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Development of the American shad Alosa sapidissima (Wilson, 1811) and implications for other actinopterygian fishes
Phylogenetic changes arise by modifications of ancestral ontogenies. However, we are restricted to studying outgroups, not ancestors, so the search for ancestral ontogenies depends on appropriate choice of taxa for study. Alosa sapidissima was chosen for detailed embryological analysis because American shad are members of an early lineage within teleosts and may retain more characters common to the ancestor of teleosts than do more derived lineages. An objective of my research is to determine ancestral ontogenies and modifications that led to modern teleostean ontogenies using outgroup comparisons. I collected a developmental series of hatchery-reared shad spanning the period from fertilization to metamorphosis. Using light and scanning electron microscopy I described embryological development of shad and formulated an anatomical staging system based on external and osteological features. New information about the early development of shad includes: (1) the unique radial polarity of the first neuromast organ to develop, (2) possession of an extensive neuromast line on the trunk in embryonic and larval shad, (3) formation of the autogenous hypural 1 in the caudal fin, and (4) development of gill filaments and operculum long after the yolk-sac larval period, almost at metamorphosis. These features have important functional and physiological implications for shad which need further study. From my description of shad and my working knowledge of embryology I chose a series of developmental events likely to be common to all taxa of teleosts and formulated a representative staging system. I made explicit comparisons between my staging system and published descriptions for other actinopterygian fishes. The staging system and comparisons are summarized in an extensive series of tables, which show heterochronic differences among taxa. Examples of heterochronic shifts include: (1) shifts in form and timing of neurulation, (2) shifts in initial organogenesis relative to epiboly, and (3) shifts in hatching relative to jaw formation. Seven characters were used to examine the phylogenetic relationships of neopterygian fishes. Basic objectives of comparative embryology have not changed but methods used for descriptive embryology and our conception of how to make phylogenetic comparisons have changed. Future work should focus not only on excellent modern descriptions of development but also on increasingly sophisticated phylogenetic comparisons.
Shardo, Judith D, "Development of the American shad Alosa sapidissima (Wilson, 1811) and implications for other actinopterygian fishes" (1994). Doctoral Dissertations Available from Proquest. AAI9434531.