The Fin Ray® Paper KitTail fins of bony fish do not bend away from the compressive force – as would be expected – in the event of a compressive load, but instead bend in the direction of the compressive force. This initially surprising feature enables highly efficient force transmission of the fish to the surrounding medium of water. When the fish moves by flapping its tail, the tail fin assumes the (concave) shape of a ladle (with the open side in the direction of the flapping of the fin). The water volume enclosed by the tail fin is accelerated to the rear, propelling the fish forwards in the process. The adaptive change in shape of the tail fin is a reason for the good force transmission. From a technical point of view, the tail fin of the bony fish is a design which has been optimised in terms of lateral force application in the course of evolution.

The Fin Ray® Paper Kit – Technology for schools – Learning Systems – Festo Didactic

Also https://www.nature.com/articles/ncomms2751

The function of fin rays as proprioceptive sensors in fish