Animal of the Week May 16, 2005 -- I appear to have a man in my ovaries

Oh hello... it's you... welcome. It's been a while, well, a week. Come in, sit down, put your feet up.... how rude of me, how terribly rude, let me introduce you to Bonellia viridis (green spoon worm), this week's Animal of the Week. Green spoon worms were brought to my attention by the recent Channel 4 tv series Dr Tatiana's Sex Guide to All Creation. These worms live in the Pacific ocean and are special for three main reasons. 1: They have the greatest sexual dimorphism of any species, but unlike many familiar species, in which the males are bigger (eg, humans), female green spoon worms are bigger than males, 1000–2000 times bigger to be approximately precise. 2: Males live inside the ovaries of females in a chamber called the androecium, they absorb nutrients through their skin because their mouths are too busy spewing sperm. 3: Even though they have this bizarrely extreme and extremely bizarre differentiation between the sexes, their sex is not genetically determined; whereas, for example, mammal males have an X chromosome and a Y chromosome and females have two X chromosomes, green spoon worms have no gender-defining genetics. Their sex is determined by the events in the first three weeks of their lives; if they don't meet another green spoon worm they settle down on a rock and become a 2 m long female; however, if they run into one of these mammoth worms they are swallowed by her and become a male, never growing bigger than 1–2 mm.