From trees to octocorals: the role of phenotypic integration, shading, and self-thinning in underwater animal forests

From trees to octocorals: the role of phenotypic integration, shading, and self-thinning in underwater animal forests


Nelson, H. R.


While it is known that octocorals and trees share many similarities in form and function due to their sessile, modular nature, and octocoral communities create dense canopies termed "animal forests," there has been little quantification of whether these two groups are shaped by the same ecological and evolutionary processes. This thesis is comprised of two different studies that take classic concepts developed in the terrestrial plant literature (e.g. phenotypic integration, self-thinning, shading) and examines how well they can be applied in octocorals. In Chapter 2, I examined the extent to which variation in octocoral morphology is constrained by covariation among traits (i.e. integration), including how traits scale with overall colony size (i.e. allometry) by comparing five morphological traits among and within eight species of the common Caribbean octocoral Eunicea. I found that polyp, branch, and colony level traits covary with one another, but the pattern of trait covariation depends on whether covariation is investigated at an intra- or interspecific scale. Species with more branches tended to have shorter, thinner branches. This pattern of integration has been well established for many higher plants, suggesting modularity may have given rise to similar constraints in both groups. This study highlights the role of covariation among traits in constraining the morphology of modular organisms. In Chapter 3, I investigated whether self-thinning models can be applied to mixed-species octocoral assemblages by exploiting natural variation in the density of octocorals at two shallow reefs in St. John, U.S. Virgin Islands. I found a negative power relationship between mean colony width and the density of colonies, which suggests that mixed-species octocoral communities experience density-dependent mortality due to competition (e.g. self-thinning) for space, waterborne resources, and/or light. Light was reduced next to octocorals, but shading could not account for differences in spacing among species. A substantial fraction (50-73%) of the variation in average colony size was not explained by variation in octocoral density. These results suggest that self-thinning models can be applied to mixed-species assemblages, but the predictive power of these size-density relationships may be weakened by asymmetries in competitive abilities or resource use among species.

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Thesis or Dissertation

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