Dyakia Godwin-Austen, 1891

Sutcharit, Chirasak, Tongkerd, Piyoros, Tan, Shau-Hwai Aileen & Panha, Somsak, 2012, Taxonomic Revision Of Dyakia Janus From Peninsular Malaysia (Pulmonata: Dyakiidae), With Notes On Other Sinistrally Coiled Helicarionoids, Raffles Bulletin of Zoology 60 (2), pp. 279-287 : 280

Genus Dyakia Godwin-Austen, 1891

Types species. — Helix hugonis Pfeiffer, 1863 , by original designation. Syntypes BMNH 1975118 (three shells; the specimen that most closely matches the description and measurements given in the original description is shown in Fig. 1A ).

Remarks. — From previous records, six Dyakia s.l. species have been reported from Indochina and peninsular Malaysia ( Laidlaw, 1963; Panha, 1996; Hemmen & Hemmen, 2001; Maassen, 2001). Three species, morphologically classified as “ Dyakia ” retrorsa ( Gould, 1843) , “ Dyakia ” salangana ( Martens, 1883) , and D. janus , were collected and examined (as interpreted by Laidlaw, 1963).

Helix retrorsa Gould, 1843 and Nanina salangana Martens, 1883 have large sinistral shells that are brown to yellowish with thin growth lines. The last whorl is large, with or without peripheral keels. These two species have similar genitalia, and Nanina salangana Martens, 1883 is figured as an example. The genitalia comprise a thin penial sheath (psh), straight epiphallic caecum (ec), short flagellum (fl), bulbous gametolytic sac (gs) and large cylindrical amatorial organ (am). The mantle collar has a large left dorsal lobe (ldl), undivided right dorsal lobe (rdl), and shell lappets are absent ( Fig. 2A–C ). The central tooth is unicuspid and triangular in shape; the lateral teeth are unicuspid whilst bicuspid marginal teeth are present towards the margins ( Fig. 2D, E ). The genitalia of these two species are clearly distinct from Dyakia , notably in the absence of an amotorial organ gland and presence of a penial sheath, epiphallic caecum, flagellum and a gametolytic sac connected by a short duct to the vagina. Clearly the sinistral shells, while showing marked similarity to true Dyakia , had misled previous workers and the assumption that such sinistral shells were uniquely derived in Dyakia (Hausdorff, 1995) is disproved. In fact the reproductive systems of Helix retrorsa Gould, 1843 and Nanina salangana Martens, 1883 demonstrate that they are better placed in the Ariophantidae sensu Schileyko (2002) .

Figures

Fig. 1. Shells of Dyakia species. A. Syntype of Dyakia hugonis (BMNH 1975118). B–F. Dyakia janus; B, neotype (BMNH 42.5.10.1463) from Malacca, Malaysia; C, examined specimen from Johor, Malaysia (CUMZ 4919); D, figures of “Helix janus bifron” after Chemnitz (1795, pl. 213, Figs. 3016, 3017); E, lectotype of Helix sannio Pfeiffer, 1854 (BMNH 20100242); and F, syntype of Nanina albersi Albers, 1864 (ZMB 57526). G. Syntype of Dyakia kintana (MNHN, 8 shells). H. Holotype of Dyakia lindstedti (BMNH).

Fig. 2. Genitalia, mantle collar, and radula morphology of Nanina salangana Martens, 1883 from Songkhla, Thailand (CUMZ 4920). A. Whole genital organ. B. Internal wall sculpture of penis, vagina and amatorial organ. C. Ventral view of mantle collar region. D, E. SEM image of radula; central tooth indicated by ‘C’. Numbers indicate tooth order from lateral to marginal end.