Phylliroe

Image: Fabien Michenet/nuditahiti.com

It's a sea slug that thinks it's a fish! And it's so good at being a fish you may well need a second look to decide if it's a weird-looking sea slug or a weird-looking fish.

Either way, it's weird-looking. And that's the main thing!

Phylliroe is a genus containing a couple species of nudibranch that have completely given up on the normal, sluggish life on the sea floor. These guys want to go places. Mostly places like 'up'. They reach about 5 to 8 cm (2 or 3 inches) long and have become completely pelagic, meaning they spend all their time swimming in the sea.

We've seen some pelagic slugs and snails before, and they're consistently in the top tier of bizarre weirdos, strange oddities and preposterous peculiarities. Sea Elephants with their one flapping wing? Bravo! Sea Butterflies with their two, flapping wings? Encore! There's also many a nudibranch who, while they spend most of their time on the sea floor, can still wriggle their body to launch themselves on a short swim when they need to. Good effort.

Phylliroe is different...

Video: liquidguru

This is a nudibranch who never sets foot on the sea floor. Instead, it swims like a fish! And it can do that because it looks like a fish! Their body is compressed and they have what looks just like a fish's tail fin. It works like one, too. A few flicks of the tail, an energetic wiggle and off they go, swimming through the sea as if it's what slugs were meant to do all along. They can even bioluminesce when they want to, almost as if they desperately want everyone to see how cool they look.

Ground-dwelling nudibranches usually feed on soft creatures that live plastered to the ocean floor - corals, byozoans and the like - not the kind of thing you find drifting around on the current. Thankfully there are quite a few softies that do drift about in the sea, particularly jellyfish, and Phylliroe just love jellyfish.

Image: Fabien Michenet/nuditahiti.com

They sniff out their prey using a pair of long rhinophores that extend from their face like tentacles. Where other slugs and snails have a long, sticky foot on their underside which they use to get around, Phylliroe has just a tiny... footlet? toe? next to their mouth. They use this to grip on to their prey so they can consume it at their leisure.

At least one species, P. bucephalum, uses a method like this all their lives. Adults lay only about 240 eggs and the tiny larvae, less than 2 mm long, stick onto a species of jellyfish called Zanclea costata. Here they stay, slowing chewing through their host, drinking its juices and even stealing its food directly from the jellyfish's digestive system. The larvae grow quickly, and soon enough they're too big to act like a parasite. So, by way of thanks, they consume their erstwhile host and swim away to find yet more jellyfish and other soft-bodied prey to eat.

It sounds like classic, deep sea fare, but nope. These guys live close to the surface and are widespread across the world. It just so happens that they're seldom encountered and there's not a huge amount known about them. It makes me wonder how many people have seen them and thought "whoa! What a weird-looking fish!"