Before starfish grow into many armed, largely stationary adults, they navigate the sea as tiny larvae (about 1 millimeter in length, or about the size of a grain of rice) and propel themselves with only 100, scientists recently discovered, 1,000 tiny hairs called cilia surrounding their bodies.
But those hard-working cilia do more than just help the larvae paddle along.
Using high-speed cameras, the researchers found that the swimming larvae were also using the cilia to create microscopic vortices that capture nearby algae predators and push them toward the hungry swimmers. This efficient hunting behavior was previously unknown in starfish larvae, suggesting that the use of cilia in marine invertebrates is far more complex than previously thought, the scientists write in a new study. [What's spinning? Starfish larvae stir up an algal dinner|video]
Free-swimming starfish larvae don't look like adults - they have tiny, transparent bodies with only the beginnings of sprouts that later turn into arms. The study's authors decided to look more closely at these very young forms to better understand the starfish larvae's unusual bodies and how they use them - "How Physics Shapes Life," study co-author and Stanford University, Calif. bioengineering assistant professor Manu Prakash, said in a statement.
Rotation cycleMicroscope magnification has revealed the arrangement of thousands of cilia in the starfish larvae, which move in a series of synchronized motions to help the larvae move forward, backward, or change direction.
But the researchers discovered another type of cilia whose movements are beautiful but puzzling.
When a group of cilia swims in the opposite direction of the larva, it creates a small vortex. The authors of this study were able to observe the movement of the water by implanting particles in the water that were illuminated against a black background, which they then captured with a high-speed camera. Multiple swirls were visible around the larvae's bodies as they were tracked by the illuminated particles.
, but what is the purpose of the swirling motion? It takes a lot of energy to stir up these swirls, and the scientists wondered how this benefited the larvae.
Further observations showed that when the larvae were in areas with large amounts of algae, they churned the vortexes, generating an electric current that transported the algae to hungry creatures, even at distances several times the larvae's length. Once the food supply is depleted, the larvae swim away.
, but producing an efficient food conveyor belt comes at a price. A larva churns its cilia to suck algae closer to it, and it swims more slowly, spreading its position in the water and making it easier for predators to snatch it up, the researchers noted.
While the larvae's hypnotic swirls of water are mesmerizing to watch - this video recently won first prize in Nikon's Small World Dynamic Micrographics competition -- but the researchers found that they also serve a very specific purpose. Their findings also suggest that cilia, which are common in other tiny invertebrates, may help them survive in a similar way, according to study lead author William Gilpin, a postdoctoral student in the Prakash lab at Stanford University, where the research was conducted.
"Evolution seeks to meet basic constraints," Gilpin said of the first effective solution that tends to win.
The findings were published online Dec. 19 in the journal Nature Physics.
It was the original article on life sciences.