The benefits of bioluminescence for nonsymbiotic marine bacteria have not been elucidated fully. One of the most commonly cited explanations, proposed more than 30 y ago, is that bioluminescence augments the propagation and dispersal of bacteria by attracting fish to consume the luminous material. This hypothesis, based mostly on the prevalence of luminous bacteria in fish guts, has not been tested experimentally. Here we show that zooplankton that contacts and feeds on the luminescent bacterium Photobacterium leiognathi starts to glow, and demonstrate by video recordings that glowing individuals are highly vulnerable to predation by nocturnal fish. Glowing bacteria thereby are transferred to the nutritious guts of fish and zooplankton, where they survive digestion and gain effective means for growth and dispersal. Using bioluminescence as bait appears to be highly beneficial for marine bacteria, especially in food-deprived environments of the deep sea.B ioluminescence is common in the marine environment, occurring in numerous organisms, from bacteria to invertebrates and fish (1, 2). Bacterial bioluminescence occurs as a continuous glow in the presence of oxygen at cell concentrations exceeding quorum-sensing levels (3-6). Luminous bacteria occur free-living in seawater (7,8), in symbiotic associations with marine organisms (most notably fish and squids; see refs. 7 and 8 and references therein), as saprophytes on suspended organic material such as marine snow (9, 10), as a major component of fecal pellets (11-13), and as parasites on crustaceans (14).Although the adaptive benefits of energetically costly bioluminescence in symbiotic bacteria are well understood (e.g., 7, 15), those benefits in nonsymbiotic bacteria and those living as ectoparasites on zooplankton are less obvious. Several different physiological and biochemical functions of bacterial bioluminescence have been proposed (7,(16)(17)(18)(19)(20), focusing mostly on antioxidative activity, enhanced DNA repair, and UV resistance, although the validity of some of these hypotheses has been questioned (21).An ecological function in propagation and dispersal also has been postulated (6,7,22). According to this hypothesis (hereafter, "bait hypothesis"), the bacteria, by glowing, visually mark the presence of a food particle for fish in order to get into their nutritious guts. So far, this hypothesis was supported by circumstantial evidence showing that luminous bacteria thrive in and survive passage through fish guts (7,12,23,24). Here we propose that the mechanism underlying the bait hypothesis is based on the following steps: (i) Quorum sensing assures that bacterial bioluminescence is a reliable signal of the presence of food aggregates, e.g., marine snow; (ii) zooplankton is attracted to luminous particles and grazes on the bacteria-rich organic matter; (iii) because of its contact with or ingestion of the luminous bacteria, the zooplankton itself becomes glowing; (iv) the glowing zooplankton is detected readily and consumed by fish; (v) once in the ...