Abstract
ABSTRACTAlthough proteins in the green fluorescent protein family (GFPs) have been discovered in a wide array of taxa, their ecological functions in these organisms remain unclear. Many hypothesized roles are related to modifying bioluminescence spectra or modulating the light regime for algal symbionts, but these do not explain the presence of GFPs in animals that are non-luminous and non-symbiotic. Other hypothesized functions are unrelated to the visual signals themselves, including stress responses and antioxidant roles, but these cannot explain the localization of fluorescence in particular structures on the animals. Here we tested the hypothesis that fluorescence might serve to attract prey. In laboratory experiments, the predator was the hydromedusa Olindias formosus (previously known as O. formosa), which has fluorescent and pigmented patches on the tips of its tentacles. The prey, juvenile rockfishes in the genus Sebastes, were significantly more attracted (P<1×10−5) to the medusa's tentacles under lighting conditions where fluorescence was excited and tentacle tips were visible above the background. The fish did not respond significantly when treatments did not include fluorescent structures or took place under yellow or white lights, which did not generate fluorescence visible above the ambient light. Furthermore, underwater observations of the behavior of fishes when presented with a brightly illuminated point showed a strong attraction to this visual stimulus. In situ observations also provided evidence for fluorescent lures as supernormal stimuli in several other marine animals, including the siphonophore Rhizophysa eysenhardti. Our results support the idea that fluorescent structures can serve as prey attractants, thus providing a potential function for GFPs and other fluorescent proteins in a diverse range of organisms.
Highlights
Autofluorescence is a common phenomenon in the natural world, and a variety of molecules can re-emit absorbed photons as light of a longer wavelength
In the family of greenfluorescent proteins (GFPs), which are found in cnidarians, crustaceans, and chordates (Deheyn et al, 2007; Matz et al, 1999; Shagin et al, 2004; Wiedenmann et al, 2002), the fluorescent signal is so strong that the proteins are assumed to be present primarily because their fluorescence serves a role, even though for most GFP-bearing organisms the natural functions remain unclear
It is possible that GFPs serve a physiological function unrelated to their fluorescence (D’Angelo et al, 2008; Palmer et al, 2009), but their quantum efficiency and the morphological and spectral diversity of fluorescent structures that have evolved indicates that this is not the case
Summary
Autofluorescence is a common phenomenon in the natural world, and a variety of molecules can re-emit absorbed photons as light of a longer wavelength. The range of wavelengths decreases rapidly with depth, until predominantly what we perceive as blue light remains In such a monochromatic environment, differential absorption by pigments can only provide coloration in the form of variations in shades of blue (Johnsen, 2005; Marshall, 2000). The presence of fluorescent proteins can be an effective way to produce undersea coloration patterns, with the excitation coming from either ambient or bioluminescent light. These pigments are so long-lived and efficient that this vivid coloration comes at a relatively low metabolic cost (Leutenegger et al, 2007)
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