Steff from Seattle sent me a link to a very cool paper describing an extreme octopus. One where the sexes are radically different from one another.

This is a female Tremoctopus violaceous. Note the scale bar, which is 5cm long. This is an immature specimen, and adult females grow to be about 2m long and weigh about 10kg.

This is a male Tremoctopus violaceous. Isn't he adorable? The scale bar is 1mm.

That is a fully adult male. It is 2.4cm long and weighs 0.25g.

The females are almost 100 times longer and weigh up to 40,000 times more than the males. That's an impressive degree of sexual dimorphism, and the authors report it as the most extreme example yet in animals this large.

Usually, this degree of dimorphism is ascribed to situations where females are sparse and hard to locate, and the male has become basically an extended sperm dispersal unit—once a male is lucky enough to find a female, he needs to cling to her and dedicate all of his time to his only important function, delivering sperm. A different strategy is proposed here. The tiny males have an effective defense mechanism: they collect stinging nematocysts from the tentacles of the Man-of-War, and hold them in their arms to flail at threats. Here's a closeup of the little guy's weapons.

This defense only works if you are small. Females, on the other hand, benefit from growing large, so that they can produce larger quantities of eggs. These two competing selection forces have produced two different growth strategies in the two sexes.

What else could this bring to mind but The Incredible Shrinking Man? Look at this still from the movie—the scale is even about right, with our one inch tall hero, and his 5-6 foot tall wife off-screen. And here he is even fighting off predators with a borrowed sting!

Of course, the one bit of verisimilitude they needed to add to the movie would be to have the little guy scrambling up his wife's leg to slap a sperm packet in her reproductive tract^*. Maybe in the remake…

^*One bummer for size fetishists: the evidence suggests that the male Tremoctopus, once he has fertilized a female, drops dead (although some seem to find that part appealing). This detail may have to be omitted in our hypothetical remake of TISM, too. It would be kind of a downer ending, not that the original's was any happier.

Norman MD, Paul D, Finn J, Tregenza T (2002) First encounter with a live male blanket octopus: the world's most sexually size-dimorphic large animal.New Zealand Journal of Marine and Freshwater Research 36(4):733–736.