Fig. 6

Development of the anal and dorsal fins of A. burtoni. a The first dorsal and anal fin Anlagen are visible at hatching day (4 dpf) as a fin fold that is continuous with the caudal fin. In the anal fin fold, the domain of the final anal fin is recognizable by the lack of vascularization (black arrowhead in lower panel), which is restricted to the part of the fin fold that will degenerate. b At 6 dpf, mesenchymal condensations become visible proximally in the posterior dorsal and anal fins (indicated with a black arrowhead for the dorsal fin). These condensations correspond to tissue surrounding the forming fin radials (see transition from 138 hpf to 156 hpf in panel c). c Alcian blue staining shows the progression of dorsal and anal fin formation. At 138 hpf, a fin fold is present without detectable skeletal structures (asterisk). By 156 hpf, fin radials have formed (blue arrowhead), but no ray structures are apparent yet in the fin fold (asterisk). By 172 hpf, fin rays start forming (black arrowhead). The last elements to appear are the distal radials in between the proximal radials and the fin rays, and these appear around 198 hpf. d Initially, fin spines and soft-rays look very similar. By 10 dpf, their differentiation has progressed and clearly shows diverging developmental trajectories. Fin spines have developed a pointy ending, and segmentation is apparent in the soft-rays (black arrowheads). e At 13 dpf, ossification in the soft-rays and spines in dorsal and anal fins has progressed, and clearly demonstrates stronger ossification of the spines compared to soft rays. Upper panel shows bright-field view, lower panel shows fluorescent imaging for Alizarin red staining in the same specimen. Abbreviations: dpf: days post fertilization; hpf: hours post fertilization; AZR: Alizarin red; S: spines, SR: soft-rays