You'd think by now we'd have given up on crying 'alien!' every time a strange-looking creature is hauled up from the deep, but this colourless pot-bellied fish has the internet abuzz once again.  

Image: Jaime Rendon/Dr. Pescado

Marine biologist Dr Brit Finucci, who studies deep-sea chondrichthyans (sharks and their relatives), identified the bizarre-looking creature as a swell shark (genus Cephaloscyllium) after she spotted images of the catch on Instagram last week. 

"Swell sharks are named for their ability to ingest large amounts of water to increase their body size and avoid predation when threatened," she explains. 

Shark scientist Dr Dave Ebert agrees. "This is clearly a swell shark, no doubt about that," he told Pete Thomas Outdoors, adding that it's likely Cephaloscyllium ventriosum, the only species of swell shark known to inhabit the Eastern Pacific.

While this is no extraterrestrial (and certainly not a Fukushima mutant), the specimen is unique in that it lacks the yellow-brown colouration typical of the species. It's possible that what we're seeing is a true albino, but because the shark's eyes appear blue-black, the more likely explanation is that this animal is leucistic.

Unlike albinism, which causes a complete lack of the pigment melanin, leucism doesn't affect the eyes. It's the same condition we saw in this squirrelthis whale and this baby great white shark.

Image: Jaime Rendon/Dr. Pescado
Image: Jaime Rendon/Dr. Pescado

The swell shark was caught in Mexico's Cabo San Lucas by 18-year-old Chicago fisherman Scott McLaughlin and local guide Jaime Rendon, who snapped a few photographs of the unusual fish before it was released.

The question on everyone's mind, of course, is whether the animal survived. While the sharks can be found up to 1,500 feet (457m) beneath the surface, they prefer to cruise the rocky shallows of the Pacific coastline, which means coming up from the depths isn't necessarily a death sentence. That said, it's impossible to be certain about the shark's fate.

Another peculiar finding is that the specimen had only three gill slits (compared to the typical five to seven). Still, this anomaly shouldn't set off any "new species" alarms, since bottom-dwelling sharks have been observed with reduced or compressed gill slits in the past. 

Swell sharks are nocturnal predators, but they certainly pose no threat to humans. They feed on a combination of small reef fishes, molluscs and crustaceans, often by remaining motionless on the seabed with their mouths open, simply waiting for prey to wander in. 

But their sluggishness doesn't mean these sharks aren't worthy of our attention. Just last year, City University of New York biologist Dr David Gruber discovered that some species of swell shark fluoresce a brilliant green when exposed to ultraviolet light. 

The current hypothesis is that swell sharks use this fluorescence as a way to attract or recognise each other, something of a secret form of communication between the sharks and other animals that have similar vision systems.

Image: Gruber/Discovery Channel

Want more strange fish in your life? Check out the rarely seen "sofa shark"!



Top header image: Stancina, Flickr