We continue our exploration of the many mysterious octopuses that live far from shore—and the eyes of humans. Today we meet the blanket octopus (Tremoctopus), a genus with four species that, until recently, had only been described based on female specimens. Why? Although they live in the vast open ocean, they are big (up to two meters long) and have fleshy, capelike webs that extend down their longest arms.
The males, on the other hand, are, well, quite a bit smaller. We're not talking Napoleon complex shorter. These guys are many orders of magnitude smaller. Some hundred times shorter—and tens of thousand times lighter—making the blanket octopus one of the most extreme cases of sexual dimorphism in the animal kingdom.
The females of many octopus species outgrow their male counterparts, a strategy that seems to work for their non-social system. These reclusive cephalopods live alone. And when it comes time to mate, the females are carrying an entire clutch of eggs—the more of which the better—and the males are contributing the sperm—which comes in small packets called spermatophores. So, bigger females, more eggs equals better odds at continuing the genetic line.
But the blanket octopus has taken this to such an extreme that it took years to even identify males of the genus. And then it was only from the dead ones that occasionally appeared in plankton nets. These measured no longer than 2.4 centimeters. It wasn't until just a decade ago that scientists finally found their first identifiable live male blanket octopus. The tiny fellow swam up toward a dive light on a night research dive[pdf] off the northern Great Barrier Reef in Australia.
To mate with the large ladies—who are carrying some 100,000 eggs—the males secrete their spermatophores into a pouch on their specialized third right arm (the hectocotylus) and then leave it behind in or on the body of the female, where it will then fertilize her eggs when she is ready to lay them. The male is assumed to die soon after he has made his contribution, and the female carries the eggs in a long strand until they hatch.
The large web of the blanket octopus helps her appear much bigger to potential predators—a tried and true defensive strategy in the oceans and on land. This web can also be rolled up when not in use. And the most well studied species of this group (T. violaceus), the common blanket octopus or the violet blanket octopus, also has the ability to drop portions of her web to serve as a distraction to potential predators.
But both males and small females have a more menacing option for defense. These octopuses are apparently immune to the sting of Portuguese man-o-wars (Physalia physalis). These octopuses have been known to collect stinging tentacles from the jellyfish-like creatures and wield them like weapons to fend off predators.
Illustration courtesy of Ivan Phillipsen