Podocerus jinbe, a newly discovered species of gammarid amphipods, is about 5 millimeters long. (Provided by Ko Tomikawa)

The world's largest fish seems to be offering a safe haven, ample food and perhaps a “free ride” to previously unknown tiny crustaceans.

That is the view of Hiroshima University zoologist Ko Tomikawa, whose team discovered a new species of shrimp-like gammarid amphipods living in the mouth of a whale shark.

Researchers first found more than 1,000 of the small invertebrates in the gills of a whale shark raised at the Okinawa Churaumi Aquarium’s preserve in waters off Okinawa Prefecture in 2017.

The researchers said the 5-millimeter-long amphipods, named “Podocerus jinbe,” apparently chose the shark's mouth as shelter from predators.

The amphipods, which apparently feed on microorganisms in seawater, can also siphon off seconds as the shark gobbles mountains of plankton.

“Gammarid amphipods don’t migrate much,” said Tomikawa, an associate professor of systematic zoology. “They may be using whale sharks, which travel wide areas, like a bullet train to expand their habitats.”

Whale sharks are known to grow to more than 10 meters in length and weigh up to 20 tons.

Gammarid amphipods usually hug rocks on the seafloor or hide between clumps of seaweed.

The researchers said the shape of the amphipod’s legs and the number of its setae, spiky protrusions that resemble short hairs, differed from any previously identified member of the species.

This is the first time a variation of the creatures has been found thriving inside a whale shark’s mouth.

Tomikawa worked with researchers from the Okinawa Churaumi Aquarium and the Arctic University of Norway.

Their findings were published in Species Diversity, a journal of the Japanese Society of Systematic Zoology.