Underwater robots help scientists see where marine larvae go and how they get there
Posted by admin on 28th September 2016

Many people who love the oceans never realize that a single drop of seawater is teeming with plankton, which means “drifters” in Greek. These organisms, which typically range in size from a pinhead down to the tip of a pin, spend their lives drifting with currents and form the base of ocean food chains.

Most larger marine organisms, from corals to crabs to fish, also begin life as tiny drifters. Females release eggs, or larvae, which join plankton and spend days to weeks adrift. Only larvae that arrive at the right habitats, at the right stage in their life cycles, will grow into a new generation. The rest will perish.

Scientists have long assumed that larvae are at the mercy of ocean currents. We cannot track such tiny creatures for weeks and months as ocean currents carry them over long distances, but many of us have wondered where they go and how they get there.

Seven crab larvae in a drop of water.
Peter Parks

Thousands of miles and decades apart, members of our team independently wrestled with this puzzle. Studying land crabs on Bermuda, Tom and Donna Wolcott of North Carolina State University wondered why larvae of one species seemed to return to the islands every year from the surrounding ocean, but those of another species did not. On the California coast, Steven Morgan at the University of California at Davis documented larval behaviors of shore-dwelling species that apparently enabled them to avoid being swept offshore by strong currents. And Morgan’s colleague John Largier explored the pattern of currents in which the larvae drifted.

We needed a new tool to address these issues. The Wolcotts reasoned that they could not put transmitters on individual larvae, but they could design something that behaved like a larva. If it could be tracked, following a group of them would reveal the path of a cloud of larvae.

Over three decades they developed a robot “larval mimic” that senses its environment, mimics the vertical swimming responses of larvae and relays its location. Following a cluster of these robots would show where larvae would be carried by currents, depending on the depths the larvae chose.

This research is providing insights into how evolution has “tuned” subtle changes in larval swimming behaviors to maximize the chances that larvae drifting in the ocean will beat the enormous odds against them and arrive at suitable habitat for settlement.

Following the currents – or not

The strong and persistent currents off the northern California coast provide an excellent opportunity to test the effectiveness of larval behaviors. Upwelling occurs along coastlines that run north-south along the western margins of continents, where prevailing winds combine with the rotation of the Earth to drive surface waters away from shore. This process draws deep, cold, nutrient-rich waters to the surface, supporting some of the most productive fishing grounds in the world.

Upwelling brings cold, nutrient-rich water to the surface along coastlines.

Scientists have long assumed that during upwelling, larvae developing near the surface were carried away from shore and lost at sea. But when Morgan and colleagues towed plankton nets at discrete depths throughout the water column and across the continental shelf of northern California, they found that larvae of most species remained within three kilometers of the shore where they originated.

How do such tiny organisms control their positions in the water? It has been known for decades that even though larvae cannot swim against currents, they can swim vertically from one layer of water to another moving in a different direction. The evolution of vertical swimming behaviors could help larvae to reliably stay close to shore, or to range different distances across the continental shelf.

By regulating their depth, larvae appear to have a surprising amount of control over how far from home they are transported. Many marine populations need this ability for successful reproduction. It also determines which populations exchange larvae sufficiently to remain a single genetically similar population along the coastline.

Knowing that larvae can regulate their movements in this way could affect how we design and evaluate networks of marine protected areas to conserve ecosystems and populations of economically and ecologically important species. It also is relevant for tracking the spread of invasive species, and for analyzing how marine species adapt to a warming planet by shifting their ranges north or south along the coast.

A robot that mimics larvae

Tom and Donna Wolcott named their robot the Autonomous Behaving Lagrangian Explorer, or ABLE – autonomous because it is deployed without a tether, and Lagrangian because it is carried along with the water parcel in which it is embedded. The Lagrangian approach to studying currents consists of following the movement of a parcel of water, typically with some sort of “drifter” – in this case, a drifting robot.

But the ABLE is not entirely passive: It mimics larval behavior by monitoring time and conditions around it, including depth, temperature, light, salinity and vertical water motion every 10 seconds. Then it calculates a new “target depth” appropriate for that larva and swims toward it by subtly adjusting its buoyancy. It can log up to 16 megabytes of environmental data from its sensors and upload them after the “bot” is recovered from the water.