Meeting Abstract
Understanding the evolution of complex structures like eyes has long fascinated biologists. Although many assume that eyes derive from simpler precursors, this hypothesis has rarely been demonstrated with explicit phylogenetic methods. We examine the hypothesis that light sensing components of eyes in Cnidaria had an earlier history, and were used in simpler functions. Light sensing structures are common among medusozoans, in which they vary from pigment spots to pigment cup ocelli and complex eyes. Molecular, behavioral and pharmacological evidence indicate that light modulates cnidocyte discharge in the hydrozoan Hydra vulgaris Pallas, 1976. By surveying literature and incorporating new experimental data, we test two predictions of the hypothesis that light sensing functions of eyes had an earlier role in modulating cnidocytes. First, we predict that light modulated cnidocyte firing can be reconstructed as an ancestral state. Second, we predict that eyes assembled later in the history of Cnidaria. We show that three lineages of non-medusozoans (Actiniaria, Octocorallia, and Corallimorpharia) exhibit significant difference between the numbers of nematocysts discharged under two blue light (470 nm) intensities (dim, 0.1W/cm²; bright, 2.8W/cm²). Based on a working phylogenetic hypothesis, we also show that eyes and ocelli were absent from the cnidarian ancestor. Our results suggest that cnidocyte discharge regulation by light predates the emergence of light sensing organs, and both potentially use a homologous opsin-based phototransduction pathway. As such, this represents an empirical case in which an opsin-based behavior might have predated this pathway’s function in light sensing organs as complex eyes.
