Geographical studies on the interaction between the pear limpet, patella cochlear, and the encrusting coralline alga, spongites yendoi

Abstract

The territorial pear limpet, Patella cochlear, occurs along the South and Southern West coasts of South Africa, while one of its primary prey, the encrusting coralline alga, Spon~ites yendoi, occurs much further north along the West coast. This has presented the ideal situation to develop and test hypotheses concerning the importance of grazing in the ecology of encrusting coralline algae along a geographic gradient. Variation in the abundance of P, cochlear has various implications for the morphology and ecology of S, yendoi. The coralline comprises the major portion (roughly 85 %) of the limpet's diet while fleshy algae (Gelidium micropterum) comprise roughly 7 % of its diet. Grazing by the limpet reduces the thallus thickness of the coralline producing a relatively smooth coralline; in the absence of herbivory, or under low grazing frequencies, the coralline is thick and highly protuberant. As far as overgrowth (interference) competition is concerned, grazing reduces the initiative ability; as far as exploitative competition is concerned, grazing enhances the coralline's competitive ability (increased growth rate means increased rate of substratum occupancy). Grazing also reduces the fecundity of the coralline. Manipulation experiments support the observed geographic trends. Although grazing is commonly perceived as a negative factor in marine ecosystems, the intimate herbivore-coralline association between P, cochlear, and S, yendoi shows characteristics of a facultative mutualistic association. Grazing increases the corallines' lateral growth rate; thinner forms of the coralline grow significantly faster (0.58 ± 0.24 mm.month.1) than thicker forms (0.10 ± 0.02 mm.month.1). An increased lateral growth also resulted in the thinner form of the coralline being very abundant on South and southern West coasts occupying as much as 79 % of all surfaces in the lower littoral zone; there is thus a positive correlation between limpet and coralline abundance. Preliminary nutrient analyses reveal that the coralline is equally high in organic content per volume as compared with the limpet's garden of G, micropterum and possibly also Herposiphonia heringii.