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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 ...

Copepods are among the most abundant and diverse groups of mesozooplankton in the world's oceans. Each species has a certain depth range within which different individuals (of the same life stage and sex) are found. Lipids are accumulated in many calanoid copepods for energy storage and reproduction. Lipid content in some species increases with depth, however studies so far focused mostly on temperate and high-latitude seasonal vertically migrating copepods and compared lipid contents among individuals either from coarse layers or between diapausing, deep-dwelling copepods and individuals found in the photic, near-surface layer. Here we examined whether lipid contents of individual calanoid copepods of the same species, life stage/sex differ between finer depth layers within the upper water column of subtropical and Arctic seas. A total of 6 calanoid species were collected from samples taken at precise depths within the photic layer in both cold eutrophic and warm oligotrophic environments using SCUBA diving, MOCNESS and Multinet. Measurements of lipid content were obtained from digitized photographs of the collected individuals. The results revealed significant differences in lipid content across depth differences as small as 12–15 meters for Mecynocera clausi C5 and Ctenocalanus vanus C5 (Red Sea), Clausocalanus furcatus males and two clausocalanid C5s (Mediterranean Sea), and Calanus glacialis C5 (Arctic). We suggest two possible explanations for the differences in lipid content with depth on such a fine scale: predator avoidance and buoyancy.