An epic battle occurs in the deep, cold waters of the Antarctic between a giant fish and the largest invertebrate alive, with the victor devouring the loser. These pitched battles happen between the massive Antarctic toothfish and the colossal squid, a new study reveals.

Considering they’re possibly the largest cephalopod to have ever lived – weighing in at an astounding 500 kilograms (1,100 pounds) and reaching lengths of over 2.5 meters (8 feet) – scientists know shockingly little about the colossal squid. Originally described in the 1920s solely on the basis of the creature's hard beaks found in the stomachs of sperm whales, little more than their size has been figured out, and even that is normally just an estimate based on their beaks.

But now scientists are starting to unravel at least a few of the secrets of how these giant sea creatures live. Researchers from the Centre for Environment, Fisheries and Aquaculture Science in Suffolk, UK, have discovered that colossal squid prey upon Antarctic toothfish in similar levels as orcas, and that the toothfish prey upon the squid in return. The study is published in the Journal of Natural History.

It had already been proposed that colossal squid feed on toothfish. However, evidence was scant: The only two intact colossal squid specimens to have ever been caught, which now reside in the Te Papa Museum, New Zealand, were both snagged on a long line by the same captain on the same toothfish boat in the icy waters of Antarctica. The last one he scooped up in 2014 was dissected live online by the museum.

Imprints of squid suckers on toothfish heads (A,B); and wounds of the squid beak on toothfish bodies (C,D). Credit: A. Remeslo.

So the researchers went out on two South Korean toothfish fishing vessels and analyzed over 8,000 of the fish caught over three years. They were looking for signs that the fish, which themselves are giants of the deep at up to 2 meters (6.6 feet) in length and 80 kilograms (176 pounds) in weight, had been preyed upon by the squid. The scientists also looked inside the stomachs of the fish to see what they had been feeding on.

The scientists found that 71 of the fish had been preyed upon by the squid, showing signs of having chunks taken out of them by the cephalopod's bony beak, and signs of the squid's hooked suckers on their skin. In return, they discovered that 57 of the fish contained lumps of colossal squid in their bellies, ranging from hunks of the squid's mantle to a tentacle that reached a whopping 2.4 meters (7.8 feet) in length.

Large tentacle and two arms recovered from a toothfish stomach. Credit: A. Remeslo.

Based on this data, they worked out that the depredation rates of the squid on the fish do not seem to be that high, at only around 1%. As the squid are not thought to be active hunters – most are probably “sit-and-float predators” – this fits in with previous theories. The authors do, however, suggest that regional depredation rates might increase to something similar to that of sperm or orcas in some Australian areas.

What's more of a puzzle is how the fish, which are much smaller than the squid, manage to sink their teeth into them. It’s most likely that they are feeding on either young, injured or dying squid whenever they get the chance. And why wouldn’t they? According to the researchers, the flesh is “rumored to be of ‘excellent quality and very flavourful’, but this needs to be reconfirmed.”

Main image credit: AI404/Flickr CC BY 2.0