Morphological diversity within the Ozark minnow (Notropis nubilus: Leuciscidae).

The Ozark minnow, Notropis nubilus, is a small stream fish that has a disjunct distribution in the Ozark Plateau and upper Mississippi River basin. Three reciprocally monophyletic and deeply divergent lineages have been hypothesized within the species based on molecular data. These lineages are allopatric and isolated from each other. The objective of this study was to test the hypothesis that these lineages and the disjunct population in the upper Mississippi River basin are morphologically distinct. Meristic and geometric morphometric data were used to identify and quantify morphological diversity within the Ozark minnow. Analyses of the meristic data and a principal component analysis of the morphometric data were unable to find any noticeable differences in morphology among groups. However, canonical variates analyses of the morphometric data and linear models were able to define statistically significant differences in shape. Analyses of all-individuals were able to identify shape differences between all groups. Males-only analyses were less conclusive, but there was some indication that males may be diverging more quickly than all-individuals. The detection of subtle variation in shape implies selection is not a strong factor in morphological divergence and observed differences are most likely due to morphological drift. This indicates that the lineages within the Ozark minnow are likely on the trajectory for speciation. The allopatric nature of these clades makes the Ozark minnow an interesting model for the study of morphological drift and speciation.

Histological development and integration of the Zebrafish Weberian apparatus.

BACKGROUND: The Weberian apparatus enhances hearing in otophysan fishes, including Zebrafish (Danio rerio). Several studies have examined aspects of morphological development of the Weberian apparatus and hearing ability in Zebrafish. A comprehensive developmental description including both hard and soft tissues is lacking. This information is critical for both interpretation of genetic developmental analyses and to better understand the role of morphogenesis and integration on changes in hearing ability. RESULTS: Histological development of hard and soft tissues of the Weberian apparatus, including ossicles, ear, swim bladder, and ligaments are described from early larval stages (3.8 mm notochord length) through adult. Results show a strong relationship in developmental timing and maturation across all regions. All required auditory elements are present and morphologically integrated early, by 6.5 mm SL. Dynamic ossification patterns and changes in shape continue throughout the examined developmental period. CONCLUSIONS: This study provides the first comprehensive histological description of Weberian apparatus development in Zebrafish. Morphological integration was found early, before increases in hearing ability were detected in functional studies (>10 mm total length), suggesting morphological integration precedes functional integration. Further research is needed to examine the nature of the functional delay, and how maturation of the Weberian apparatus influences functionality.

Histology and structural integration of the major morphologies of the Cypriniform Weberian apparatus.

The Weberian apparatus, a diagnostic feature of otophysan fishes, is a novel hearing adaptation integrating several developmental and morphological systems (ear-vertebral column-swim bladder). Otophysan fishes are one of the largest and most successful freshwater clades, with over 10,000 species across most continents. The largest otophysan order, Cypriniformes, dominates the freshwaters of Asia, Europe, North America, and Africa. Spanning such a wide variety of environments, the Weberian apparatus undergoes morphological modifications to maintain functionality. Within Cypriniformes, we propose three distinct morphological classes of the Weberian apparatus based on the level of skeletal expansion around the swim bladder: simple (typical of most Cyprinidae), anterior plate (found in families such as Gyrinocheilidae, Catostomidae, and Botiidae), and encapsulated (either single-capsule as found, e.g., in Gobionidae and Cobitidae, or double-capsule as found, e.g., in Nemacheilidae and Balitoridae). Little ontological or comparative data exists regarding the construction or integration of these different morphologies, and less is known about the tissue level integration and variation within these morphologies. We used paraffin histology to document the hard and soft tissue anatomy of the Weberian apparatus in six species representing all morphological classes. We found sites of similarity across the morphologies including size and structure of the saccule, aspects of ossicle ossification, and swim bladder tunica composition, indicating potential sites of developmental and functional constraint. In contrast, we found differences across both auditory and nonauditory features in otic chamber size, ossification within ossicles and other vertebral elements, and composition of ligaments, indicating likely sites of adaptability. Some of these changes are likely evolutionary (taxonomic), but may be influenced by the environmental niche occupied by the clade. These results show a clear need for increased ontological and comparative study of the complete cypriniform Weberian apparatus, particularly histologically, as well as increased auditory studies across morphological types.