The Philippines seabed is one of the most biodiverse in the world, and with so many species living in close quarters, interesting relationships are bound to crop up in the area. During a recent dive, lab supervisor Chet King spotted a scuttling crab adorned with a jellyfish "top hat".

The disguise was so effective, in fact, that it took the experienced diver several seconds to figure out what he was looking at. 

"When I turned my video light on to video the motion of a jellyfish, it began to shake and turn," King told Caters News. "The jellyfish emerged from the bottom and started 'running', as witnessed in the video. It was alive and well but attached tightly to a carry crab."

As their nickname suggests, "carry crabs" (a colloquialism used to describe many members of the family Dorippidae) use modified hind legs to grip any items that might make for good cover. The strategy yields incredibly effective camouflage – like the marine equivalent of a ghillie suit.

Claims that this crab is exploiting the jelly's "stinging tentacles" for protection, however, are not entirely accurate. The billowing lobes you see in the clip aren't tentacles; they're "oral arms" – frilled masses of spongey tissue used for filter-feeding. 

And while it might look like the crab in the footage is holding its hostage in an abnormal position, this jelly actually prefers things to be topsy-turvy. 

Upside-down jellyfish (genus Cassiopeia) like the one in King's video spend their days planted on the seafloor, where they rest on their bells and feed on tasty plankton. Filter-feeding can be done right-side up, but there's a good reason behind the jelly's contrary orientation – and it's hidden within the oral arms. Much like corals, Cassiopeia jellies harbour symbiotic algae within their tissues. In exchange for free room and board, the algae provide their jellyfish hosts with extra nutrients – as long as they get enough sunlight to photosynthesise. Inverted living, then, ensures the jellies' tiny tenants get exactly what they need.

Spot the crab! Here, a decorator crab hides in plain sight under Cassiopeia cover!

Upside-down jellies tend to congregate in large groups (like the one in the header image of this article) and stay put, so it's likely that the crab simply plucked one for the taking. 

Of course, crabs cart around more than just jellyfish! Banana peels, tissue boxes and other invertebrates are among the mobile disguises used by crafty crustaceans. Back in 2016, Earth Touch contributor Mark Thorpe managed to film this one – and its "star-butt" urchin veil – in Indonesia's Lembeh Strait:

Super Macro Related 2016 09 14


Top header image: Adam Fagen/Flickr