Abstract
ABSTRACTFish larvae are the smallest self-sustaining vertebrates. As such, they face multiple challenges that stem from their minute size, and from the hydrodynamic regime in which they dwell. This regime, of intermediate Reynolds numbers, was shown to affect the swimming of larval fish and impede their ability to capture prey. Prey capture is impeded because smaller larvae produce weaker suction flows, exerting weaker forces on the prey. Previous observations on feeding larvae also showed prey exiting the mouth after initially entering it (hereafter ‘in-and-out’), although the mechanism causing such failures had been unclear. In this study, we used numerical simulations to investigate the hydrodynamic mechanisms responsible for the failure to feed caused by this in-and-out prey movement. Detailed kinematics of the expanding mouth during prey capture by larval Sparus aurata were used to parameterize age-specific numerical models of the flows inside the mouth. These models revealed that for small larvae which expand their mouth slowly, fluid entering the mouth cavity is expelled through the mouth before it is closed, resulting in flow reversal at the orifice. This relative efflux of water through the mouth was >8% of the influx through the mouth for younger ages. However, similar effluxes were found when we simulated slow strikes by larger fish. The simulations can explain the observations of larval fish failing to feed because of the in-and-out movement of the prey. These results further highlight the importance of transporting the prey from the gape deeper into the mouth cavity in determining suction-feeding success.
Highlights
Most marine fish reproduce by broadcasting small (~1 mm in diameter) eggs into the open ocean, providing no parental care from the hatching larvae (Blaxter, 1988; Cowen, 2002; Houde, 1987)
Young larvae live in a domain of intermediate Reynolds numbers (Re), in which viscous forces are non-negligible compared to inertial ones
To facilitate the comparison between fish species in which the growth rates can differ, we hereafter report on the scaling of suction feeding kinematics and dynamics with buccal length
Summary
Most marine fish reproduce by broadcasting small (~1 mm in diameter) eggs into the open ocean, providing no parental care from the hatching larvae (Blaxter, 1988; Cowen, 2002; Houde, 1987). It is estimated that >90% of the brood is eradicated during the “critical period”, extending from the time of first feeding until the larvae is ready to settle in its juvenile habitat During this period, larval fish undergo dramatic morphological and developmental changes, including the ossification of the cranium and vertebrae, the degradation of the fin fold and development of fin rays, as well as the continuous growth and development of the eyes (Blaxter, 1988; Kavanagh and Alford, 2003). Young larvae live in a domain of intermediate Reynolds numbers (Re), in which viscous forces are non-negligible compared to inertial ones This hydrodynamic regime was shown to impede the feeding rates of larval fishes, with 8 Days Post Hatch (DPH) Sparus Aurata larvae failing to capture non-evasive prey in ~80% of their feeding strikes (China and Holzman, 2014). Transition into higher Re improves the larvae’s ability to capture highly evasive prey such as copepods (Jackson and Lenz, 2016; Sommerfeld and Holzman, 2019; Yaniv et al, 2014)
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