What do you get if you cross a knobbly ribcage with a blob of spilled jelly? This nudibranch …

Meet Melibe viridis, a carnivorous sea slug with a gelatinous vacuum cleaner for a head. The fascinating sea-dweller was filmed by Emeric Benhalassa off the coast of Bali in 2016, and unsurprisingly, it generated a fair amount of interest (and confusion) online.

"I'm pretty sure that's a scrotum-headed nightmare sponge," one Reddit user proclaimed. The description is perhaps not that far off. Melibe viridis is equipped with a large, net-like mouth that it spreads out to snag its prey in an action similar to the plastic grabby-jaws of a Hungry Hungry Hippo (for some reason Hungry Hungry Nudibranchs never really took off).

The inner edge of the expandable hood is fringed with sensory papillae that help the nudi detect when a tiny crustacean has strayed into the death net. Most nudibranchs (and many other molluscs) are equipped with a rasped, tongue-like structure called a radula, which they use to cut and break down food before it enters the oesophagus. But members of the Melibe genus are not like other sea slugs, and their gooey head-nets set them apart from the crowd.

Other Melibe species, like the hooded sea slug (Melibe leonina), will attach themselves to blades of kelp and sweep their raised hoods downward or to the side. If an unfortunate planktonic creature settles on this translucent "fishing net", the two sides of the hood are slammed together like an underwater Venus flytrap, and tentacles ensure prey is locked in.

Similar to other nudibranchs, Melibe are hermaphrodites and fertilisation of eggs happens internally. Unlike other nudibranches, though, they smell a bit like fruit. In the case of the hooded sea slug, the fruity odour is most noticeable when the animals are removed from the water or when several individuals are grouped together. It's believed to be a kind of pheromone that the sea slugs use to communicate with each other. Nudibranch perfume, if you will. 

Melibe viridis can be found throughout the tropical Indo-West Pacific, with a small population hanging out in the Mediterranean. Although the jelly head may look a little unsettling, the species typically grows to a maximum length of just 130 mm (five inches), so your fingers and toes are probably safe.