Red abalone (Haliotis rufescens) live deep in the cracks and crevices in rocky reefs, and have probably retreated there because of predation pressure from sea otters, who like how they taste almost as much as humans do. Abalone are external fertilisers, releasing eggs and sperm into the water, which flows at speeds 2–3 times slower than the exposed sea above. But how do the sperm find eggs? Because the gametes are so small, they live in a microscopic world of viscous forces, dominated by laminar shear flow. This is where layers of fluid slide past each other and don't mix, and looks like `stirring honey in a pot', says Jeff Riffell. With his colleague Richard Zimmer, Riffell investigated how shear, the linear change in water velocity with distance, affects sperm-egg interactions, and hence fertilisation success ().
Having measured water flows in the field, the team created shear flow conditions in the lab, using an apparatus consisting of a smaller cylinder placed inside a larger one, with a layer of water in the gap in between them. By turning the two cylinders in opposite directions, the water layers turn in the direction of each cylinder. To find out how shear flow affected fertilisation success, the team filled the gap with seawater and sperm at different concentrations. They rotated the cylinders at different speeds to create different shear flows, and then added the eggs, taking a water sample 15 s later and filtering out the eggs. They found that the percentage of fertilised eggs at each of the sperm concentrations peaked at the low shear of 0.1 s–1 and then declined as shear increased up to 10.0 s–1.
To find out how shear affects sperm behaviour and gamete interactions, the team added eggs and sperm to the apparatus at the same time. To monitor these encounters, the team relied on the fact that there is a predictable cross-over point between the two water flows created by the cylinders, where the two shear flows travelling in opposite directions effectively cancel each other out. The gametes get stuck in this cross-over point for up to 30 s at a time,experiencing shear on both sides in equal but opposite directions. Using a laser sheet to illuminate the stationary gametes, they recorded their interactions onto video and used tracking software on a computer to plot the movements, taking into account the flow of the water. The sperm swam fastest and were most likely to encounter eggs and fertilise them at 0.1 s–1 shear; they were less successful at higher shears. This is because as shear increases, so does egg rotation, which decreases fertilisation success as the sperm are more likely to slip around the egg's surface. At low water speeds, sperm swam faster than water flow, so they could overcome the effects of rotation.
`The fluid environment is playing a very important role in the evolution of gamete morphology and behaviour', says Riffell. This explains why the greatest fertilisation success occurred in conditions closest to the natural environment, suggesting that the abalone gametes have evolved to make the most of where they live. This could also have conservation implications for the endangered abalone, allowing scientists to recommend where it is best to transplant them based on an environment's shear flow, which would maximise fertilisation success and therefore survival.
