FUNCTIONAL MORPHOLOGY OF PASSIVE SUSPENSION FEEDING IN COMPOSITA (BRACHIOPODA, ATHYRIDIDA)

Throughout the Phanerozoic Eon, diverse invertebrate taxa have subsisted by suspension feeding - the process of capturing and consuming food particles entrained in water. Suspension feeding entails both active feeding, in which fluid flow is induced via pumping by an organism, and passive feeding, in which animals (with or without pumping organs) capture particles entrained in an ambient current. For active suspension feeders such as brachiopods, efficient passive suspension feeding is thought to be advantageous because it can significantly reduce the metabolic expense of biological pumping. Passive suspension feeding depends strongly on morphology; only animals with streamlined body plans that function to induce fluid circulation through the feeding organs are able to capture food in this manner.

The brachiopod Composita is ubiquitous in the Upper Devonian through Permian strata of the North American Mid-Continent. To test the hypothesis that the shell morphology of Composita functioned as an effective passive suspension feeding body plan, we used a two-part experimental approach incorporating morphometric analysis and flume tank observation. Three-dimensional Bookstein shape coordinate analysis was used to examine shape and size trends in specimens of Composita (n=136) from the Late Mississippian through Late Pennsylvanian of Texas, Kentucky, and Iowa. These specimens form a single shape distribution corresponding to progressive vertical displacement of the medial anterior sulcus from the posterior and lateral sections of the commissure with increasing shell size, indicating an ontogenetic shape change trend. To examine the relationship between this trend and passive feeding, we constructed gaping models of three specimens (with a model lophophore in the largest specimen) representative of different ontogenetic stages for observation in the flume. All three models showed medially-inhalant and laterally-exhalant passive circulation when oriented with the sulcus facing upstream, with more vigorous flow tracking increasing shell size and separation of the inhalant and exhalant points (a pattern consistent with predictions based on viscous entrainment). These results suggest that passive feeding influenced the development and evolution of Composita, and probably brachiopods in general.