Fishing lines: Hi, Mike, love the dress...

I didn't recognise my old friend Mike when we met by chance in a London street. We used to fish together regularly, share bait and sandwiches, even phone each other with angling yarns. But I didn't recognise him. That said, I had a small excuse. I didn't expect him to be wearing lipstick and a dress.

He said: "Hello, Keith." I stopped and smiled, because these days, strange women don't often talk to me in the street. I did a double take. "Hi. Mike?"

"It's Michelle now." He (sorry, she) handled it a lot better than I did. I stood with mouth open, making strange squeaky noises, while Michelle chatted happily about a new life. Fishing, sadly, was no longer on the agenda, which was probably a good thing.

I'm not too sure my wife would have understood if I said I was going night fishing with an old mate who turned out to be a transsexual. She doesn't understand me going night fishing as it is, and I'm darn sure she wouldn't have been convinced that we merely wanted to talk about old times on the Thames.

I've thought about that encounter several times. We never swapped phone numbers, and we have never met again. I felt I handled the whole thing very badly, instead of being witty and modern about it, as if my fishing mates changed sex every day.

But there's a lot of it about - especially among fish. Just this week, the National Environment Research Council's quarterly, Planet Earth, revealed that many species of fish change sex. I'm not talking about the situation on the Thames and certain other rivers, where oestrogens and other chemicals washing off the land have made dating highly problematic.

More than 90 per cent of the river's roach are now female, which is great if you're a male roach, but can't be a good long-term thing (especially for those who drink that treated Thames water. But I don't want to worry you...).

Anyway, the author of Transsexual Fish From Dimensionless Space, David Allsop, worked with 52 species of coral reef fish. He found that every one of them changed sex at the same relative size and age, when they were about 80 per cent of their maximum body size.

The reasons are quite complex and Allsop can only hazard a few guesses, with species survival being at the head of the list. He also found it didn't matter whether the fish was a male that turned female, or the other way round: the change took place just the same.

I had heard about wrasse doing this when there was a shortage of males or females, but it seems the habit is common. Fish, it seems, are the only vertebrates capable of playing both mum and dad. And then again, maybe not...

'Planet Earth' is available free from the National Environment Research Council, Polaris House, North Star Avenue, Swindon, Wilts SN2 1EU.