Contributions to Zoology, 79 (2) – 2010Bastian T. Reijnen; Bert W. Hoeksema; Edmund Gittenberger: Host specificity and phylogenetic relationships among Atlantic Ovulidae (Mollusca: Gastropoda)
Discussion

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Paedomorphosis

In the phylogeny reconstruction, Simnia patula is the sister group of the combined Simnialena uniplicata and Cyphoma species group. This is surprising because, at first sight, fully grown individuals of Cyphoma species clearly differ from S. uniplicata in shell morphology. However, when shells of S. uniplicata are compared to a series of shells of C. gibbosum in various growth-stages (Fig. 4), it turns out that S. uniplicata closely resembles juvenile C. gibbosum (as well as juvenile ovulids in general). Both species lack a clear funiculum (narrow ridge of callus at the ventral side of the shell close to the aperture) and have rounded, tapering ends, character states that are absent in fully grown C. gibbosum. Identification of juvenile ovulid shells is difficult, if not impossible, due to a lack of diagnostic shell characters. However, some specimens that were collected alive showed a mantle colour pattern diagnostic for C. gibbosum, viz. bright orange spots, encircled with a black line at a whitish background (Fig. 1). Cymbovula acicularis had a nearly transparent mantle, sometimes with white protuberances. In Simnialena uniplicata the mantle is entirely black, whereas in Cyphoma signatum it had a distinct yellow/black fingerprint pattern. This leads to the conclusion that the mantle colour and pattern may be diagnostic in Ovulidae (Mase, 1989) and that this character can be used to distinguish fully grown Simnialena uniplicata from juvenile C. gibbosum. Therefore, we hypothesize that S. uniplicata exemplifies paedomorphosis.

FIG2

Fig. 4. Shells in dorsal and ventral view. Simnialena uniplicata (d) compared to a series of shells of Cyphoma gibbosum (a-c, e-k) in different developmental stages. Scale bar 1 cm.