Monday, February 02, 2009
A special arm for a mating octopus
During the breeding season, a male octopus develops a modified third right arm, called a hectocotylus, Initially he may use it to caress his mate, then after a while he reaches into his body with it and pulls out a mass of spermatophores - large sperm packages shaped like baseball bats. Each has spring-like ejaculatory organ held by a cap at one end. In one octopus species, spermatophores may be more than 3 ft. (1 m) long.
Using his hectocotylus, the male pushes the sperm package into the female's body through her breathing siphon. Sometimes the arm breaks off and stays inside the female. If the male completes this manoeuvre successfully, the sperm package takes up water through a syringe-like system. The water pressure inside eventually pushes off the cap and the spring uncoils, pulling out the mass of sperm and spilling it all over her eggs.
With common squids, a female has more control. She stores the male's sperm in a cavity under her mouth until her eggs are ready for fertilisation. Then she reaches inside her own body and removes her eggs in a string. She makes sure they are fertilised by pressing them one by one against the stored package of sperm, and then she attaches them to the ocean floor in a cluster of 10-50 long strings.
In primitive insects such as the springtail, a male has to trust to luck. He leaves his sperm package on the ground hoping a receptive female will stumble across it. The moisture within her genital cavity melts the protective skin of the package, allowing the sperm to swim free and fertilise her eggs.
If a male finds another male's sperm package, he will eat and then substitute one of his own.