Fig. 1

Overview of the setup showing the basic principle of measurement. (A) Side view of a lined force attack from the deflection pin onto the fin surface (D, distal end of the fin; P, proximal end of the fin). (B) Bottom view showing the pin (red arrow) at a certain effective beam length from the fixation (black foam pads). (C) Overall view of the measurement setup showing the water basin, two camera positions (1 and 2) as in A and B, respectively, a fish-holding device (3), as also shown in F, and the sensor construct (4) mounted onto piezo-based positioners (5). (D) Design of the sensor construct with strain gauges (2) mounted onto a spring sheet (3), with the deflection pin (1) that leads to the base of the strain gauges to increase the effective bending moment together with the specialized geometry of the spring sheet (3). (E) Measurement principle illustrating the deflection pin (2) applying a force () at a particular proximal–distal position (effective beam length, L_eff) leading to a particular displacement y. The fish peduncle is held by two opposing foam pads (1), as also shown in B. (F) Zebrafish-holding device showing an anaesthetized adult specimen.