Investigation of Ontogeny, Homology and Heterochrony in the Siluriform Skeleton

Abstract

Catfishes (Otophysi: Siluriformes) is a remarkably diverse assemblage (~4000 spp) that is distributed across the globe in fresh and marine waters. Catfishes are united by several modifications of the skeleton, including the extreme modification of certain elements (e.g., the pectoral-fin spine) and the presence of several bones that are currently presumed to be the result of fusion (e.g., the parieto-supraoccipital). In light of this, there have been a vast number of anatomical investigations of the adult skeleton in catfishes; however, comprehensive information on early development of the skeleton remains scarce. This dissertation reports detailed information on the development of the skeleton in catfishes to address issues of homology, provide ontogenetic information on a systematically important character complex, and assess the role that heterochrony may have played in the evolution of the skeleton. I provide a detailed description of skeletal development for two species of North American catfish, Ictalurus punctatus and Noturus gyrinus. Development of the skeleton was complete by 22.4 mm SL in I. punctatus and 14.1 mm SL in N. gyrinus, excluding the dorsal- and anal-fin distal radials in the latter. No major differences were identified between the ossification sequences compiled for each species. No signs of ontogenetic fusion were observed in previously purported compound elements. I also examined the development of the pectoral-fin spine across the order to determine if it develops from a single ontogenetic pathway and standardize terminology of the spine ornamentation for use in systematic studies. The earliest stages of pectoral-fin spine development were highly conserved across the order and most of the morphological diversity of the structure can be attributed to the presence/absence of five traits (distal rami, anterior/posterior serrae, denticuli, and odontodes). Finally, I compare the ossification sequences of four catfishes and 3 non-siluriform otophysans in order to determine what heterochronic shifts (changes in the relative timing of developmental events), if any, are characteristic of catfishes. Eight different bones were found to be shifted in their appearance within the ossification sequence of catfishes, including the morphologically diverse and functionally important pectoral-fin spine.