Tremoctopus, the blanket octopus
The male blanket octopus spends his existence drifting along waiting to meet with a female. If the male meets a female, he fills one of his tentacles with sperm and tears it from his body. He gives this sperm-filled tentacle to the female which she then uses to fertilize her eggs. Afterwards, the female leaves the male who floats away and dies.
These species have an extreme degree of sexual dimorphism. According to the Melbourne Museum, "there's no other non-microscopic critter that has such a significant size difference between the male and female." Measuring only 2.4 cm long, the male blanket octopus is incongruously smaller than the female who can grow to over 2 m. This means that the female can be 100 times larger than her mate, and up to 40,000 times heavier. The male can afford to be small, since his chance of mating with a female and passing on genes does not depend on size, though the female's size is necessary to pass on as many healthy offspring as possible. The male's small size in fact allows him to reach sexual maturity faster, raising the chances that when a female stumbles upon him, he is ready to copulate.
An unusual defense mechanism in the species has evolved: blanket octopuses are immune to the poisonous Portuguese man o' war, whose tentacles the female rips off and uses later for defensive purposes.
Unlike most octopuses that squirt thick clouds of black ink to confuse their predators, the blanket octopus unfurls a giant sheet of webbing that trails behind like a cape. The webbing breaks apart rather easily when attacked — much like a lizard’s tail — and it gets wrapped around the predator’s face, giving the octopus a chance to flee.