VISUAL WAVELENGTH DISCRIMINATION BY THE LOGGERHEAD TURTLE, CARETTA CARETTA by Morgan Young A Thesis Submitted to the Faculty of The Charles E. Schmidt College of Science in Partial Fulfillment of the Requirements for the Degree of Master of Science Florida Atlantic University Boca Raton, Florida May 2012 Copyright by Morgan Young 2012 ii VISUAL··WAVELENGTH··DISCRIMINATION BY·THE LOGGERHEAD·TURTLE, CARETTA CARETTA by This thesis was prepared under the direction of the candidate's advisor, Dr. Michael Salmon, Department of Biological Sciences, and has been approved by the members of her supervisory committee. It was submitted to the faculty.()f the-Charles E. Schmidt College of Science and was accepted in partial fulfillment ofthe requirements for the degree ofMaster ofScience. Michal Salmon, Ph.D. Thesis Advisor ~~.tJ~ Gary .PerrPh. ~CharlesE.Sc. .dtCollege Of.• ·Science ~ /~':?7 r.~.~ BarryT. R08s0n, Ph.D. Dean, Graduate College 111 ACKNOWLEDGEMENTS I would like to thank my graduate advisor, Dr. Michael Salmon, for his support, guidance and patience throughout my graduate studies. I would also like to thank my committee members, Jeanette Wyneken and Tammy Frank. I thank FAU undergraduate students Mrs. Alexandra Kogan and Mr. Justin Vining for their assistance. I greatly appreciate Mark Royer’s help in construction of the training apparatus. I would also like to thank T. Warraich, L. Bachler, A. Lolaver, A. Loson, Z. Anderson, M. Rogers, B. Resnick, I. Pokotylyuk and M. Glider for their help with the husbandry of turtles used in this study. I would like to acknowledge and thank the National Save the Sea Turtle Foundation for funding this research. The hatchlings used in this study came from nests managed by the Gumbo Limbo Nature Center, Boca Raton, Florida, U.S.A; their assistance is greatly appreciated. Permits to complete this study were issued by the Florida Fish and Wildlife Conservation Commission (TP 173) and the FAU Institutional Animal Care and Use Committee (A10-12). iv ABSTRACT Author: Morgan Young Title: Visual Wavelength Discrimination by the Loggerhead Turtle, Caretta Caretta Institution: Florida Atlantic University Thesis Advisor: Dr. Michael Salmon Degree: Master of Science Year: 2012 Little is known about the visual capabilities of marine turtles. The ability to discriminate between colors has not been adequately demonstrated on the basis of behavioral criteria. I used a three-part methodology to determine if color discrimination occurred. First, I exposed naïve, light-adapted hatchlings to either a blue, green or yellow light. I manipulated light intensity to obtain a behavioral phototaxis threshold to each color, which provided a range of intensities we knew turtles could detect. Second, I used food to train older turtles to swim toward one light color, and then to discriminate between the rewarded light and another light color; lights were presented at intensities equally above the phototaxis threshold. Lastly, I varied light intensity so that brightness could not be used as a discrimination cue. Six turtles completed this task and showed a clear ability to select a rewarded over a non-rewarded color, regardless of stimulus intensity. Turtles most rapidly learned to associate shorter wavelengths (blue) with food. v My results clearly show loggerheads have color vision. Further investigation is required to determine how marine turtles exploit this capability. vi VISUAL WAVELENGTH DISCRIMINATION BY THE LOGGERHEAD TURTLE, CARETTA CARETTA LIST OF TABLES ............................................................................................................. ix LIST OF FIGURES ............................................................................................................ x INTRODUCTION .............................................................................................................. 1 METHODS ......................................................................................................................... 5 Hatchlings: Study site and turtle acquisition .................................................................. 6 Hatchling experiments .................................................................................................... 6 Phototaxis testing procedure ........................................................................................... 7 Training juvenile turtles .................................................................................................. 9 Apparatus and single light training ............................................................................... 10 Paired light training....................................................................................................... 11 Wavelength discrimination ........................................................................................... 11 RESULTS ......................................................................................................................... 13 Stimulus measurements ................................................................................................ 13 Hatchling phototaxis trials ............................................................................................ 13 Single light training....................................................................................................... 14 Paired light training....................................................................................................... 15 Wavelength discrimination ........................................................................................... 15 DISCUSSION ................................................................................................................... 16 Experimental approach ................................................................................................. 16 Training efficacy and stimulus “bias” ........................................................................... 20 APPENDIX ....................................................................................................................... 24 Anatomy and physiology of the vertebrate eye ............................................................ 31 Visual ecology .............................................................................................................. 32 Evolution of color vision .............................................................................................. 34 vii LITERATURE CITED ..................................................................................................... 37 viii LIST OF TABLES Table 1. Hatchlings’ response in phototaxis trials ........................................................... 24 Table 2. Numbers of “assisted”, “unassisted”, and total trials during single light training .............................................................................................................................. 25 ix LIST OF FIGURES Figure 1. Light training apparatus .................................................................................... 26 Figure 2. Spectra for 3 colors used in phototaxis and light training ................................ 27 Figure 3. Number of trials required to complete criterion ............................................... 28 Figure Legends ................................................................................................................ 29 x INTRODUCTION Many animals respond behaviorally to differences in object color. These responses range from phototaxes (orientation towards or away from the light source) to discrimination between objects differing in color. Color vision is defined as the ability to distinguish between light stimuli on this basis, independently of differences in light intensity (Hailman and Jaeger 1971; Cronin 2007). Such a capacity is exploited by animals for many functions such as finding food, avoiding predators, orienting toward favorable habitats, reducing conspicuousness (e.g., camouflage), increasing conspicuousness (distasteful and/or poisonous species, or their mimics), species and/or individual recognition, and mate choice (Bradbury and Vehrencamp 1998; Kelber et al. 2003; Siddiqi et al. 2004; Cronin 2007). Presumably, these capabilities evolve if doing so improves probabilities of survival and/or reproductive success. If color is to be perceived, two requirements must be met. First, the animal must possess at least two different types of retinal (cone) photoreceptors; second, the animal must have neural circuits in the brain designed to contrast signals received from those two groups of photoreceptors so that they can be perceived and responded to behaviorally (Jacobs and Rowe 2004). Thus, physiological study alone is unable to provide conclusive evidence for color vision; that can only be done on the basis of appropriate 1 behavioral studies (Bradbury and Vehrencamp 1998; Birgersson et al. 2001; Kelber et al. 2003). The visual capabilities of marine turtles have been studied primarily from a physiological perspective (Reviewed by Bartol and Musick 2003). Most of the behavioral studies, centering on visual orientation of hatchlings from the nest to the ocean, have been done at night (e.g., Mrosovsky and Carr 1967; Mrosovsky and Shettleworth 1968; Mrosovsky 1972; Witherington and Bjorndal 1991a, b), when rod vision dominates. Thus, there is no conclusive evidence that these animals perceive color (see Appendix). However, it is likely that they do as their closest relatives do. Freshwater turtles