Hierarchical skeletal architecture and ecological tradeoffs in Pacific Northwest sea stars

Sea star armor comes in the form of a highly articulated endoskeleton made up of individual elements called ossicles. Many descriptive studies have been conducted on the basic patterning of sea star skeletons, with differences in ossicle shape forming the basis of some echinoderm phylogenies. However, ossicle function is not related only to individual element morphology, but rather the whole system. In this study, we use micro-computed tomography (CT) to describe and compare skeletal anatomy of nine sea star species from the Salish Sea, Washington, USA. We quantified 14 morphological traits and tested whether or not they were predictors of ecology. We expected to see that differences in the amount of armoring (relative volume of skeleton) arise from varying arrangement and shape of ossicles across distinct regions of the body. For broad comparability, we grouped skeletal elements into five basic types of ossicles. The amount of skeletal armoring across the body varied by at least an order of magnitude across species and differed in its distribution across ossicle types. Heavily armored sea stars invest in larger, boxy body wall ossicles, whereas a reduction in armor volume was often paired with more intricately-shaped body wall ossicles and an increase in the number and complexity of spines.