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These Newly Discovered Shrimp Call a Whale Shark’s Mouth Home

Found in a whale shark off Okinawa, hundred of amphipods were living it up in the giant fish’s gills

(Species Diversity/Ko Tomikawa et al.)
smithsonianmag.com

Amphipods, a large group of small shrimp-like creatures, can be found all around the globe, pretty much anywhere that holds enough moisture. From the deep-sea dwellers chomping on Cold War nuclear bomb particles to humble swimmers living in cave streams in Illinois, these crustaceous scavengers have adapted to all sorts of environments.

Now, another strange locale can be added to the list—with a new amphipod species to go with it. Researchers found a new kind of amphipod living in the gill rakers of a female whale shark's mouth, according to a paper describing the new species in the journal Species Diversity.

While scuba diving in a fish preserve off Yomitan Village on the Japanese island of Okinawa, researchers used a suction pump to collect samples from the big fish, including material from its gills. Lead author Ko Tomikawa of Hiroshima University tells Agence-France Presse that he was surprised to find the creatures inside the whale shark’s mouth. In total, the scuba divers collected 357 male amphipods and 291 female crustaceans, and estimate there were at least 1,000 critters partying inside.

“This creature, which is usually 3-5 centimeters long [1-2 inches], is amazing because they can live in so many different kinds of environment,” Tomikawa tells AFP. “But I didn't expect we would find one inside the mouth of a whale shark.”

The team examined the species under a scanning electron microscope and sequenced its DNA, establishing that it was indeed a new species in the genus Podocerus. They officially named it Podocerus jinbe, since jinbe is the Japanese term for whale shark, which are the largest species of fish in the ocean.

According to the paper, it’s not too unusual for amphipods to be associated with one particular animal. Other species of the crustaceans have been found living on the surface of fish, sea turtles and some marine mammals. They are also known to live on other invertebrates.

“The mouth of the whale shark is probably a good habitat because fresh seawater, which is necessary for them to breath comes in regularly, and food flows in too,” he says. “And it also provides a safe place without any predators.”

It’s not entirely clear if this new species is associated with whale sharks specifically or if it was an unknown local species that saw an opportunity, as Tammy Horton of the National Oceanography Centre in the United Kingdom tells Ryan F. Mandelbaum at Gizmodo. According to the paper, the sheer number of Podocerus jinbe on the fish's gills appeared to make it difficult for the whale shark to breath. The animal ended up dying from unknown causes about three months after the researchers collected the amphipods.

Some amphipods are known to swarm creatures that enter their territory, including human scuba divers. It’s possible they glommed onto a wandering whale shark. But it will take a few more glimpses into whale shark gills to find out.

“It highlights just how little we know about Amphipods and how there are so many new species to describe,” Horton, who was not associated with the study, tells Mandelbaum.

Whale sharks are pretty mysterious as well. Though they are the world’s largest fish, with some reaching 45 feet in length, finding and studying the animals is difficult. Researchers are only now mapping their migrations, feeding areas and mating grounds. They have also begun taking blood samples and ultrasounds from the endangered mega-fish, trying to understand the unique anatomy of the gentle, ginormous plankton eaters.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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