Fish Scales: Morphology, Evolution, and Function

Abstract

Fish scales are plates of dermal bone that are embedded in the dermis and epidermis of most fish species. Scales occur in many forms and past research has provided a framework to categorize scale types and a foundation for describing scale morphology. This thesis seeks to further our knowledge of scale morphology, evolution, and function using a combination of established, modern, and novel methods to quantify scale morphology in both intra- and inter-species contexts. In my first chapter, I describe how a method called gel-based stereo-profilometry can be used to image and quantify the three-dimensional topography of biological surfaces of interest, especially scales. In my second chapter, I use both gel-based profilometry and micro-computed tomography (μCT) to show that scale morphology of the bluegill (Lepomis macrochirus) is different qualitatively and quantitatively among regions of its body. In my third chapter, I use μCT and histology to study scale morphology in bigeye tuna (Thunnus obesus) and I show that regions on the anterior body of bigeye tuna have large scales made of cellular bone that are filled with adipocytes. In my fourth chapter, I quantify scale morphology across 59 damselfish (Pomacentridae) species that exhibit multiple transitions between pelagic and benthic feeding ecologies. With this comparative approach, I ask if scale morphology fits models of ecologically-driven evolution, thus seeking to link scale morphology to function. Finally, in my fifth chapter I briefly review our knowledge of elasmoid scale morphology and function, and I also demonstrate the quantitative effect mucus and epidermis have on scale morphology by in vivo imaging the surface topography of seven fish species.