Patterns of Stimulated Bioluminescence in Two Pyrosomes (Tunicata: Pyrosomatidae).

Pyrosomes are colonial tunicates that, in contrast with typical luminescent plankton, generate brilliant, sustained bioluminescence. They are unusual in numbering among the few marine organisms reported to luminesce in response to light. Each zooid within a colony detects light and emits bioluminescence in response. To investigate the luminescence responsivity of Pyrosoma atlanticum and Pyrosomella verticillata, photic, electrical, and mechanical stimuli were used. Photic stimulation of 1.5 x 109 photons{middot}s-1{middot}cm-2, at wavelengths between 350 and 600 nm, induced bioluminescence, with the maximum response induced at 475 nm. The photic-excitation half-response constant was 1.1 x 107 photons{middot}s-1{middot}cm-2 at 475 nm for P. atlanticum; P. verticillata had a significantly higher half-response constant of 9.3 x 107 photons{middot}s-1{middot}cm-2. Individual zooids within a colony, however, appeared to have different half-response constants. Stimulus strength influenced recruitment of zooids and, in turn, luminescent duration and quantum emission. Image intensification revealed saltatory propagation of luminescence across the colony, owing to photic triggering among zooids. Repetitive, regular mechanical or electrical stimulation elicited rhythmic flashing characterized by alternating periods of high and low light intensities.