Geographic studies on the interaction between the limpet, Patella cochlear, and the encrusting coralline alga Spongites yendoi.

By Gavin W. Maneveldt & Derek W. Keats

Botany Department,
University of the Western Cape,
P. Bag X17, Bellville 7535
South Africa

Running Head: Geographic studies of grazing on corallines

Key words: competitive ability; facultative mutualistic association; fecundity; geographic gradient; growth rate; herbivore-coralline association; herbivory; morphology; Patella cochlear; Spongites yendoi; thallus thickness.

Key phrases: Effects of grazing on: thallus thickness, lateral growth rate, general physiognomy, fecundity, competitive ability, and abundance; Facultative mutualistic association; Grazing as a positive factor; Nutrient status of coralline vs that of alga from limpet's garden; Results of limpet manipulation vs geographic trends.

Abstract. The territorial gardening limpet, Patella cochlear, occurs along the South and southern West coasts of South Africa, while one of its primary prey, the encrusting coralline alga, Spongites 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 limpets diet while fleshy algae (Gelidium micropterum) comprise roughly 7 %. 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. Grazing weakens S. yendoi's interference (overgrowth) competitive ability and also reduces the fecundity of the coralline. Manipulation experiments support the observed geographic trends.

The intimate herbivore-coralline association between P. cochlear and S. yendoi shows characteristics of a facultative mutualistic association. By reducing its thallus thickness, grazing by P. cochlear increases the coralline's 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 results 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 eulittoral zone; there is thus a positive correlation between limpet and coralline percentage cover. Spongites yendoi's success at exploitative (faster growth rate) competition is thus enhanced by the higher grazing frequencies of P. cochlear. 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.

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