Monday, September 22, 2014

Following Nemo: Clownfish make epic ocean journeys

From National Geographic by Jason Bittel 

Turns out finding Nemo could take a while. A new study reveals that baby clownfish can travel up to 250 miles (400 kilometers) in search of a new reef—an almost unthinkable distance for a creature just a few millimeters long.

Scientists already knew that clownfish larvae hatch in the safety of their parents’ sea anemone, but then leave this sanctuary in search of a home of their own.

A spine-cheek clownfish nestles in its bulb tentacle sea anemone. 
A spine-cheek clownfish nestles in its bulb tentacle sea anemone. 
Photograph by David Doubilet, National Geographic Creative

This is the opposite of the plot in Pixar’s Finding Nemo, in which a father clownfish, Marvin, sets out across the open ocean to find his son Nemo after he was caught in an amateur aquarist’s net.
(Read more about clownfish in National Geographic magazine.)
But until recently, scientists didn’t know just how truly epic the voyage was.
“This study is the first to directly measure long-distance dispersal [of clownfish larvae] over hundreds of kilometers,” study co-author Stephen Simpson, a marine biologist at the U.K.’s University of Exeter, said by email.

Fishy Accents

Tracking larvae has always been extremely difficult—after all, it’s not as if you can attach a GPS tag or GoPro camera to such a tiny organism.
So instead of tracking a single larva’s trek, Simpson and his team caught hundreds of Omani clownfish (Amphiprion omanensis) living in two coral reefs that are hundreds of kilometers apart off the southern coast of Oman. (See a map of where clownfish live in the world.)
The team removed a tiny part of each fish’s fin before releasing them unharmed into the ocean.
They then ran a DNA analysis on these fins, which revealed that fish living on Reef A have a different genetic signature than a fish living on Reef B, according to the study, published September 17 in the journal PLOS ONE.
The scientists liken these signatures to accents that are as easily recognizable as the difference between someone from New York City and someone from London.
By comparing the DNA signatures from the fin samples, the team proved what they’d long suspected—the two reefs were swapping clownfish despite being so far apart.
(See more coral reef pictures.)

“That larval fish can disperse between remote reef locations is impressive,” said David Coughlin, a professor of biology at Widener University in Pennsylvania who wasn’t involved in the study.
“However, it seems likely that some small number of larval fishes would end up at other reef locations as a matter of chance.”

In other words, the ocean’s currents probably have a greater impact on where the little fishies end up than which way they point their minuscule fins.
In fact, the team’s research showed that greater numbers of larvae traveled from the northern reef to the southern reef, and this mirrored the predominant current of the ocean.

Into the Great Wide Open

If you’re picturing a tiny, bright orange clownfish larvae plunging headlong into the high seas—stop.
Nearly see-through and smaller than a grain of rice, clownfish larvae don’t look much like Nemo. These defenseless youngsters also have it tough from the get-go.
The larvae can’t metamorphose into adults until they find a host anemone.
Other animals, like lobsters, may spend nearly a year in this open-ocean phase, called the pelagic larval duration (PLD).
However, the PLD of this species of clownfish lasts just two to three weeks, limiting the amount of time they can travel and find a new home. (See beautiful pictures of clownfish and anemones.)
And they’re easy prey: Of the many thousands of clownfish that hatch, only a lucky few will ever reach a reef—and most of those will be eaten within the first 24 hours, study co-author Hugo Harrison, a research fellow with the Australian Research Council Centre of Excellence for Coral Reef Studies, said by email.

“These kids don’t have it easy,” Harrison said.
“Pretty much everything is out there to eat them!”
For instance, clownfish larvae must escape hungry mouths large and small, from enormous filter feeders like baleen whales to tiny jellyfish, shrimp, copepods, and even zooplankton.

Why Risk It?

If there’s so much danger in the open ocean, why do clownfish venture out at all?
Because dispersing to new environments is “crucial” for the animals, said Harrison.
Traveling far from home ensures genetic diversity among the fish, and mixing up genes serves as a buffer against extinction.
It also allows species to colonize new habitats as they become available or to recolonize areas that were previously disturbed or depleted.
(Also see “Do You Know Where Your Aquarium Fish Come From?”)
Overall, the paper’s authors think the research could shed new light on just how connected even seemingly isolated marine populations are and may even help scientists develop and better manage marine reserves in the future.
In any event, the next time your kids want to watch Finding Nemo, you can tell them all about the real-life struggles of clownfish and their young.
Though you might need to censor it a bit.  

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