About 99% of fish larvae die within weeks of hatching, but what causes this extreme death toll? Disease, predation, lack of food and inhospitable habitats are possible culprits, but most larvae die even when raised with plenty of food and safety. Recently, Roi Holzman and colleagues at Tel Aviv University and Afeka College of Engineering, Israel, explored this paradox of ‘starvation despite food abundance’ through the lens of biomechanics. Whereas most theories of larval mortality have focused primarily on ecological factors, Holzman and colleagues tested whether feeding success was influenced by the hydrodynamic forces acting upon larvae. In a previous study, they found that recently hatched larvae experienced ‘hydrodynamic starvation’ whereby strong viscous forces in water make many larvae physically incapable of capturing prey. In a new study led by Victor China working with colleagues at Tel Aviv University and the Inter-University Institute for Marine Sciences, Israel, they continue to delve into the mechanisms that drive hydrodynamic starvation by evaluating how suction feeding is affected by the development of physical traits and the associated hydrodynamic forces that work with, or against, them.
Larval fishes primarily use suction feeding, where they rapidly expand their mouth cavity to create a pressure gradient that draws water (and the prey within it) into the mouth. How water moves can be described by its Reynold's number (Re), which is a ratio of the viscous forces that resist movement and the inertial forces that keep the fluid moving. Small organisms experience a low Re environment, where viscous forces predominate and water feels as thick as molasses. By the end of the larval period, Re increases 100-fold and prey capture improves 4-fold. This improvement could be due to changes in the hydrodynamic regime that make it easier to move water, or developing anatomical traits that make larvae better predators (e.g. better vision). To disentangle the contributions of hydrodynamics and development, the team used high-speed videos to compare the feeding manoeuvres of gilt-head bream (Sparus aurata) ranging between 8 and 23 days old in hydrodynamic environments that were experimentally manipulated to have higher viscosities.
They found that increased feeding success was not necessarily a matter of being older and more developed, but instead depended on the correct combination of physical traits that allowed individuals to escape the danger zone of low Re and high viscosity. Larger body sizes associated with older individuals and decreased viscosities improved feeding success, but only indirectly through the alteration of physical characteristics that conferred higher Re, such as larger body/mouth sizes and faster attacks. At low Re, the team also observed events in which the prey was engulfed but then inadvertently spat out as the fish closed its mouth. These ‘in&out’ events suggest that timing is of the essence and that larvae face unique challenges in the viscous world compared with adult fish living in a higher Re environment. Consequently, high larval mortality may be due not to intense predation but to being too small and slow to eat properly.
The discovery that feeding success depends on escaping the danger zone of low Re brings us closer to solving the mystery of massive larval die-offs and helps inform wildlife management decisions. The team suggests that selection for larger larvae at hatching could increase survival rates because bigger individuals feed better, faster and on a greater range of prey by bypassing low Re. With many fishes decreasing in size as a result of over-fishing, these results also serve as a cautionary note that being smaller can be big trouble for fishes.