al., 1961; Pretorius and van Eeden, 1969). Some variations were observed within the present material in the dimensions of the
spermathecal duct, penial sheath and praeputium, but these were attributed to different degrees of contraction and relaxation during preservation.
Hubendick (1951) drew attention to the fact that the different structures of the male reproductive anatomy varied so much in shape and intra-species variation within single species of Lymnaea, that this system could not be considered as being of taxonomic importance in the Lymnaeidae. In their study of the male reproductive anatomy of L.
natalensis, Pretorius and van Eeden (1969) observed that in some specimens the penis was much shorter than its penial sheath (Pretorius and van Eeden, 1969; see Figure 84, page 100), while in other specimens it extended beyond the velum well into the
praeputium (Pretorius and van Eeden, 1969; see Figure 85, page 100). These conditions reflected the different degrees of retraction and eversion respectively, of the penis at the time of fixation. Similarly, the praeputium was also subject to varying degrees of contraction and relaxation (Pretorius and van Eeden, 1969).
In this study, conchological and anatomical characters were used to identify the snail population from the UKZN Pond as L. natalensis Krauss, 1848. Despite some observed variation, the individuals sampled from this study site conformed to accounts made by other authors.
Historically gastropod taxonomy has been based purely on morphological characters, however the large degree of morphological plasticity exhibited by the Lymnaeidae has confused the taxonomy of this group. In the last few decades, the use of molecular techniques in taxonomic studies (Márquez et al., 1995; Bargues and Mas-Coma, 1997;
Bargues et al, 1997; Remigio and Blair, 1997; Stothard et al., 2000; Bargues et al., 2001;
Remigio, 2002; Bargues et al., 2003; Puslednik, 2006), often in conjunction with more traditional morphological approaches, has provided increased taxonomic clarity about relationships within molluscan groups. In a study of lymnaeids from the Amatikulu Hatchery and the UKZN Pond, KwaZulu-Natal, South Africa (J. Lamb and K. Pillay, unpubl. data), molecular data were obtained by sequencing three gene regions: two mitochondrial DNA genes (cytochrome oxidase subunit I and 16S rRNA) and one
nuclear DNA gene (18S rRNA). In this molecular study, the cytochrome oxidase subunit I (COI) gene was chosen as it was useful in analysing variation among closely allied taxa (Black et al., 1997; Attwood and Johnston, 2001; Attwood et al., 2003; Remigio and Herbert, 2003; Staton, 2003; Genner et al., 2004). The 16S rRNA region was selected as it had both slow and rapidly evolving regions, allowing for family and genus level
delineation (Hillis and Dixon, 1991; Reid et al., 1996; Remigio and Blair, 1997; Attwood et al., 2003). In addition the slowly evolving 18SrRNA gene was included to resolve any deeper phylogenetic divergences.
Figures A1.1 - A1.3 in the Appendix to Chapter 3 represent the trees that were constructed using the neighbour-joining method. The results of this comparative molecular study established a clear distinction between the lymnaeids from the UKZN Pond and the Amatikulu Hatchery. The results of the DNA sequencing identified the UKZN Pond population as Lymnaea natalensis Krauss, 1848, an indigenous lymnaeid, and the population from the Amatikulu Hatchery as Radix rubiginosa (Michelin, 1831), a lymnaeid from southeast Asia.