VAN LEEUWEN, JL; MULLER, UK; LIU, H; Wageningen University; Wageningen University; Chiba University: Fast starts mechanics in larval fish

We studied fast starts of larval fish because we expect that maximum mechanical loads on the musculo-skeletal system occur during this behaviour. When startled, fish larvae perform a fast start followed by a period of cyclic swimming. During the initial phase, when adopting a C shape, the larva must generate the appropriate body geometry to maximise thrust using pressure forces during the subsequent acceleration phase. We studied the fast starts of zebrafish larvae (age 3 days post-fertilisation; body length BL 3.9 mm) using motion analysis and two-dimensional flow visualisation. Our kinematic analysis revealed that zebrafish larvae assume a C-shape within 5 to 6 ms, their midline reaching maximum curvatures of 8 to 9 BL^-1 (0.03 m^-1). The tail tip reaches speeds of up to 0.36 m/s during the initial phase and 0.56 m/s during the straightening phase of the fast start. Midway through the main acceleration phase and 12 to 14 ms after initiating the fast start, larvae reach accelerations of up to 40 000 BL s^-2 (measured at the transition between head and yolk sac), which corresponds to up to 16g. During the consecutive phase of cyclic swimming, larvae maintain swimming speeds of up to 50 BL s^-1. Our flow visualisations revealed that the initial C-bend of the body generates a strong suction flow towards the concave side of the body. During the initial phase, much kinetic energy is imparted to the water, but the centre of mass of the body travels relatively little. The lateral momentum generated in the water during this first half tail beat, when the larva assume the C-shape, is consistent with the observed change in heading of the larva. During the main acceleration phase, a backward jet becomes clearly visible in the wake, attesting to the net thrust production of the larva. Our next step will be the quantification of the loads and the implications for the muscular system.