Bottom-heaviness stabilizes pelagic fish eggs and generates advective transport at their surface

Pelagic fish eggs are small spheres (~1 mm) that drift in the open ocean, carried by large-scale currents. Embedded in these currents are egg-scale turbulent eddies that impose local rotational forcing on the eggs. Accordingly, eggs are considered passive tracer particles that move with these eddies, implying that mass transfer of oxygen and waste are controlled only by molecular diffusion. However, eggs are bottom heavy, characterized by a dense embryo and a buoyant oil globule at opposing poles. We hypothesized that such internal structure generates a gravitational restoring torque, stabilizing the egg and generating relative egg-water motion. To test this hypothesis we exposed fish eggs to controlled flows with shear levels typical of upper mixed-layer and estuarine environments, and analyzed their dynamics. Eggs displayed one of two repeatable behaviors: (i) steady orientation, with a near-constant egg angle, and (ii) revolving motion, with egg rotation at time-varying angular speed. Both behaviors generated relative motion between the eggs and the surrounding water. A rigid-body rotational model balancing external flow-induced torque and gravitational restoring torque showed that transitions between these states depend on local flow fields and developmental stage, with older eggs shifting to revolving behavior at lower external fluid torque. Together, these findings suggest that advective transport is important and that diffusion-only models underestimate exchange rates, which are a major component in predicting the effects of environmental factors on embryo survival.