Thread by @NSF

Hi 👋 I'm @beatrixparis from @UMiamiRSMAS.

I study fish larvae, the early life stage of fishes: one challenge is to collect them alive to be able to observe their behavior. A second challenge is to observe them in their natural environment without human disturbance. #takeoverNSF

The European eel is a migratory species that crosses the Atlantic Ocean twice during its life. After hatching in the Sargasso Sea, eel larvae move >3,000 miles to the freshwater streams of Europe and eventually navigate back to the Sargasso Sea to spawn and die. #takeoverNSF

During their journey, glass eels encounter many distinct environments, with dramatic differences in physical, chemical and biological features. This journey is complex and they need to rely on multiple orientation mechanisms. “Magnetic imprinting” is one of them. #takeoverNSF

Glass eels use an “internal compass” to memorize the magnetic direction of tidal flows in the estuaries where they are juveniles, which may help them orient in moving water during migration.

#takeoverNSF

They can sense the Earth's magnetic field and use it like a compass controlled by an internal biological clock to orient themselves. Since this finding, we have shown that other haddock larvae have similar abilities that they use to orient towards the coast. #takeoverNSF

The odyssey of this eel begins when they hatch in the Sargasso Sea. #takeoverNSF

As little larvae, they ride the Gulf Stream, traveling thousands of miles across the Atlantic Ocean, hopefully making it to the European continental shelf where they metamorphose into a small, transparent form called “glass eel.” #takeoverNSF

At some point during this journey from the Sargasso Sea, the eel larvae “hop off” the Gulf Stream and migrate towards the coast, heading for estuaries. How they know to leave the Gulf Stream and find estuaries is a longstanding mystery. #takeoverNSF

When they arrive at estuaries, glass eels' internal compasses imprint the directions of local currents. Those memories help orient them in moving water – including knowing if they’re moving upstream or downstream.

#takeoverNSF

That’s important because they need to keep moving upstream to continue their migration and life cycle. #takeoverNSF

It might not have occurred to you that this kind of navigation would pose a challenge for fish, but picture how big an estuary is to a tiny eel. Trying to use just visual cues in such a huge, cloudy environment would result in them getting lost – and probably dying. #takeoverNSF

We have found that their magnetic memory imprints upon arrival in estuaries, but it’s also possible that it continues to learn and remains flexible, adjusting as the eels head upstream and the magnetic direction of the current could change. #takeoverNSF

European eels are catadromous, which means they’re born in the ocean, live most of their lives in freshwater, then return to the ocean to spawn --kata in Greek means down, so adult eels run down the river to the ocean to spawn. #takeoverNSF

Eels can spend a long time in these estuaries could be very long. Thus, they might be using this magnetic memory for a significant portion of their lives before they reach their next stage of metamorphosis. #takeoverNSF

The eels continue metamorphosing, losing their transparency and turning into “pigmented elvers,” before ascending into freshwater & growing into adult yellow eels.

#takeoverNSF

After up to 50 years, they undergo another metamorphosis into silver eels, then migrate for thousands of kilometers to the Sargasso Sea where they spawn and die. #takeoverNSF

But magnetic imprinting is not the only behavioral strategy glass eels use to migrate. They use chemical cues such as salinity, water currents and other orientation mechanisms depending on what’s available in any given environment. #takeoverNSF

Since studying glass eels in their natural environment is difficult, researchers used a Drifting In Situ Chamber (DISC) invented at the Paris lab @umiamirsmas. This chamber drifts through the fjords of Norway, equipped with a compass, camera, and other sensors. #takeoverNSF

When a glass eel is deployed into the DISC’s behavioral chamber, researchers can study its movement and orientation during the migration over the continental shelf, from the ocean to the estuary where they ingress. #takeoverNSF

The international research team of the Institute of Marine Research (@Havforskningen) and @UMiamiRsmas, collected over 200 glass eels from various estuaries in the archipelago of Austevoll, Norway to test the hypothesis of magnetic memory. #takeoverNSF

We also deployed glass eels in a magnetic lab that allowed us to rotate “magnetic north” to a different compass direction. We observed that the eels orient themselves to the magnetic direction of the prevailing tidal currents in the estuaries where they arrive. #takeoverNSF

... these eels are amazing. They migrate over 3,000 miles from the Sargasso Sea to Europe, where they mature for nearly five decades and return home where they spawn and die. #takeoverNSF

📷: Alamy

Studying how eels navigate and adapt can show us that fish, even in their earlier forms can use large-scale external cues to navigate in the open ocean –changing the paradigm of simple larval dispersal by the currents to larval migration #takeoverNSF

The European eel is imp to study since it is a commercially important species that is critically endangered @IUCN. Eel populations have declined precipitously since 1980. We seek to understand the ecological roles of eels, and how climate may shape these roles. #takeoverNSF

If young people are interested in doing this kind of work, science and math classes are key, together with natural history.

Follow me @beatrixparis and my institution @UMiamiRSMAS to learn more!

#STEMed #takeoverNSF

📷: Eimear Egan, NIWA