An animation showing this beautiful spider in Normal light and in UV light.
The evolution of fluorescence is largely unexplored, despite the newfound occurrence of this phenomenon in a variety of organisms. We document that spiders fluoresce under ultraviolet illumination, and find that the expression of this trait varies greatly among taxa in this species-rich group. All spiders we examined possess fluorophores in their haemolymph, but bright fluorescence appears to result when a spider sequesters fluorophores in its setae or cuticle. By sampling widely across spider taxa, we determine that fluorescent expression is labile and has evolved multiple times. Moreover, examination of the excitation and emission properties of extracted fluorophores reveals that spiders possess multiple fluorophores and that these differ among some families, indicating that novel fluorophores have evolved during spider diversification. Because many spiders fluoresce in wavelengths visible to their predators and prey (birds and insects), we propose that natural selection imposed by predator–prey interactions may drive the evolution of fluorescence in spiders.
Fluorescence occurs when molecules called fluorophores absorb light at one wavelength and then emit light at a longer wavelength. In recent years, fluorescence has been described from a disparate array of living organisms (e.g. Mazel et al. 2004; Gandia-Herrero et al. 2005; Haddock et al. 2005). Still, we understand little about the taxonomic distribution, evolutionary history or function of this trait.
In corals, fluorophores are widely distributed taxonomically, and a variety of fluorescent proteins have evolved from a common ancestral protein (Labas et al. 2002; Ugalde et al. 2004). However, the possible functional role(s) of fluorescence is unknown. Likewise, all known species of scorpions have cuticles that fluoresce, suggesting that fluorescence may not play an ecological role (e.g. Fasel et al. 1997; Frost et al. 2001). In contrast, fluorescence is known from only a few types of birds (parrots) and crustaceans (mantis shrimp), but has been suggested to function as a visual signal in intraspecific communication in each of these organisms (Arnold et al. 2002; Mazel et al. 2004). Thus, fluorescence appears to be distributed haphazardly across the tree of life, but has the potential to function adaptively in at least some organisms.
We document that spiders, a species-rich and ecologically diverse group of organisms, possess fluorophores and can fluoresce. Remarkably, the externally visible expression of UV-induced fluorescence varies considerably both among portions of the body in individual spiders and from species to species. The expression of fluorescence appears to be controlled by sequestration of fluorophores in different regions of the body (figure 1a,b), suggesting that natural selection may act to control expression. We provide evidence that spiders are the only group known in which fluorescence is (i) taxonomically widespread, (ii) variably expressed, (iii) evolutionarily labile, and (iv) probably under selection and potentially of ecological importance for intraspecific and interspecific signalling.