2194 INTRODUCTION In Caribbean Elacatinus (Jordan, 1904) (previously Gobiosoma) gobies, evolutionary shifts from non-mutualists to obligate generalist mutualists coincide with shifts in color patterns associated with advertising this derived status. This lineage of species provides a model framework in which to test signal evolution in a mutualism. Elacatinus (E.) gobies (sensu lato-s.l.) are small (approximately 2–4 cm in length) and habitat restricted, with limited mobility (Rüber et al., 2003; Colin, 1975). The striped pattern of the subgenus (sensu stricto-s.s.) E. is notably different from the most closely related sister species’ body patterns, which are more disruptive or banded (Rüber et al., 2003). These distinctive stripes are potentially conspicuous to visual detection, which makes it of particular interest that a lineage within the subgenus has evolved highly specialized parasite-cleaning behavior. Molecular phylogenetic analysis shows that E. (s.s.) gobies diverged into two behaviorally distinct – obligate sponge-dwelling and cleaning – clades (Taylor and Hellberg, 2005; Rüber et al., 2003). Cleaners derive much or all of their nourishment by cleaning parasites from the bodies of visiting ‘clients’. Limited larval dispersal may contribute to the rapid speciation within the clade (Taylor and Hellberg, 2003) and may be an important mechanism by which selection on fitness-related traits varies and evolves in the marine environment. Sponge dwellers often associate with chemically defended sponge species such as Xestospongia spp. (Pawlik et al., 1995) and Agelas spp. (Assmann et al., 2000) and tend to retreat into sponge cavities if disturbed, while cleaners tend to be found on corals or on the outside of sponges, where they are both more exposed and more visible to passers by and potential predators (L.L., personal observation). All species within the clade have distinctive stripe patterns, which set them apart from other E. (s.l.) species. Basal species within the cleaner lineage have a lateral yellow stripe along both sides of the body, while more recently evolved species have derived green (broad spectrum reflectance that appears iridescent white) or blue lateral stripes (Taylor and Hellberg, 2005). In addition, two species of cleaners exhibit color stripe polymorphism, and the order of evolution within these stripes appears to follow this same trend (Taylor and Hellberg, 2005). Whereas bold black dorsal and sublateral stripes with a yellow or green lateral stripe can be found in both cleaning and non-cleaning gobies, the blue lateral stripe is only observed in cleaners and is significantly associated with a morphological change in mouth position that may be adaptive to cleaning (Taylor and Hellberg, 2005). It has been suggested that the black striped pattern is a conspicuous signal evolved to advertise cleaning behavior (Côté, 2000; Arnal et al., 2006; Stummer et al., 2004) and is a clear example of profile enhancing high contrast edge (Stevens, 2007). Paired with a blue, green or yellow stripe, the pattern may increase signal contrast against typical backgrounds (Marshall, 2000). Interactions where fitness may be tied to visual signaling have the potential to exert selective pressure on color and pattern phenotypes (Stevens, 2007). Traits thought to have fitness consequences tied to contrast or chromatic distinctiveness may appear differently to many potential onlookers in the same microenvironment. Recent studies have used visual models to The Journal of Experimental Biology 212, 2194-2203 Published by The Company of Biologists 2009 doi:10.1242/jeb.025478 Cleaner gobies evolve advertising stripes of higher contrast L. Lettieri 1, *, K. L. Cheney 2 , C. H. Mazel 3 , D. Boothe 1 , N. J. Marshall 4 and J. T. Streelman 1 1 School of Biology and Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, GA 30332-0230, USA, 2 School of Biological Sciences, The University of Queensland, Brisbane, QLD 4072, Australia, 3 Physical Sciences, Inc., 20 New England Business Center, Andover, MA 01810, USA and 4 Sensory Neurobiology Group, School of Biomedical Sciences, The University of Queensland, Brisbane, QLD 4072, Australia *Author for correspondence (e-mail: liliana@gatech.edu) Accepted 27 April 2009 SUMMARY Elacatinus gobies of the Caribbean have undergone rapid speciation along ecological axes, and particular species from this genus act as ‘cleaners’ that remove ectoparasites from larger coral reef fish, termed ‘clients’. Evolutionary shifts in habitat use, behavior and lateral body stripe colors differentiate cleaners from ancestral sponge-dwelling lineages. High-contrast stripe colors associated with cleaning behavior on coral reefs may have evolved as a signal of cleaning status. We asked whether cleaner gobies with blue stripes are more conspicuous than ancestral yellow- and green-stripe phenotypes to a diverse set of potential client visual systems in the tropical reef environment where cleaning stations are commonly observed. Using spectrophotometric measurements of cleaners with blue and yellow stripes and their F 1 hybrid, we tested the contrast of each color stripe to both potential dichromatic and trichromatic reef fish visual systems, against typical coral and sponge microhabitat background colors. Blue stripes provide the highest average chromatic contrast across a range of possible microhabitat colors to the majority of fish visual systems tested. The contrast provided by yellow and hybrid green stripes are comparable across habitats to dichromatic visual systems. The green stripe is less contrasting than both blue and yellow to many potential trichromatic visual systems. We suggest that the evolution of blue stripes in Elacatinus gobies could be a result of natural selection for signals of high color contrast, driven by the sensory biases and visual systems of diverse reef fish clients. Supplementary material available online at http://jeb.biologists.org/cgi/content/full/212/14/2194/DC1 Key words: specialization, mutualism, adaptive radiation, color, vision model. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY