Photogrammetry for coral structural complexity: What is beyond sight?

Coral function largely depends on its structural complexity; for instance, corals with complex shapes can potentially provide more and diverse shelter than those with less complex shapes. However, the accurate determination of complexity traits has been historically difficult, so underwater photogrammetry has become popular in coral reef studies. Although the usefulness of photogrammetry in marine studies is undeniable, understanding the limitations and capabilities of this novel technique is crucial. Here, we compared the performance of photogrammetry against a high-precision structured light scanner in determining six morphological traits, four of which are related to structural complexity. We obtained 3D models from coral skeletons of 12 Caribbean species using both techniques, and then we performed a series of discriminant analyses to identify differences in size and morphological traits as well as species-specific biases. Our results show that there were small though significant differences between the two techniques in the determination of morphological traits, especially those that related to variation in volume compactness (sphericity and packing) and surface complexity (fractal dimension). Therefore, photogrammetry tended to underestimate coral complexity, but rendered similar values regarding areas and volumes of the corals. Nevertheless, the morphological traits obtained from photogrammetry models allow for the correct classification and separation of coral species with different morphologies, even of those with more complex shapes. These results demonstrate that photogrammetry may provide the correct shape description of coral species and is able to quantify coral complexity.