Meeting Abstract
Chitons (Mollusca: Polyplacophora) display a diversity of sensory structures embedded within their eight overlapping shell plates. All chitons have non-pigmented clusters of multimodal sensory cells called aesthetes. In addition to aesthetes, some chitons have image-forming eyes. Still other chitons have modified aesthetes that include an eyespot with a pigmented retina-like structure. To ask how eyes and eyespots may have evolved from aesthetes, we examined the relationships between the molecular, structural, and functional complexities of these sensory organs. We studied molecular and structural complexity by comparing the expression of molecular components of sensory transduction in chitons that represent each character state: a chiton with aesthetes, a chiton with modified aesthetes that include eyespots, and a chiton with aesthetes and eyes. We learned that aesthetes express many molecular components of sensory transduction; that the retina-like structures of eyespots express a subset of the proteins found in aesthetes; and that the photoreceptors in the eyes express fewer molecular components of sensory transduction than aesthetes and eyespots. To address the relationship between structural and functional complexity, we tested how chitons with eyespots respond to changing overhead stimuli and static visual landmarks. Like all chitons, those with eyespots respond to changes in the overhead light field. However, in contrast to chitons with eyes, chitons with eyespots do not distinguish between objects and equivalent, uniform changes in light levels. We also found that chitons with eyespots have the ability to orient to static visual landmarks. We conclude that the eyespots of chitons may represent molecular and structural intermediates between aesthetes and eyes, but they do not appear to represent functional intermediates.
