Archachatina natalensis (Pfeiffer, 1854)

Sirgel, W. F., 2004, A further note on Archachatina churchilliana and a description of the genital anatomy of Archachatina natalensis (Mollusca: Pulmonata: Achatinidae), African Invertebrates 45, pp. 271-278 : 273-277

Archachatina natalensis (Pfeiffer, 1854)

Observations: The above mentioned specimen of Archachatina natalensis (Natal Museum accession no. L5564) was collected by Miss Pam Cairns on Inhaca Island (26º05'S 32º55'E), Mozambique, on 25 March 2001. It was kept alive in the Natal Museum for a year and then preserved in April 2002 (Mrs Linda Davis pers. comm.). This specimen was kindly made available to me for examination. The shell ( Fig. 4) corresponds very well with the type specimen illustrated by Connolly (1939) and has

3 4

the following morphometrics; whorls 7.5+; length 60.5 mm; diameter 30.5 mm; last whorl 41.5 mm; length/diameter ratio 1.98; total length/length of last whorl ratio 1.46. During dissection, eleven eggs were found in the free oviduct and spermoviduct, proving that the individual had reached sexual maturity. Four of these eggs were removed to make it possible to draw a clear figure of the relationships between the various parts of the genital system.

Genital anatomy: Numerical data on genital structures are given in Table 1. Penis extends above thin­walled penis sheath ( Fig. 5), slightly shorter than vagina; tripartite consisting

TABLE 1

Measurements of genital system of A. natalensis (mm).

Penis length 10.2

Basal part of penis diam. 1.2

Penis sheath length 8.2

Free oviduct length 19.0

Basal vas deferens length 7.5

Basal vas deferens diam. 0.5

Apical vas deferens length 28.4

Apical vas deferens diam. 1.2

Vagina length 13.7

Spermathecal duct length 18.3

Spermathecal duct diam. 1.0

Spermatheca length 4.5

Spermatheca diam. 2.3

of a tubular basal part (length 4.2 mm, diameter 1.2 mm) followed by a subapical part (length 4 mm) which gradually expands in diameter as it extends apically to reach a diameter of about four times that of the basal part; the third or apical part extending above penis sheath is bean­shaped (4.5 x 2 mm), its longitudinal axis orientated transversely in relation to the two more basal parts ( Fig. 6); wall of basal part of penis relatively thin, that of subapical part about twice as thick while that of the apical part is even thicker; internal surface of penial wall not thrown into longitudinal rugae, but thin threadlike fibres, arranged in a net­like pattern, can be perceived in this almost smooth inner surface of the penial wall; basal vas deferens emerges from the ventral side of the apical part of the penis ( Fig. 6) and extends in a basal direction along the penis to pass through the penis sheath close to its basal end ( Fig. 6); it is not embraced by a fold of the penial wall on its course. Muscle fibres originate from a thin strip running along the length of the inner surface of the penis sheath. These fibres continue along the length of the basal vas deferens towards the apex of the penis, forming a thin muscular curtain connecting the penis sheath to the vas deferens, but not to the penis itself in the specimen dissected ( Fig. 6). On reaching the apex of the penis these fibres merge with others originating from the central region of the apical part of the penis to form the penial retractor muscle. This muscle traverses the sagittal myoseptum to join the columellar muscle band of the right side some distance apically of the septum. It should however, be kept in mind that the point of insertion of this muscle is very variable amongst individual members of Achatinidae (Mead 1995 a) . Apical vas deferens emerges from the penis sheath close to the peniovaginal angle, with distinctly larger diameter than basal vas deferens, continued along vagina and free oviduct; spermathecal duct long; carries a clavate spermatheca which is attached to the spermoviduct apical to the junction of the apical vas deferens and free oviduct. Each of the eleven eggs found in the system measured 8.9 x 7.5 mm.

Remarks: The general pattern of the genital system of Archachatina natalensis ( Fig. 5) conforms to that described by Mead (1988, 1991, 1995 a) for the South African members of the genus Archachatina , subgenus Tholachatina . He points out that the subgenus Achatina s.s. of Achatina differs by typically having a short spermathecal duct with a relatively large sacculate spermatheca attached to the free oviduct, thus not reaching the junction of the vas deferens and free oviduct. The presence of a penial atrium also elevates the emergence of the apical vas deferens from the penial sheath well above the common genital atrium in the subgenus Achatina . These characters are absent in A. natalensis . Neither does A. natalensis have a muscular bulboid enlargement of the basal vagina nor an elevated emergence of the apical vas deferens, as described for Achatina (Lissachatina) by Mead (1991).

These characters indicate that A. natalensis belongs to the southern African section of the genus and subgenus Archachatina (Tholachatina), sensu Mead (1991) . But subsequently Mead (1995 b) suggested that the subgenus name Tholachatina should be reserved for a group now largely restricted to the Lake Region of central eastern Africa. He believes that the ‘Southern African branch’ originated from the ‘East African branch’ of Achatinidae , and gave origin to a genus, for which he suggests that the name Cochlitoma Férussac, 1821 should be resurrected. Should these suggestions of Mead (1995 b) be accepted, the southern Africa species previously referred to the genus and subgenus Archachatina (Tholachatina) , will have to be re­allocated to the genus Cochlitoma .

The genital system of A. natalensis deviates in a few minor respects from that normally found in those few members of the so­called Southern African Archachatina (Tholachatina) group ( Cochlitoma, sensu Mead 1995 b ) for which the genital anatomy is known. The penis for instance resembles that described for Archachatina (Tholachatina) livingstonei ( Sirgel 2000) in extending above the penis sheath, but differs from the condition described for all other species in this group. The emergence of the basal vas deferens from the ventral side of the apical part of the penis, instead of from its base, seems to be a unique character, found within this group only in A. natalensis . Furthermore, the basal vas deferens, not being embraced in a longitudinal fold of the penial wall along its course, differs from the condition usually found in this group (Mead 1991). A. livingstonei , however, shows only slight signs of such a condition, in having a short and very shallow concavity in a corresponding position ( Sirgel 2000). This seems to suggest that both A. natalensis and A. livingstonei present a primitive condition in this respect.