How Blue Planet II filmed the fish that changes sex in episode one

We filmed them near Sado island off the west coast of Japan. There is a large wreck just off-shore which is in a protected area. It’s about 30m down and these fish have taken up residence.

The water was very dark and there were often strong currents coming through. They would come out of the gloom and they would really surprise you. You’d be filming near a port hole and they’d just swim out of the wreck. Those big males were pretty grotesque. We called them the Shrek fish because their heads looked far too big for their bodies.

We spent three weeks filming them looking at how temperature affects behaviour. In Japan the cue for mating, and for females to start trying to become males, is 16 degrees Celsius which is normally in June and it’s only about two-three week window each year.

If there are a lot of dominant males then the female fish doesn’t change sex. But if a male is starting to lose his hold on the group, and the social demographic allows it to happen, then a female of a certain age and size can turn into a male. The female hormones switch off and male hormones start to move through the body – Japanese scientists believe it can happen in a matter of weeks. The reason they do it is all to do with succession - they’re likely to produce more offspring as a male than as a female.

The males have big chins and heads and when they’re mating their jowls wobble. It’s not a great look, but fascinating nonetheless.

Interview by Terry Payne