Asymmetry in intraspecific competition in the limpet Cellana tramoserica (sowerby)

Abstract

Most studies of competition have classified interactions simply as interspecific or intraspecific. In doing this, individuals within a species are all assumed to be similar in their competitive ability. Consequently, the importance of variation in competitive ability among conspecific individuals is overlooked. At the Merret Rifle Range, in Williamstown (Victoria), the limpet Cellana tramoserica (Sowerby) occurs in high densities. We examined competitive interactions between limpets of different sizes from this population to test assumptions about competitive abilities among conspecific individuals. The effects of competition between and within size-classes were investigated by manipulating biomass in field experiments. Large limpets were affected by competition with both size-classes. Increased biomass (whether of small or large limpets) resulted in reduced growth rates of large limpets, but no mortality. Survivorship and growth of small limpets decreased markedly when caged with other small individuals, but were not affected by competition with large limpets, except at very high densities. The abundance of microalgae within enclosures was reduced more by small limpets than large limpets, regardless of biomass. The effect of small limpets on microalgal abundance increased with increasing biomass, whereas large limpets showed no such pattern. These results indicate strong asymmetrical, exploitative competition between different size-classes of the limpet C. tramoserica. The mechanisms determining competitive ability in different sized limpets were also investigated. Small and large limpets had different sized radulae. The radular teeth of large limpets were longer and more widely spaced than those of small limpets. At the Rifle Range, the surface of the feeding substratum is highly pitted, and these pits are a source of microalgae. We propose a model in which smaller radulae allow small limpets better access to food within small pits, and thus a greater amount of the microalgal resource, to explain the superior competitive ability of small limpets.