A living scarf: The blanket octopus

I post on cephalopods as rarely as I do on dogs, but at least I don’t have an “anti-cephalopod Tuesday.” But some of them are so weird that I can’t resist. A recent post by Katherine Harmon at Octopus Chronicles of Scientific American describes a small group of octopi that go under the name “blanket octopus” (four species in the genus Tremoctopus).  Besides their strange vampire-ish appearance, with the tentacles enclosed in a “cape,” they have an unusual defensive behavior (co-opting the tentacles of a jellyfish) and, especially, the most extreme sexual dimorphism and bizarre mating behavior I’ve heard of in any mollusc. From Wikipedia:

These species exhibit an extreme degree of sexual dimorphism. Females may reach 2 m in length, whereas the tiny males are at most a few centimeters long. The males have a specially modified third right arm which stores sperm, known as a hectocotylus. During mating, this arm detaches itself and crawls into the mantle of the female to fertilize her eggs. The male dies shortly after mating. The females carry over 100,000 tiny eggs attached to a sausage-shaped calcareous secretion held at the base of the dorsal arms and carried by the female until hatching.

As National Geographic describes, the walnut-sized males, weighing only 1/40,000th as much as the females, weren’t even seen alive in the wild until 2003:

Blanket octopuses are rarely seen. They spend their entire life drifting in the open oceans of warm regions worldwide. Females have the odd appearance of a “big pink drifting blanket” said Tregenza, explaining the origins of the octopus’s name.

Among their more unusual behavior, the octopuses employ a unique defense mechanism by tearing off the tentacles of passing Portuguese man-of-war jellyfish. The octopuses are immune to the tentacle’s painful sting. When they encounter potential predators, the octopuses waft the captured man-of-war tentacles in two pairs of its upper arms as an effective deterrent.

The male blanket octopus recently photographed by researchers was shown to clutch tentacle segments in his suckers, said Tregenza [Tom Treganza from the University of Leeds].

. . . If a male does chance across a female, it uses all its resources in an attempt to mate, “as he’s unlikely to encounter another one,” said Tregenza. A male blanket octopus fills a modified tentacle with sperm, tears it off, presents it to its prospective mates, and then drifts off to certain death.

Females store the tentacles inside large internal body cavities until they are ready to lay their eggs. At that time, the female pulls the tentacle out and “squeez[es it] like a tube of toothpaste,” over the eggs, said Tregenza.

This video showing the females was embedded in Harmon’s piece; there’s a really nice shot at the end of a school of fish in an antipredator formation:

And here’s another video taken in the Gulf of Mexico (warning: annoying music!):

It’s not so easy to find pictures of those diminutive males, but here’s one:

Image: Dhugal Lindsay/JAMSTEC/CMarZ

The mating behavior remind me of the anglerfish, deep-sea creatures in which the males are also very tiny, but also parasitic.  When a male finds a female, he attaches himself permanently to her body, absorbs his head, and fuses his tissue and bloodstream with that of the female.  He becomes, in effect, just a lump on the female’s body, ready to fertilize her eggs. (See a nice video of that here.) This behavior is obviously adaptive for a deep-sea creature, for females are so rare that when you find one, it’s best to never let her go (h/t: to South Pacific).

I often describe anglerfish to my undergraduate students, noting that the male is “nothing more than a parasitic sack of gonads—much like a male undergraduate.” The women students love that.