Anaglyphula Rensch, 1932

Chen, Zhe-Yu & Páll-Gergely, Barna, 2023, A new species of Anaglyphula Rensch, 1932 from an island off the coast of West Indonesia, with redescription of the type species (Gastropoda: Caenogastropoda: Assimineidae), Raffles Bulletin of Zoology 71, pp. 597-605 : 599

Genus Anaglyphula Rensch, 1932

Anaglyphula Rensch, 1932: 122 .

Anaglyphula – Zilch 1959: 216; Vermeulen, 1996: 149; Vermeulen & Whitten, 1998: 140; Vermeulen et al., 2015: 17.

Type species. Anaglyphula cancellata Rensch, 1932 , by original designation.

Remarks. Vermeulen et al. (2015) proposed using the presence of a constriction and the thickened peristome to distinguish Anaglyphula from the most similar genus Acmella . Although the neotype of Acmella tersa ( Benson, 1853) (the type species of Acmella ) had a sharp peristome ( Das et al., 2021), a thickened aperture can also be observed in very similar shells ( Fig. 5A , Acmella cf. tersa, NHMUK, Cherra Poongee , India, leg. H.H. Godwin-Austen, id. W. Blanford). This suggests that the thickened aperture is variable even within a single Acmella species, or at most between closely related Acmella species. Therefore, this feature cannot be used to distinguish the above genera. The constriction Vermeulen et al. (2015) found in Anaglyphula sauroderma seems to be more useful as a diagnostic trait, as this structure can also be observed in Anaglyphula cancellata and Anaglyphula whitteni ( Figs. 4B–C ). The constriction was formed by a thin radial palatal internal fold approximately ½ whorl away from the aperture, mostly visible from the aperture, but no structural change can be found on the outer surface of the shell. Notably, the position of the constriction is not stable even in the same population. In the type series of Anaglyphula cancellata , although the constriction of two specimens is too deep to be observed from the aperture, the constriction of the other specimens is visible through the aperture when observed at an angle. In contrast, the constriction in other cyclophoroidean groups (i.e., Alycaeidae and Diplommatinidae ) is mostly clearly visible from the outside, since it is a reduction of the diameter of the whorl where the constriction is situated (see e.g., Páll-Gergely et al., 2017; Chen & Wu, 2020; Páll-Gergely, 2020). In the new species, the presence of a constriction is somewhat uncertain due to the difficulty in discerning whether a lighter-coloured line approximately halfway behind the aperture represents the constriction or merely a growth line, as this position is too deep to be visually examined from the aperture.

Meanwhile, the type species of Anaglyphula , Anaglyphula cancellata , is characterised by a rib-mesh (=pitted) surface structure, and that trait was mentioned to be the most diagnostic one in the original description of the genus. A similar one is present in Anaglyphula sauroderma , and Anaglyphula yanseni , new species. Since this character is nearly unique in microsnails (the only similar genus is the stylommatophoran genus Acinolaemus F. G. Thompson & Upatham, 1997 ; see Changlom et al., 2019), this character could be used as a diagnostic feature for Anaglyphula . However, Anaglyphula whitteni , which has converging ribs ( Fig. 2C ) similar to the Himalayan type species of Acmella , is very similar in shell shape to the geographically adjacent A. cancellata . If only shell sculpture is taken into consideration, Anaglyphula whitteni should be moved to Acmella . However, it is more likely that Anaglyphula whitteni is more closely related to Anaglyphula cancellata than to Acmella tersa based on the presence of a constriction and its geographical distribution. Furthermore, the fine sculpture of Anaglyphula cancellata contains ribbed and pitted parts as well, which may indicate that the ribbed and pitted sculptures are states of the same character, and differ only by the absence of the spiral element in the first state between the ribbed and pitted sculpture in Acmella and Anaglyphula . Lastly, the genus Acmella currently contains species with a wide variety of shell sculptures, and the sculpture of several species is not sufficiently known ( Das et al., 2021), suggesting that Acmella is a wastebasket taxon. For these reasons, at the moment, we are not in the position to clarify the diagnostic characters of Anaglyphula and Acmella , since it could only be done with more detailed morphological and preferably molecular evidence. Nevertheless, the new species described herein is evidently an Anaglyphula due to the similar sculpture and shell shape to the type species of this genus.

Figures

Fig. 5. Acmella species. A, Acmella cf. tersa, NHMUK; B, Acmella moreletiana G. Nevill, 1878, NHMUK 1891.3.17.944–7; C, Acmella mellilla Godwin-Austen, 1895, NHMUK 1903.7.1.1698.

Fig. 4. A, Putative constriction position of Anaglyphula yanseni, new species, holotype MZB.Gst. 23.829; B, Constriction of Anaglyphula cancellata Rensch, 1932, syntype NHMUK 1897.8.3.150; C, Constriction of Anaglyphula whitteni Vermeulen, 1996, paratype NHMUK 20000247. Not to scale.

Fig. 2. A, Anaglyphula cancellata Rensch, 1932, syntype NHMUK 1897.8.3.149; B–C, Anaglyphula whitteni Vermeulen, 1996, Paratype. NHMUK 20000247; B, Shell; C, Ribbing structure details.