Quantifying coral morphology

The morphology of coral colonies has important implications for their biological and ecological performance, including their role as ecosystem engineers. However, given that morphology is difficult to quantify across many taxa, morphological variation is typically shoehorned into coarse growth form categories (e.g., arborescent and digitate). In this study, we develop a quantitative schema for morphology by identifying three-dimensional shape variables that can describe coral morphology. We contrast six variables estimated from 152 laser scans of coral colonies that ranged across seven growth form categories and three orders of magnitude of size. We found that 88% of the variation in shape was captured by two axes of variation and three shape variables. The main axis was variation in volume compactness (cf. sphericity) and the second axis was the trade-off between surface complexity and the vertical distribution of volume (i.e., top heaviness). Variation in volume compactness also limited variation along the second axis, where surface complexity and vertical volume distribution ranged more freely when compactness was low. Traditional growth form categories occupied distinct regions within this morpho-space. However, these regions overlapped due to shape changes with colony size. Nonetheless, four of the shape variables were able to predict traditional growth form categories with 70 to 95% accuracy, suggesting that the continuous variables captured much of the qualitative variation inherently implied by these growth forms. Distilling coral morphology into geometric variables that capture shape variation will allow for better tests of the mechanisms that govern coral biology, ecology and ecosystem services such as reef building and provision of habitat.