Indirect evidence for ecophenotypic plasticity in radular dentition of Littoraria species (Gastropoda: Littorinidae)

In examination of radulae from all but one of the 36 species of the littorinid genus Littoraria we found extraordinary intraspecific variation in those occurring on a range of substrates. Radulae from rock showed a less well developed 'hood' on the rachidian tooth, a strikingly enlarged major cusp on each of the five central teeth, fewer cusps on the outer marginal teeth and the radular ribbon was longer, when compared with radulae of conspecifics from plant substrates. The radulae of species found exclusively on rock differed in similar ways from those restricted to plant substrates (mangroves, driftwood and saltmarsh). We suggest that this may be an example of phenotypic plasticity of radular form, induced by substrate and/or diet, as recently shown experimentally in another littorinid genus. The mechanism of inducible plasticity deserves further study. Ecotypic variation in the radula may be widespread in littorinids, and radular characters should therefore be used with caution in studies of taxonomy, phylogeny and adaptation.