Lopsided lovers. A male blanket octopus (inset) smaller than the eye of a female.

New Clue to Octopus Sex Oddity

If a sparrow tried to mate with a fighter jet, that would fairly describe the kamikaze sex life of a bizarre seafaring octopus. Among the rarely encountered blanket octopus (Tremoctopus violaceus), females outweigh males by up to 40,000 times.

During a night dive in deep water off the northern Great Barrier Reef, zoologist Mark Norman of Museum Victoria in Melbourne and his colleagues had what they say is scientists' first-ever encounter with a live male blanket octopus. The adult male is about the size of a jellybean, weighing about a quarter-gram--making it about the size of the pupil in a female's eye. Although the males are small, they are not harmless--they carry stinger-laden tentacles, apparently stolen from Portuguese man-of-war jellyfish, and bare them whenever approached, the team observed.

Stealing a self-defense system instead of making one from scratch, may have enabled the males to stay small and devote more resources to sex. Blanket octopuses are pelagic, never touching the sea floor, Norman explains, which makes it hard for males to find females in the open ocean. He suggests one reason the males miniaturized was to cut down on development time and beat competitors to the punch. (Females, on the other hand, grow large to produce as many eggs as possible so at least a few will survive in the vast sea.)

When a male does find a female, he gives her his all. After a male uses his special reproductive arm, the sperm-loaded limb breaks off and crawls into the female's gill cavity, and the males usually die. Scientists have found females containing multiple arms from males in them--evidence of male competition, Norman says.

"The extreme nature of the size dimorphism is staggering," says cephalopod specialist John Forsythe of the University of Texas Marine Biomedical Institute in Galveston. More extreme examples can be found among several barnacle species, but there the male lives on the female and is basically "reduced to a sperm generator," Norman says. The scientists will describe their findings in the December New Zealand Journal of Marine and Freshwater Research.

Related sites
CephBase--images and taxonomic data on octopuses and other cephalopods
Lecture on octopus diversity by Mark Norman