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

This paper will examine the comparative adaptive significance of solitary and colonial existence on marine hard substrata with an emphasis on features important to interference competition for space. Solitary animals are distinct individuals which usually are capable of performing all individual functions. Colonial animals are those in which "members of the colony must be physically connected and ... have common ancestry through asexual reproduction" (Boardrnan et al. 1973, pp. vi-vii). The major groups of nonvagile marine animals which live attached to hard substrata are listed in table 1. Phyla are subdivided so far as is necessary to include only sessile attached forms and to separate solitary and colonial groups. Space on which to live is often the most important limiting resource in marine hard substratum environments (Connell 1961a; Dayton 1971; Paine 1974). Solitary and colonial animals differ fundamentally in their growth patterns and other life-history attributes and thus in their ability to use space. I will attempt to demonstrate the functional significance of competitive strategies of solitary and colonial animals for space. The results provide an ecological basis for classification of animals as solitary or colonial, help to explain differences in diversity of the two functional groups, and lead to testable hypotheses about recruitment rates, geographic range, and palatability to predators of spacelimited organisms. I will first document the general distribution of solitary and colonial animals in Jamaican reef environments, then argue that the observed patterns are of general occurrence, and finally analyze the attributes of these organisms which are responsible for their distributions. The adaptive significance of varying morphological strategies of colonial animals are considered elsewhere (Jackson 1978).