Most squid snatch their prey with a pair of extendable tentacles, which can shoot out at high speed. These muscular limbs are longer than the other eight arms, and end in a bulbous “club” that’s covered in suckers or hooks. Once they latch onto something, they rapidly contract and bring the prey in towards the squid’s sharp beak.
But one deep-sea squid—Grimalditeuthis bonplandi—is very different. Its tentacles are thin and fragile, and almost always break off when it’s captured. For ages, people thought it lacked tentacles altogether until a full specimen was found in the stomach of a fish. Weirder still, its clubs have neither suckers nor hooks. Instead, they are flanked by a pair of leaf-shaped membranes. Why?
Now, after observing a live individual off the coast of California, Hendrik-Jan Hoving from the Monterey Bay Aquarium Research Institute (MBARI) in California thinks he knows what how the squid uses its feeble tentacles. They’re not grasping limbs, but fishing lures. By waving the membranes, the squid uses its clubs to mimic the movements small animals and attract its prey.
For around 25 years, MBARI has been using remotely controlled subs to explore the waters around California’s Monterey Bay. Hoving frequently pilots these vehicles and he sees a lot of squid. But on 22 September 2005, he stumbled across his first specimen of Grimalditeuthis bonplandi. It was the first time that anyone had seen the creature alive in its natural environment.
G.bonplandi lives all over the world, but this particular one was floating in a deep underwater canyon off the coast of California, 1000 metres or so below the surface. Its 14-centimetre body was largely transparent but flashed with neon pulses of red and violet. It hung there in the darkness at a slight tilt, beating its heart-shaped tail fins, arms splayed apart and tentacles outstretched.
“When we zoomed in for a close-up of the tentacle club, we saw it wiggling and undulating by flapping its membranes,” says Hoving. “We were really excited. It so strikingly resembled the movements of a small marine organism.”
The team collected and dissected the Monterey specimen, and confirmed that its tentacle muscles were extremely thin and poorly developed compared to those of a related squid. There’s no way the animal could have used these limbs to handle prey. Indeed, when the squid wanted to bring its club towards its head, it seemed to swim towards the club rather than contracting the tentacle muscles.
Over the next five years, Hoving’s team observed six more of these squid in the Gulf of Mexico, and all of them behaved in the same way. Their clubs tips all gently undulated like some small swimming animal.
Judging by their stomach contents, we know that G.bonplandi eats shrimp, other small crustaceans, and even other small squid. Hoving thinks that the squid could use its undulating clubs to lure in these victims in three possible ways: by disturbing glowing creatures in the water and creating temporary flashes; by creating attractive low-frequency vibrations; or by creating waves of moving water that mimic small swimming animals. Of course, how the squid then grabs any incoming prey is an open question.
Many predators use lures to attract their victims. Anglerfish, viperfish and dragonfish have lures projecting from their heads. Alligator snapping turtles wriggle their pink tongues like wriggling worms. Many vipers do the same with their tails, and one even has a tail that looks like a spider. The assassin bug (Stenolemus bituberus) kills spiders by strumming their webs in the style of a trapped and struggling insect. In a fit of perverse irony, the cookie-cutter shark uses a glowing collar to lure in larger predators, which it then eats.
It’s entirely plausible that some squids use similar tactics. The stubby squid buried itself in sand but wiggles one outstretched arm. The deep-sea squid Chiroteuthis calyx has glowing organs on its tentacles, which might help to attract fish. Another squid, Octopoteuthis deletron might use the glowing tips of its (freakishly detachable) arms in a similar way. None of these possible examples of luring have been proved, and the same goes for G.bonplandi.
Edith Widder, a marine biologist and deep-sea explorer, says the idea is plausible. “I wish they had had some way to check for the amount of bioluminescence in the water column where they observed the squid,” she says. “I also think the idea that it could produce a hydrodynamic signal has merit. If you think about the enormous challenge these animals face, locating food and mates in such vast volumes of darkness you have to believe they must have come up with some pretty clever solutions.”
“We are very far from doing experimental work on these animals as we encounter them so rarely,” admits Hoving. “We had moments of good fortune which allowed a peek into the lives and behaviour of this poorly known species. Hopefully, with increasing application of underwater technology and exploration of the water column, we will be able to learn more about Grimalditeuthis and other inhabitants of the world’s largest ecosystem.”
Reference: Hoving, Zeidberg, Benfield, Bush, Robison & Vecchione. 2013. First in situ observations of the deep-sea squid Grimalditeuthis bonplandi reveal unique use of tentacles. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2013.1463