In the underwater forests off northern Japan,
the residents of this sunken wreck
are waiting for the summer temperatures
to reach 16 degrees 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's 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 10 years old
take little notice of his advances.
This is because when any large female reaches a critical body size,
she can begin a dramatic transformation.
Over just a few months,
particular enzymes inside her body cease to work,
and male hormones start 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.
The more bulbous the head,
the more it intimidates an opponent.
The territory has a new ruler.
Only the largest females transform themselves in this way.
But the change enables them to have more mates,
so they will have many more offspring carrying their genes.
But a new male can't afford to be complacent.
Inside the body of every kobudai female,
there is a new male in waiting.