Do viscous forces affect survival of marine fish larvae? Revisiting the ‘safe harbour’ hypothesis

Aquatic organisms physically interact with the water that surrounds them, and this interaction is fundamental in shaping many aspects of their biology. General characteristics of the hydrodynamic interactions between organisms and the flow around them can be captured by the dimensionless Reynolds number (Re), depicting the ratio between inertial and viscous forces operating on the organism. The characteristic flow regime of larval fish that cruise at slow speeds is a regime of low Re, where viscous forces dominate. In this study, we experimentally test the ‘safe harbour’ hypothesis, which proposes that increasing larval body size facilitates an ‘escape’ from the detrimental effects of low Re. Larval gilthead seabream (Sparus aurata) were reared during early ontogeny under artificially manipulated water viscosities to expose larvae to low Re regimes. Larval survival decreased significantly with increasing water viscosity, and increased with increasing standard length. Surviving larvae exceeded the mean length of mortalities by ~1 mm, on average. Our findings provide direct experimental support for the ‘safe harbour’ hypothesis, indicating that marine larvae incur a fitness cost when operating under low Re conditions. Moreover, the results highlight the need to recognize the hydrodynamic environment when considering the a-biotic characteristics that may influence organismal performance and fitness.