Colonial radiolaria (Protozoa: Spumellarida) were a conspicuous feature in surface waters of the Sargasso Sea during the April (1985) Biowatt cruise. The abundance of colonies at the sea surface at one station was estimated to be 23 colonies · m−2.
Bioluminescence by colonial radiolaria, representing at least six taxa, was readily evoked by mechanical stimuli and measured by fast spectroscopy and photon-counting techniques. Light emission was deep blue in color (peak emissions between 443 and 456 nm) and spectral distributions were broad (average half bandwidth of 80 nm). Single flashes were 1–2 s in duration at ≈23 °C, with species-dependent kinetics which were not attributed to differences in colony morphology, since colonies similar in appearance could belong to different species (even families) and display different flash kinetics. Although the presence of dinoflagellate symbionts was confirmed by the presence of dinoflagellate marker pigments in the colonies, luminescence in the radiolaria examined most likely did not originate from symbiotic dinoflagellates because of (1) differences in the emission spectra, (2) unresponsiveness to low pH stimulation, (3) differences in flash kinetics and photon emission of light emission, and (4) lack of light inhibition.
The quantal content of single flashes averaged 1 × 109 photons flash−1, and colonies were capable of prolonged light emission. The mean value of bioluminescence potential based on measurements of total mechanically stimulated bioluminescence was 1.2 × 1011 photons · colony−1. It is estimated that colonial radiolaria are capable of producing ≈2.8 × 1012 photons · m−2 of sea surface. However, this represented only 0.5% of in situ measured bioluminescence potential
