New species of crustacean discovered in a whale shark’s mouth

Researchers in Japan have discovered a new species of amphipod living behind the gills of a captive whale shark. It is the first of its kind to ever be found living inside such a large animal and has seriously surprised some scientists.

whale shark
The whale shark is the largest fish species in the world, but its playing host to one of the smallest crustaceans

The mouth of the world’s largest fish might not seem like the most obvious home for a tiny shrimp like creature. But it turns out there is no place like it for one brand new species of crustacean. The recently discovered critter is known as Podocerus jinbe, named after the behemoth it was found inside. It is an amphipod, a type of crustacean with no hard carapace and very compressed bodies. It is the first of its kind to be found living inside a whale shark and was first described by a team of researchers in Japan. But the discovery raises a lot more questions than answers. Such as how did they get there? Why would they want to stick around? And are they affecting their unaware hosts?

A surprising discovery

The story begins in 2007 when a female whale shark was caught off the coast of a small village on Okinawa Island in Japan. Unfortunately she spent the next decade in captivity in an ‘open ocean’ aquarium on the island. In 2017 her keepers found multiple mini crustaceans inside her mouth and around the inside of her gills. They decided to contact researchers from Hiroshima University to find out what these unusual creatures were. A team of researchers were then dispatched to the island where they removed the hitchhikers using suction pumps whilst scuba diving. Lead researcher Ko Tomikawa said he was “very surprised” when he realised thecreatures were amphipods which he claims he “would never have expected to find in a whale shark”. Upon later inspection at the lab he and his team realised that this species was in fact previously unknown to science.

podocerus_jinbe
P. jinbe photographed by electron microscope from the paper by Tomikawa et al

Podocerus jinbe

In their paper describing the new species the researchers named it as Podocerus jinbe after the Japanese translation for whale sharks ‘jinbe zame’. It is an amphipod which is a fairly common type of crustacean with no hard carapace and a very short and compressed body. But out of the over 10,000 species known to science the samples they collected did not match with any of them. P. jinbe are brown and yellow in colour and only grow to a max size of five millimetres making them a particularly small amphipod. Closely related species have been known to live on small fish, sea turtles and even larger crustaceans. But it is the first time an amphipod has been found living in a large vertebrate like a shark. Which makes this discovery particularly important as it is not just a new species to study but also a brand new relationship. It is unkown if P. jinbe are globally distributed or localised to Okinawa but closely related amphipods can be found all over the world.

What are they doing there?

Although it is still a mystery as to how P. jinbe managed to end up living inside the whale shark. Researchers have suggested it may have been caused by the shark’s limited movement having been confined to a small sea pen. But one thing that researchers can agree on is that the inside of whale shark’s mouth is actually an ideal location for these crustaceans. That is because they feed on small pieces of detritus in the water. This could include phytoplankton, dead scales or other decomposed organic material. Luckily seawater is rich in this material and gets pumped in by the gallon load every time the whale shark takes a breath. So by setting up shop just behind the gill rakers the amphipods not only gain free food and water to breathe, but also protection from any of its natural predators.

mouth wide open
The whale sharks wide mouth is the perfect place for a tiny amphipod

Crawling with crustaceans

Given that P. jinbe only grow to less than a centimetre and feed on waste materials in the water. You would be forgiven for thinking their host wouldn’t even know they were there at all. However the researchers believe the whale shark were definitely taking note of their hidden hitchhikers. That is because they actually found over 600 of them attached to the gills. It is believed that the sheer number of them were actually making it very hard for the shark to breathe. Which is why the keepers felt compelled to contact researchers in the first place. Sadly the whale shark also died a couple of months after having the amphipods removed, although an exact cause was not determined. Which is probably why more species of amphipod and other crustaceans don’t try this tactic. Although the story has a sad ending it does show us that life in the ocean can survive in even the most surprising of places.


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