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This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

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Fish larvae develop a fin fold that will later be replaced by the median fins. I hypothesize that fin fold reabsorption is part of the initial patterning of the median fins, and that caspase-3, an apoptosis marker, will be expressed in the fin fold during reabsorption. I analyzed time series of larvae in the first 20-days post hatch (dph) to determine timing of median fin development in a basal bony fish - sturgeon- and in zebrafish, a derived bony fish. I am expecting the general activation pathway to be conserved in both fishes but, the timing and location of cell death to differ. The dorsal fin fold is the first to be reabsorbed in the sturgeon starting at 2 dph and rays formed at 6dph. This was closely followed by the anal fin at 3 dph, rays at 9 dph and only later, at 6dph, does the caudal fin start forming and rays at 14 dph. In zebrafish, reabsorption of the anal fin fold began to occur around 6 dph but, the dorsal fin did not begin until around 12dph, while in the caudal fin it did not occur until 18 dph. Contrary to the sturgeon larvae, there were no rays formed by day 19 dph, but all fins had started forming, and reabsorption of the fin fold was underway. In both species the timing of fin folding reabsorption is distinct and could be one of the drivers of differences in median fin shape. Zebrafish larvae were incubated with caspase-3 mouse antibody and then with a secondary antibody - fluorescent marker. Caspase-3 activity was seen at the edges of what would later become the anal fin at 6 dpf, in the same regions of the dorsal fin at 10 dph, and in the caudal fin edge at 6 dph. Caspase-3 and thus cell death can clearly be linked to fin patterning in fish. A histological analysis was performed on muscle fibers of bluegill sunfish fins to determine size and orientation of the fibers in the different locations. Collagen was also observed in developing zebrafish larvae fins at 12 dph where actinotrichia was expected to be present.