In the underwater forests of northern Japan, the residents of the sunken wreck are waiting for the summer temperatures to reach 16 Celsius.

That, for some, is the time for mating. A kind of giant wrasse called a Kobudai.

This is a male. And in female terms, he is particularly handsome.

He's a metre long and weighs 15 kilos. Much larger than the diminutive female. And he is ready to breed.

He attempts to mate with her -- and with any of the other dozen or so females that live in his territory whenever he gets the chance.

But females from around ten years old take little notice of his advances.

This is because when any large female reaches a critical body size, she could begin a dramatic transformation.

Over just a few months, particular enzymes inside her body cease to work. And male hormones started to circulate.

As time passes, her head expands and her chin gets longer. A she has changed into a he. And with this comes a change in temperament.

The old male who ruled all the females here is challenged to a face-off.