The water whorls of baby starfish
Tiny starfish larvae – each smaller than a grain of rice – spend 60 days and 60 nights paddling the open ocean, feeding to accumulate the energy they need to metamorphose into the familiar star shape.
The video above describes the beautiful and efficient mechanism that they use to swim, feed and grow without becoming exhausted by the journey.
We have shown that nature equips these larvae to stir the water in such a way as to create vortices that serve two evolutionary purposes: moving the organisms along while simultaneously bringing food close enough to grab.
The researchers had wondered about the evolutionary underpinnings of the starfish larva’s shape – why did it end up looking this way? Prakash said:
When we see strange and beautiful shapes in nature we bring them back to the lab and ask why they evolved this way. That is the perspective we bring to biology: to understand mathematically how physics shapes life.
The researchers studied the organisms in a systematic way, feeding the larvae nutrient algae and observing their movements with video-enabled microscopes. Vivek Prakash (no relation) is a postdoctoral scholar in bioengineering and member of the research team. Prakash said:
Our first eureka moment came when we saw the complex vortices flowing around these animals. This was beautiful, unexpected and got all of us hooked. We wanted to find out how and why these animals made these complex flows.
The vortices made by the starfish larvae were puzzling to the scientists, because it seemed to make no evolutionary sense. Creating spiral flows of water required a lot of energy. Why would a larva with just three imperatives – feed, move and grow – expend such effort?
It was figuring out how larvae made the water swirl that led the researchers to understand why. The experiment zeroed in on one of evolution’s most structures called cilia, (from the Latin word for “eyelashes”) From a Stanford statement:
Imagine that the cilia on a starfish larva are like the oars that might be used to row an ancient galley – except that each larva has about 100,000 oars, arranged in what researchers call ciliary bands that gird the organism in a pattern far more complex than any galley’s oars.
The rowing metaphor hints at the complexity the researchers found as they studied how these 100,000 eyelashes paddled the larva through water.
Like oars, the cilia had three potential actions: forward, reverse and stop. And just as with oars, the cilia moved in different synchronized patterns to create different motions. Presumably orchestrated by its nervous system, the larva beats its 100,000 eyelashes in certain patterns when it wants to feed, so as to swirl the water in a way that brings algae close enough to grab. Then, with a different flutter of eyelashes, the larva creates a new pattern of whorls and speeds off.
The researchers realized that they were observing an active and previously unknown mechanism that improved the larva’s odds of survival. The physical structure of the starfish larva, controlled by its nerves, allows it to make feed-versus-speed tradeoffs – lingering whenever algae are plentiful, then darting off when nutrients grow scarce.
Bottom line: Video shows the beautiful and efficient mechanism that starfish larvae use to swim, feed and grow.