A couple of years ago, Dr. Kubodera, who has a remarkable talent for filming large, unusual squid underwater, got some footage of Taningia danae. This octopoteuthid squid is notable for a) being large, b) lacking tentacles as an adult, d) having big, bright photophores on the tips of two of its arms and c) swimming with fins rather than jet propulsion.
Addressing points a)-c), we have a 2007 article from National Geographic:
The human-size squid were filmed at depths of 780 to 3,100 feet (240 to 940 meters) off the Ogasawara Islands during a scientific expedition led by Tsunemi Kubodera of the National Science Museum in Tokyo, Japan.
T. danae lacks the two long feeding tentacles of most other big squid. But the species has suckers with sharp claws and light-producing organs on the ends of two of its arms called photophores. The size of lemons, these photophores are the largest found in the animal kingdom.
Nice analogies! "Human-size" should be used more often for Humboldt squid; it's much more calmly informative than "huge." And lemon-size photophores are pretty darn cool.
But I found myself forgetting the photophores as soon as I heard how this animal swims (point d). Nat Geo goes on:
The squid reached speeds of 5.6 miles (9 kilometers) an hour when attacking the baited lures, swam both forwards and backwards, and rapidly changed direction.
"Some people have said all deep-water squid are pretty sluggish, because their muscles are not real firm when you catch them," Vecchione said. "But this particular family has got very muscular fins, and that's what it's using for swimming."
Other types of deep-sea squid, including the giant squid, propel themselves by squirting jets of water, he added.
When I first read this, I was totally confused. Not just deep-sea squid, but all squid, as far as I knew, use jet propulsion to some extent. Of course, most squid use their fins to some extent as well. When the fins are small relative to body size, they're assumed to be mere stabilizers, and when they are relatively large, they're thought to contribute more in terms of propulsive force. But this is always propulsive force added on top of the jet.
Taningia danae has the largest possible fin-to-body-size ratio: its fins are the entire length of its body. Could this be the only known squid who doesn't use jets at all?
Going to one of my favorite sources, the Tree of Life, I found this account of Kubodera's work, and the accompanying picture:
The squid swam using the large fins. Large amplitude waves in the fins passing from head to tail propelled the squid forward and reverse waves propelled it backwards.
Beautiful! Amazing! It looks like a swimming ray--something I've often thought while watching the smaller fins of my Humboldt squid, but in Taningia the convergence has come nearly full circle.