Laboratory Evidence for a Size Refuge in Competitive Interactions Between the Hydroids Hydractinia Echinata (Flemming) and Podocoryne Carnea (Sars)

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Biology (HMC)

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Size refugia from competition, whereby one organism may grow too large to subsequently be overgrown by a superior spatial competitor, have rarely been documented in marine benthic communities. Size-symmetrical and size-asymmetrical competitive interactions were established between colonies of two hermit crab-associated hydroids, Hydractinia echinata and Podocoryne carnea, to assess the outcome of competition for space between the two species and its possible size-dependence. In size-symmetrical interspecific contests, P. carnea overgrew and killed H. echinata in 100% of 74 observed encounters. In size-asymmetrical contests in which H. echinata was always the larger colony, P. carnea was able to overgrow H. echinata in only 55% of 76 contests. H. echinata reaches a size refuge from overgrowth by P. carnea, but this "safe" size depends on the position occupied by a colony of H. echinata on a substrate with respect to physical or biological barriers to growth. The outcome of intraspecific competition for space between P. carnea colonies depends on the relative growth rates of the competitors. In 23 intraspecific contests, the P. carnea colony with the highest rate of stolonal growth was always competitively dominant, and also overgrew H. echinata most rapidly in the size-symmetrical interspecific encounters. The ability of P. carnea to overgrow H. echinata in size-asymmetrical contests, however, did not depend on the growth rate of the P. carnea colony. Data on the distribution and abundance of these two species suggest that P. carnea recruits to hermit crab shells at a low frequency and is thus a rare member of the hermit crab epifaunal community. The observed differences in interspecific competitive ability may reflect asymmetry in the frequencies with which these species encounter one another. The probability that a colony of H. echinata will encounter P. carnea is low, hence there will be little selection for interspecific competitive ability in H. echinata. The probability that a colony of P. carnea will encounter the common H. echinata is high; P. carnea, therefore, should maintain a mechanism for recognizing and overgrowing this important spatial competitor.

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© 1986 Biological Bulletin