The numbers are staggering: for every oyster that makes it onto a dinner plate, 10 million siblings perished as larvae, and the factors that lead to such shocking statistics fascinate Heidi Fuchs from Rutgers, The State University of New Jersey, USA. Having realised early in her career that water turbulence can determine where the larvae of bottom-dwelling species settle, Fuchs discovered recently that oyster larvae swim harder – propelled by microscopic hairs (cilia) covering two fleshy oval wings that protrude out of the larva's shell – when they are tumbling in strong currents: ‘[which] made me wonder about the energetic cost to the larvae and how much they would have to eat to make up for it’, says Fuchs. Could agitated larvae consume enough in choppy conditions to sustain their turbulent lifestyle?
Despite their minute size, Fuchs reckons that oyster larvae (Crassostrea virginica) are some of the easiest plankton to work with; ‘We get them delivered overnight from a hatchery’, she explains. However, measuring the oxygen consumption of the minute 2-week-old molluscs and observing their swimming manoeuvres as they swirled around in water simulating eddies in a gentle stream through to smashing storm breakers was technically challenging. ‘Fortunately, Adam Christman perfected some techniques in advance and my graduate student, Jackie Specht, was able to assist’, says Fuchs, recalling how Specht took charge of the oxygen measurements that would tell them how much energy the larvae consumed, while she monitored the larvae's swimming movements. The duo also added microscopic algae (Isochrysis galbana) to the swishing water during some of the trials, to investigate how much the turbulence affected the larvae's ability to feed.
Monitoring the larvae, the team could see that although the tiny youngsters spent more time swimming downward in the roughest conditions, on the occasions when they swam upward, their energy consumption doubled, even though their energy use became more efficient – thanks to the turbulence counteracting the effects of gravity. And the larvae that were provided with an algae diet used more energy than the unfed larvae as they beat their propulsive cilia to produce a feeding current, in addition to propelling themselves through the water.
However, Fuchs and Diane Adams were most surprised at the impact that the churning water had on the larvae's ability to feed. Although the young oysters bumped into more algae in the choppiest conditions, they grasped hold of less food. ‘Our interpretation is that the water motion… made it more difficult for the cilia to handle or retain food particles. We suspect that the larvae can increase their ciliary activity for either swimming or feeding, but they cannot do both at full capacity at the same time, so in strong turbulence they focus most efforts on swimming’, says Fuchs.
In fact, the larvae were so seriously impaired that they were unable to consume enough food to survive in the most turbulent waters, no matter how many algae were available. Fuchs says, ‘Our observations suggest that many of these larvae could be lost to turbulence-induced starvation’, admitting that she is surprised by how badly the young oysters are affected. However, she suspects that smaller (younger) larvae may be at less risk of starvation, as they have not developed their sense of balance and should conserve more energy by spending less time swimming against the turbulent conditions.