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Fig. 6 b13a−/−; c13a−/− double mutants have coiled tails that lack a caudal fin. (A) Adult double mutant zebrafish showing a coiled tail and posterior repositioning of the anal fin. Scale bar, 5 mm. (B) Segmented micro-CT images of the double mutant tail with each vertebra colored differently. The posterior vertebrae are abbreviated and highly fused. (C) Tail vertebral number in double mutants, single mutants, and WTs. ANOVA revealed a significant increase in tail vertebrae [F3,53 = 101.91, P < 0.0001]. Multiple comparisons indicated that, except for the b13a-c13a pair, all comparisons were statistically significant, indicating that both genes act additively contributing to tail vertebrae number. *P < 0.0001. The line indicates the mean and circle size is proportional to the number of individuals. (D) Cleared and stained skeletal preparation of an adult double mutant tail, revealing a spiral tail without caudal fin rays at the tip. Scale bar, 1 mm. (E) Lateral view of a WT tail. (F to H) Ontogenetic sequence of tail transformations in b13a−/−; c13a−/− double mutants. Scale bars, 1 mm. (I) A cleared and stained skeletal preparation of 6.22-mm NL double mutant larva reveals the absence of any cartilaginous or bony element unique to the caudal fin, including no hypural or fin rays. (J) 6.64-mm NL double mutant larva shows two cartilaginous elements and three rudimentary fin rays. (K) Diagram of the same specimen depicting the presence of few fin rays and abbreviated caudal fin, consisting of a single hypural and a single preural element. Scale bars, 0.1 mm [(I) to (K)]. For anatomical terminology, see Fig. 1.