This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Eastern oyster larvae are only 100–300 micrometers long, but their dense calcium carbonate shells make them substantially heavier than the surrounding seawater. A new study led by the Woods Hole Oceanographic Institution (WHOI) shows that this excess weight allows gravity to drive the feeding currents the larvae use to bring food to their mouths, a finding that reveals a critical role for gravity in how these tiny animals feed and survive.
The study, published in Physical Review Fluids, challenges the long-standing assumption that larvae this small rely primarily on the drag produced by swimming to generate the feeding currents that carry food toward their mouths. While it may be true for many small plankton, oyster larvae are an exception, placing them in the same gravity-dominated feeding regime as much larger copepods and revealing an unexpected role for their shells in helping them feed. The key difference is that drag depends primarily on the larva's movement through the water, but gravity depends on the difference in density between the larva and the surrounding seawater.
"This tells us that the shell is doing more than just protecting the animal," explained Houshuo Jiang, a senior scientist at WHOI and sole author of the study. "It is actually helping the larva feed. That means anything that changes the shell could also change how the larva gets its food." Jiang captured the larvae's feeding behavior using a high-speed microscale imaging system (HSMIS) developed in his laboratory.
Rather than observing larvae under a conventional microscope, the system uses long-working-distance optics and a high-speed camera to record animals swimming freely in a larger volume of seawater. He also added tiny tracer particles to the seawater and used a technique called micro-particle image velocimetry to track how those particles moved around the larvae, allowing him to measure the invisible currents generated as the larvae feed. "Usually, when you use a microscope, you need a lot of light, and if you put a lot of light on these tiny animals, you can heat the water and change their behavior," Jiang said.
"And conventional cameras are not fast enough to capture these very rapid movements. So we developed a system that uses a low-heat red LED and a high-speed camera that can record 2,000 frames per second. That lets us watch the larva in a larger volume of water without disturbing it and actually see how the water moves as it feeds." These findings offer a possible explanation for results from earlier experiments that raised bivalve larvae in space.
In a 1999 study, scientists raised bivalves in microgravity and found that larvae reared without normal gravity tended to have lower feeding and growth rates and were in poorer condition than larvae kept under normal gravity. At the time, the physical reason for those differences was not clear. "Thanks to high-speed imaging, we now see that gravity is essential for their feeding," Jiang said.
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