Species identity rather than colony size partitions shape variation in six Caribbean octocorals

Abstract

Caribbean reefs are increasingly dominated by arborescent octocorals as scleractinian cover declines. The physical structure of these octocoral forests is established by the manner in which octocoral species generate and modify colony form as they grow. We tested whether colony shape exhibited species-specific architecture and whether it changed allometrically with size. Two-dimensional projections of colony form were modeled as a series of stacked bands, whose areas were measured from scaled in situ images of colonies of six sympatric Caribbean octocorals from St John, US Virgin Islands. Principal component analysis (PCA) of relative band areas resolved a single dominant axis, accounting for 67.9% of the variation, that separated top-heavy from bottom-heavy colony profiles. Species identity explained 42% of the total variation in relative form, whereas colony height only explained 2% and had no detectable effect on either PCA axis. The proportion of the colony outline occupied by branches increased with height, from a predicted 41% at 20 cm to 52% at 100 cm. Colonies therefore grew without a detectable redistribution of area along their vertical axis, filling a species-specific outline more densely rather than moving along a common allometric trajectory. The vertical geometry of an octocoral canopy may therefore depend more on which species are present than on the sizes its colonies have reached.