Fig. 1. 5 dpf zebrafish larva (wildtype) stained for cartilage (blue) and bone (red). A total of nine parameters for cartilage were assessed as shown in panels A (ventral view) and B (lateral view): 1 Length ceratohyal to Meckel's cartilage, 2 Length ceratohyal to posterior end of the head, 3 Width at Meckel's cartilage and palatoquadrate joint, 4 Width at ceratohyal and palatoquadrate joint, 5 Length of ceratohyal superior, 6 Length of ceratohyal inferior, 7 Length of ethmoid plate, 8 Lateral length of Meckel's cartilage. The long central arrow indicates total head length (THL). Scale bar is 200 μm. C: The nine mineralized parameters scored for presence or absence (ventral view): 1) Parasphenoid, 2) Otoliths (4 in total), 3) Cleithrum, 4) Notochordal sheath, 5) Ceratobranchial 5, 6) teeth (on ceratobranchial 5), 7) opercles, 8) Branchiostegal ray 1, 9) Entopterygoid bone. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. A–D: Control and examples of VPA treated larvae at 5 dpf. Larvae are stained with Alcian blue (cartilage) and Alizarin red (mineralization). A: Control larvae. B: Larva treated with 100 μM VPA during 1–13 hpf, showing reduced size of cartilage elements and lacking mineralization of the notochord sheath, branchiostegal ray 1 and the entopterygoid bone (see arrowheads). C: Larva treated with 50 μM VPA during 25–37 hpf, showing small reductions in the length of the ceratohyals (asterisks). D: Larva treated with 100 μM VPA during 25–37 hpf with reduced size of cartilage elements and lacking mineralization of the notochord sheath, ceratobranchial 5 (arrows) and the entopterygoid bone (arrowheads). Scale bars are 100 μm.