3.2.1 The Study Site
In his original description of L. natalensis, Krauss (1848) stated, “In stagnis natalensibus, frequens” (in still waterbodies in Natal, frequent). However, Herbert and Warén (1999) found probable syntypes of this species in the Stockholm Museum with the locality given as Port Natal. Taking this into consideration and to provide topotypical material, various waterbodies in and around the Durban Metropolitan Area were investigated for the presence of populations that refer morphologically to L. natalensis. Of the habitats visited, surprisingly few yielded L. natalensis and of these, the UKZN Pond (Cato Manor) was designated as a suitable site (Figure 3.1).
Figure 3.1: Map of KwaZulu-Natal showing the UKZN Pond study site (U), selected for the redescription of L. natalensis.
U
UKZN Pond (Cato Manor, Durban)
This is a permanent but isolated waterbody (S 29o 52‟ 02.6” E 30o 58‟ 03.6”, the altitude is 24 m), located close to a highway and a sports and recreational stadium (Figure 3.2).
The aquatic macro-flora comprised of Marsilea sp., Typha capensis, Pistia stratiotes, Nymphaea nouchali and several representatives of the Cyperaceae. About 75% of the surface area was covered by P. stratiotes, Marsilea sp. and N. nouchali, with some areas covered by dense mats of filamentous green algae. Albizia adiantifolia trees and dense shrubs along the periphery of the pond limited accessibility and provided partial shade.
The temperature in the Durban Metropolitan Area is mild in winter (June to August) and warm to hot in summer (December to February). Mean monthly figures for the minimum and maximum daily air temperatures were 10.3oC to 21.5oC and 22.8oC to 28.9oC
respectively (Source: South African Weather Service, Durban). The total annual rainfall usually exceeds 1000 mm, of which the most falls in spring and summer (September to February). The pond is dependent on rainfall and its depth varied between 2.1 m in winter to 4.5 m in summer. It has not been seen to dry out. The water chemistry
parameters for the UKZN Pond were measured by the author at a depth of 30 cm, using a YSI 6920 multi-probe data logger. A summary of selected water chemistry parameters is presented in Table 3.1.
Table 3.1: Selected water chemistry parameters for the UKZN Pond. All values measured are indicated as mean (± standard deviation), n = 35.
Parameter Mean (± standard deviation)
pH 8.13 (± 0.12)
Conductivity (mS/cm) 0.57 (± 0.01) Dissolved Oxygen (mg/L) 8.08 (± 0.81)
Figure 3.2: The UKZN Pond.
3.2.2 Shell Morphology
The snails were identified based on conchological characteristics according to the original description by Krauss (1848) and subsequent authors (Connolly, 1939;
Hubendick, 1951; Mandahl-Barth, 1954; de Azevedo et al., 1961; Brown, 1994;
Appleton, 1996; Herbert and Warén, 1999). Due to the variable nature of the shell of L.
natalensis wherever it was found, a molecular study of snails from this site was conducted in 2007 (J. Lamb and K. Pillay unpubl. data). This was the same molecular study that identified the Amatikulu lymnaeid as R. rubiginosa. DNA analyses confirmed that the lymnaeid from the UKZN Pond was L. natalensis (see Appendix to Chapter 3).
Fifteen adult L. natalensis from the study site were selected to measure the shell
thickness. In order to compare the snails without size bias, only adult snails of 15-22 mm
in shell length were used. The shell thickness was measured at three points on the shell and expressed as the mean shell thickness (Figure 3.3).
Figure 3.3: Schematic drawing of the shell indicating the points (●) where shell thickness was measured.
The mean shell thickness was presented as μm (± standard deviation).
3.2.3 Anatomical Morphology
3.2.3.1 Radula
Before dissection, 15 adult L. natalensis from the UKZN Pond were relaxed for 24 hours using menthol crystals. The specimens were fixed in 10% formalin for a further 24 hours and then stored in 75% alcohol. To study the radula, the buccal mass of each relaxed snail was removed and put into a test tube of 5% NaOH solution for 48 hours. This procedure removed the tissues surrounding the radula, leaving only the radular ribbon in the test tube.
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The contents of the test tube were emptied into a watch glass and the radula ribbon removed under the dissecting microscope. The extracted radula was then washed in three changes of distilled water, three minutes per rinse and stored in 75% alcohol. The radula was then mounted on a specimen stub using two way laboratory tape and allowed to dry at room temperature. After coating with gold, the radula was viewed using a LEO scanning electron microscope.
The number, shape, size and position of the cusps on the central, lateral and marginal teeth were noted and where appropriate, photographed.
3.2.3.2 Mantle pigmentation patterns
Fifteen adult L. natalensis from the study site were relaxed for 24 hours using menthol crystals and then immersed for approximately a minute in hot water (70oC), from which they were transferred to water at room temperature. The soft tissues were separated and dissected following the methodology proposed by Paraense (1976). By holding the cephalopedal mass with a forceps and gently twisting the shell in an anti-clockwise motion, the soft parts were drawn from the shell. Drawings of the mantle pigmentation were made using a camera lucida fitted on a stereomicroscope.
3.2.3.3 Reproductive Anatomy
Fifteen adult L. natalensis from the study site were prepared as outlined in section
3.2.3.2. In order to compare the snails without size bias, only adult snails of 15-22 mm in shell length were used. Snails were dissected under a stereoscopic microscope and drawings of the reproductive system were made using a camera lucida.