Competition on Marine Hard Substrata: The Adaptive Significance of Solitary and Colonial Strategies

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Abstract
Most solitary and colonial animals inhabiting marine hard substrata differ fundamentally in their ability to use space. Colonial animals are superior space competitors because indeterminate growth allows continuous lateral substratum occupation without requiring intervening stages of sexual reproduction and recruitment, and they are less susceptible to fouling and overgrowth. Solitary animals survive in the seas because various morphological and behavioral attributes (escape in size, aggregative behavior) protect them in spatial competition with colonial animals or because predation, physical disturbance, or competition with plants prevents monopolization of substrata by colonial animals. Focus on ecological strategies circumvents arguments regarding the solitary or colonial identity of problematical groups such as sponges. The evolution of specific competition mechanisms by colonial animals has provided the basis for competitive networks which have further favored the evolution of higher diversity within colonial rather than solitary groups. Recognition of the generalist-specialist and early-late successional roles of most solitary vs. colonial animals leads to predictions that solitary animals should recruit more heavily and be more palatable to predators than are colonial animals. Preliminary evidence supports these predictions.