Ecological change and convergence; morphospace of suspension feeding tentaculate metazoans through deep time

Abstract

A diverse set of taxa such as brachiopods, bryozoans, annelids, echinoderms, hemichordates and phoronids, have sub-cylindrical, often ciliated, suspension-feeding structures, here referred to as tentacles. Theoretical models and simulations of these tentacles imply they may be optimised either to maximise flow or interception with suspended food particles. However, no quantitative studies have compared tentacles across phyla, explored how their morphology may be influenced by ecological niche, or tracked how these structures have changed through deep time in different phylogenetic lineages. This study demonstrates the morphological changes in suspension feeders resulting from different ecological conditions in the Cambrian and the Recent. I show that the tentacular morphology of different ecological categories (motility, tiering, feeding, coloniality and phyla) do overlap in places, but may also segregate in distinct regions, suggesting the influence of these factors on the tentacular morphology. Further, the tentacular structures of Cambrian brachiopods, phoronids, entoprocts and hemichordates are more similar to one another than to the tentacles of extant representatives of those phyla. I suggest that different aspects of the striking cross-phylum convergence are due to changes in the constitution of phytoplankton, energy availability and ecological changes through deep time.