Axial muscle-fibre orientation in developing larval zebrafish.

The fast axial muscles of larval bony fish power rapid escape responses, crucial for survival under high predation pressure. In adult and juvenile teleosts, these muscles follow a pseudo-helical pattern, while those of larval zebrafish (Danio rerio) exhibit a helical arrangement. However, its developmental time line remains unclear. We analysed muscle-fibre orientation in genetically modified zebrafish from 2 to 13 days post-fertilization (dpf) using 3D fluorescence confocal microscopy. Fibre angles were quantified relative to the notochord, and a model of circular, concentric helices was fitted to assess helical trajectories across development. Our results show that a helical pattern is already present at 2 dpf, tapering towards the tail. The pattern remains stable over the first 11 days post-hatching, with decreasing projection angle variation toward the tail, particularly in younger larvae. Across developmental stages, helix centres align at corresponding normalized positions along the notochord. This study highlights the early presence of a helical fibre arrangement in larval fish, with no evidence of pseudo-helical deviations up to 13 dpf.