Light-Dependent Electrical Activity in Sea Urchin Tube Feet Cells.

Sea urchins can detect and respond to light, and many species of sea urchins are negatively phototaxic. Light detection is hypothesized to occur via photoreceptors located on sea urchin tube feet, and opsins have been detected in tube feet, spines, and the test. However, the molecular mechanisms underlying light detection are, for the most part, unknown. Individual tube feet disc cells were isolated from purple sea urchins (Strongylocentrotus purpuratus), and the electrical responses of these cells to varying levels of illumination were quantified using the patch clamp technique. No currents were observed under bright illumination, whereas under dark conditions, large, slowly activating currents were consistently observed. Two types of cells were functionally identified based on their responses to darkness. Type I cells sustained currents indefinitely in the dark, whereas Type II cell currents spontaneously decayed after several seconds. The large currents observed were composed of the summation of many smaller events that were characterized by a rapid onset and an exponentially decaying component, which may be indicative of direct vesicular release from the tube feet disc cells in response to the dark conditions.