title: The consistent difference in red fluorescence in fishes across a 15 m depth gradient is triggered by ambient brightness, not by ambient spectrum.
abstract: BACKGROUND
  Organisms adapt to fluctuations or gradients in their environment by means of genetic change or phenotypic plasticity. Consistent adaptation across small spatial scales measured in meters, however, has rarely been reported. We recently found significant variation in fluorescence brightness in six benthic marine fish species across a 15 m depth gradient. Here, we investigate whether this can be explained by phenotypic plasticity alone, using the triplefin Tripterygion delaisi as a model species. In two separate experiments, we measure change in red fluorescent brightness to spectral composition and ambient brightness, two central parameters of the visual environment that change rapidly with depth.
  
  
  RESULTS
  Changing the ambient spectra simulating light at -5 or -20 m depth generated no detectable changes in mean fluorescence brightness after 4-6 weeks. In contrast, a reduction in ambient brightness generated a significant and reversible increase in mean fluorescence, most of this within the first week. Although individuals can quickly up- and down-regulate their fluorescence around this mean value using melanosome aggregation and dispersal, we demonstrate that this range around the mean remained unaffected by either treatment.
  
  
  CONCLUSIONS
  We show that the positive association between fluorescence and depth observed in the field can be fully explained by ambient light brightness, with no detectable additional effect of spectral composition. We propose that this change is achieved by adjusting the ratio of melanophores and fluorescent iridophores in the iris.
authors.affiliation: Animal Evolutionary Ecology, Institution for Evolution and Ecology, Department of Biology, Faculty of Science, University of Tuebingen, Auf der Morgenstelle 28, 72076, Tuebingen, Germany. ulrike.harant@uni-tuebingen.de.
authors.forename: Ulrike Katharina
authors.initials: UK
authors.lastname: Harant
authors.affiliation: Animal Evolutionary Ecology, Institution for Evolution and Ecology, Department of Biology, Faculty of Science, University of Tuebingen, Auf der Morgenstelle 28, 72076, Tuebingen, Germany. nico.michiels@uni-tuebingen.de.
authors.forename: Nicolaas Karel
authors.initials: NK
authors.lastname: Michiels
authors.affiliation: Animal Evolutionary Ecology, Institution for Evolution and Ecology, Department of Biology, Faculty of Science, University of Tuebingen, Auf der Morgenstelle 28, 72076, Tuebingen, Germany. nils.anthes@uni-tuebingen.de.
authors.forename: Nils
authors.initials: N
authors.lastname: Anthes
authors.affiliation: Animal Evolutionary Ecology, Institution for Evolution and Ecology, Department of Biology, Faculty of Science, University of Tuebingen, Auf der Morgenstelle 28, 72076, Tuebingen, Germany. MMeadows@francis.edu.|Biology Department, Saint Francis University, P.O. Box 600, Loretto, PA, 15940-0600, USA. MMeadows@francis.edu.
authors.forename: Melissa Grace
authors.initials: MG
authors.lastname: Meadows
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D003118:Color Perception / Q000502:physiology*; D003867:Depth Perception / Q000502:physiology*; D005260:Female; D005399:Fishes / Q000502:physiology*; D005453:Fluorescence; D007498:Iris / Q000502:physiology* / Q000648:ultrastructure; D008297:Male; D020460:Melanosomes / Q000502:physiology* / Q000648:ultrastructure