Ontogeny of the Oromandibular Structures in the Algivorous and Federally Threatened Devils River Minnow, Dionda diaboli, with an Overview of Keratinized Oral Surfaces in North American Minnows (Teleostei: Cyprinidae)
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Algivory is a widespread phenomenon in aquatic animals that involves the consumption of nutrient poor algae. Algivorous fishes often exhibit several common morphological adaptations to contend with this type of diet, including an elongated alimentary tract and the presence of unicuspid or multicuspid teeth in dentulous fishes to facilitate removal of algae from the substrate. Keratinization of the epithelium covering the mouthparts has been hypothesized to facilitate oral scraping of algae from the substrate in the edentulous algivorous fishes of the family Cyprinidae (carps, minnows, and their relatives). Keratinization of the mouthparts has been well studied in Old World members of the Cyprinidae but remains understudied in the North American (New World) members of this successful group. This study represents the first to investigate the nature and extent of oral keratinization in the New World Members of the Cyprinidae. In the first part of this study, I use a combination of scanning electron microscopy (SEM) and histology to investigate the ontogeny of keratinization on the epithelium covering the jaw bones in Dionda diaboli, a federally threatened algivore that is currently the focus of a USFWS captive propagation program with high levels of mortality. Individuals of D. diaboli ≥9.0 mm SL exhibit a transverse band of keratinized epithelium covering the jaw bones. Histochemical investigation using A-S and H&E staining indicates that keratin is restricted to cells of superficial layer of the transverse band, which exhibit polygonal truncate unculi in individuals of 9.0-25.0 mm SL. In larger individuals (≥25.0 mm SL), keratinized cells of the transverse band lack unculi and exhibit a flaky appearance. Expansion of the transverse band on the jaws occurs concurrently with elongation and coiling of the gut, and the addition of algal matter into the diet of hatchery individuals to coincide with the first appearance of a keratinized jaw epithelium could be beneficial to the captive rearing of this threatened species. In the second part of this study, I use SEM and histological techniques to investigate the nature and extent of oral keratinization in 55 species (representing 50 genera) of North American minnows, including members of both the ‘Shiner’ clade and the ‘Western’ clade. A phylogenetic comparative approach was used to investigate the relationship between the structure and length of the gut tract relative to oral keratinization in members of the ‘Western’ and ‘Shiner’ clades. Three general states of keratinized oral epithelia were observed among the species of North American minnow examined, including non-keratinized, keratinized squamous, and unculiferous. A non-keratinized epithelium was observed only in one species (Hybognathus nuchalis). The remaining character states were observed in both algivores and non-algivores, and a positive correlation was detected between gut coiling and keratinized oral epithelium within the ‘Western’ clade but not the ‘Shiner’ clade. No correlation exists between the percent of keratinized oral epithelium covering the surface of the jaws and gut length within the ‘Western’ clade but a negative correlation was detected between these two traits within the ‘Shiner’ clade. The high prevalence of keratinized oral surfaces in non-algivorous fishes suggests that this character may have more complex roles in the life history of many species.
Mock, Iliana Marie (2017). Ontogeny of the Oromandibular Structures in the Algivorous and Federally Threatened Devils River Minnow, Dionda diaboli, with an Overview of Keratinized Oral Surfaces in North American Minnows (Teleostei: Cyprinidae). Master's thesis, Texas A & M University. Available electronically from