Fish larvae exploit edge vortices along their dorsal and ventral fin folds to propel themselves

Larvae of bony fish swim in the intermediate Reynolds number (Re) regime,
using body- and caudal-fin undulation to propel themselves. They share a
median fin fold that transforms into separate median fins as they grow into
juveniles. The fin fold was suggested to be an adaption for locomotion in the
intermediate Reynolds regime, but its fluid-dynamic role is still enigmatic.
Using three-dimensional fluid-dynamic computations,we quantified the swimming trajectory frombody-shape changes during cyclic swimming of larval fish. We predicted unsteady vortices around the upper and lower edges of the fin
fold, and identified similar vortices around real larvaewith particle image velocimetry. We show that thrust contributions on the body peak adjacent to the
upper and lower edges of the fin fold where large left–right pressure differences
occur in concert with the periodical generation and shedding of edge vortices.
The fin fold enhances effective flow separation and drag-based thrust. Along
the body, net thrust is generated in multiple zones posterior to the centre of
mass. Counterfactual simulations exploring the effect of having a fin fold
across a range of Reynolds numbers show that the fin fold helps larvae achieve
high swimming speeds, yet requires high power. We conclude that propulsion
in larval fish partly relies on unsteady high-intensity vortices along the upper
and lower edges of the fin fold, providing a functional explanation for the
omnipresence of the fin fold in bony-fish larvae.