Neurophysiological characterization of the photoreceptor system in a brittlestar, Ophiocoma wendtii (Echinodermata: Ophuroidea)

1. 1. The photosensitivity of Ophiocoma wendtii can be detected neurophysiologically by recording from single units within the nerve cord of isolated arms. 2. 2. Individuals show a strong light “ON” response and a relatively weak light “OFF” response, consistent with whole organism behaviour. 3. 3. The presence of an intraskeletal primary photoreceptor is corroborated by monitoring nervous response to light after the chemical disruption of successive layers of the integument. 4. 4. Results support the hypothesis that microscoplc, transparent structures of the skeletal plates (EPT), and their associated chromatophores and nerve bundles comprise a photoreceptor system. 5. 5. Analogous specializations in other echinoderms are suggested to account for “extraocular photo-sensitivity” that characterizes the phylum.