Understanding the association between the non-geniculate coralline red alga <i>Spongites discoidea</i> and the mollusc <i>Oxystele sinensis</i>

Abstract

Nearly all adult winkles Oxystele sinensis encountered in the Kalk Bay (Western Cape province, South Africa) shallow subtidal zone were observed to bear a thick, convoluted form of the non-geniculate coralline red alga Spongites discoidea. To understand this association, various data were collected. The results show that O. sinensis is for the most part restricted to the shallow subtidal zone and rockpools, whereas the morphologically similar O. tigrina is largely restricted to the intertidal zone. Fleshy macroalgae were absent from all habitats sampled, with much of the primary substratum being occupied by encrusting algae. Spongites discoidea was restricted to intertidal rockpools and the shallow subtidal zone where O. sinensis was the most abundant winkle. Subtidal specimens of S. discoidea were generally thicker (7.24 mm [SE 0.55] on shells, 6.02 mm [SE 0.67] on boulders) than intertidal specimens (2.39 mm [SE 0.21] on boulders, 1.02 mm [SE 0.09] on bedrock), were more heavily burrowed into (44% [SE 3.32] for subtidal shells, 36% [SE 3.94] for subtidal boulders), and were more weakly attached (2.2 kg [SE 0.58] for subtidal shells, 8.4 kg [SE 0.77] for subtidal boulders). Experimental evidence showed that the coralline has no preference for shells of O. sinensis and that, contrary to our assumption, there is no apparent advantage to the winkle bearing the coralline, other than perhaps for camouflage to avoid predation. The high frequency of the association is more likely due to the substantial overlap in the niches of the coralline and the winkle.

Keywords: attachment strength, burrowing, cover, diversity, epizoic, recruitment, settlement preference, South Africa, thallus thickness