First Footage of Deep-Sea Squid Solves Mystery of Lame Tentacle Club

This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American

Say what you want about squids - too many arms, pointy head, not enough elbows: 2/10 would not take dancing - but when it comes to killing things, they certainly aren't lacking in creativity.

The Caribbean reef squid (Sepioteuthis sepioidea) waves its arms from side to side before attacking its prey, which researchers have suggested is a kind of hypnotising or luring technique, and it's thought that the stubby squid (Rossia pacifica) from the Pacific and Northern Atlantic Oceans buries its tiny body into the sand except for one arm, which it wriggles intermittently to attract prey. Deep-sea Pacific Ocean squid, Chiroteuthis calyx, appears to use light-emitting organs called photophores on its arm tips to lure its prey, as does Octopoteuthis deletron, which adds vigorous arm wriggling to the equation. (This species also breaks off its own arms to distract predators, so it's got some ideas about how to defend itself too). All of which means: 10/10 would equip in an underwater siege.

And now the deep-sea squid species Grimalditeuthis bonplandi has been filmed for the first time by researchers at the Monterey Bay Aquarium Research Institute (MBARI) in California, revealing a new tentacle behaviour not seen in any other squid.

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Slender, slow-moving and gelatinous, G. bonplandi is found all around the world in tropical and temperate oceans at depths of around 900 to 1000 metres. Like all squids, they have two long, thin tentacles and eight arms, the distinction being that arms have suckers running along most of their length, while the tentacles generally only have suckers near their clubbed ends. Except in G. bonplandi's case, there are no suckers on its tentacle clubs. Plus no hooks, adhesive pads or photophores. This makes the species entirely unique among its squid peers, and its lack of luring and grasping apparatuses has so far baffled scientists. Because how does Batman catch criminals without his utility belt?

Publishing today in Proceedings of the Royal Society B, a team led by Henk-Jan Hoving (of vampire squid food and squid gay sex fame) was able to observe seven G. bonplandi squids in their natural environment thanks to footage collected by special a robot submarines (ROVs). One was observed at a depth of 1000 m in the Monterey Submarine Canyon off central California in 2005, while six others were spotted in the Gulf of Mexico in 2010. That same year the team also managed to find the second known G. bonplandi specimen with an intact tentacle - a young female with a 140-mm-long mantle.

In examining the physical specimen, the team confirmed that not only is the species lacking in luring and grasping features on its tentacle clubs, it also lacks the musculature in its tentacles to rapidly extend and retract them. Which is odd, because this plays an important role in the prey capture of most squid species that have so far been observed. But what G. bonplandi can do - and what each individual did do in the ROV footage - is wave both tentacles and tentacle clubs to resemble the swimming movements of small prey animals such as worms, fish or shrimps. The team figured that this was key for G. bonplandi's unique hunting method: "We hypothesise that G. bonplandi exploits this resemblance, using the tentacle clubs to attract potential prey towards the squid. How prey is subsequently engulfed by the arms and handled by the suckers remains subject to speculation," they report.

The technique of mimicking a non-threatening or tasty species to lure prey is known as aggressive mimicry, and it's seen in other deep-sea creatures such as anglerfish, viperfish and dragonfish. But all of these fish have appendages that can glow with bioluminescence, and G. bonplandi does not, so this squid has to be a little bit more creative for its aggressive mimicry technique to work.

The team suggests that G. bonplandi's waving behaviour could help lure its prey in a number of ways. Firstly, they suggest that by waving its tentacle clubs around, G. bonplandi could instigate a flash of bioluminescence by other organisms in the surrounding water to lure its prey into its vicinity. Secondly, the club movements create low frequency vibrations that could be detected by certain crustaceans, fishes and cephalopods that are tuned into such signals, just as the assassin bug lures a spider into its grasp by making vibrations on its web. And finally, the tentacle club movements create a wake, just like the wake an animal leaves behind as it swims through the water, which could cause prey to move towards it thinking it's been produced by its own prey objects or a potential mate.

Exactly how G. bonplandi grabs hold of the prey once it's been lured into arm's length remains unclear, but hopefully one of those ROVs will capture it some time soon.

Here's the footage:

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