THE ROCK-SCRAPING RADULA, A STRIKING CASE OF CONVERGENCE ()

by A. S. H. BREURE and E. GITTENBERGER (Rijksmuseumvan NatuurlijkeHistorie, P.O. Box 9517, 2300RA Leiden; The Netherlands)

Dedicatedto Prof. Dr. W. Vervoort, on the occasionof his retirementas Directorof the Rijksmuseumvan NatuurlijkeHistorie

ABSTRACT

In nine subfamilies, belonging to four superfamilies of the order (Mollusca, ), radulae are found in which series of uniform cusps, without side cusps, occupy at least the middle of the transverse rows. This type of specialized radula has only been encountered in species living on rock-faces; it is called the rock- scraping radula. A rock-scraping radula is found to be at most a character of generic value. Higher taxa for which it is typical are not known.

INTRODUCTION

The two authors of the present paper, while studying the systematics of Neotropical and Palaearctic pulmonate snails, respectively, indepen- dently discovered a conspicuous case of convergence in radular mor- phology. While comparing their findings and subsequently studying the literature, it turned out that this appears to be a case of wide-spread convergence in quite unrelated families of . Because previous authors apparently did not realize this, a preliminary report upon the phenomenon is given below.

THE MOST FREQUENTLY FOUND RADULAR TYPE

By far the most common type of radula in pulmonate snails is the one that is characterized by transverse rows with a tricuspid middle tooth, which has a large central cusp. Next to the middle tooth, the bicuspid or tricuspid lateral teeth also have single, large main cusps. In the mar- ginal teeth the main cusp is often not clearly discernible. Several specializations for different ways offeeding are known to exist in radular morphology. The best known adaptation might be the long claw-like cusps found in carnivorous snails. 308

For a well-illustrated, more detailed introduction to radular mor- phology and functioning we refer to SOLEM (1974).

THE ROCK-SCRAPING RADULA

In the present paper attention is called to what is termed the rock- scraping radula. As far as known, all species with this particular type of radula are feeding on rock-faces, where food is distributed as a film over a hard substratum. In the rock-scraping radula at least the middle part of a transverse row is occupied by a series of uniform cusps. In the cases studied in detail this regular pattern proved to be brought about by complete reduction of the side cusps of the central and several lateral teeth, which, consequently, have all become unicuspid. This has been as- certained in Chondrinidae (GITTENBERGER, 1973), Clausiliidae (NORD- SIECK, 1978; LoosJES & LOOSJES-VAN BEMMEL, 1966), Bulimulinae (BREURE, 1978, 1979) and Camaeninae (SOLEM, 1974). The middle tooth always remains recognizable as such by the structure of its basal plate, which is symmetrical, with two supporting ridges, as in less specialized radulae (pl. I figs. 1, 2). One could imagine a situation in which the side cusps are not re- duced, but enlarged to the dimensions of the original main cusps. This would also imply a radula with series of uniform cusps in the middle. However, a basal plate then would support more than one cusp. Maybe a process of this kind did start in Rupestrella (Chondrinidae: see below), where the middle tooth is unicuspid and the lateral teeth have com- paratively large side cusps next to the main cusp, which, however, remains recognizable (pl. I figs. 3, 4). The last mentioned fact implies that the Rupestrella radula only suggests a mode of development towards a rock-scraping radula, which has not (yet) been realized in that genus. Nevertheless it illustrates that detailed studies should be performed to establish whether a rock-scraping radula as defined above can originate in various ways. A rock-scraping radula is known to exist in at least two suborders of the order Stylommatophora, aiz. in and . In the former group it has been found in one superfamily (Pupillacea), in the latter in three (Clausiliacea, Bulimulacea and Helicacea). Altogeth- er we may say that a rock-scraping radula has developed by con- vergent evolution in at least nine subfamilies, listed in the next chapter. It should be emphasized that large, bare looking rock-faces constitute an extreme habitat for snails. Only a limited number of species and genera are found exclusively in that habitat, at least if a not too large mountainous region is surveyed. As far as known, a rock-scraping