Lightening strike perhaps?

Sean is right, electic shock is the most common cause of that, lighting strike near to the pond or faulty electrics, dont put your hands in the water untill you have checked out the later.

The ones I have seen with twisted spines has mostly been caused by chemicals.
They are particularly susceptable to these and all medications used in the pond should be checked first to see if they are suitable for Orfe.
Sue

my thought would also be a medication as the curvature happened over the course of several weeks - lightning strike/electric damage is generally pretty much instant. It's one of the reasons i dont keep Orfe any more (i live at the top of a hill with a large tower block nearby - any storm and we get plenty of grounding electricity - the butyl liner helps a lot -much better than the acrylic stuff - but the orfe seem especially succeptible)

Thanks for that everyone. The electrics are OK, and there's no sign of lightening strikes anywhere in the garden. I'll go along with the medication theory as earlier this year I treated the pond with anti fungus dye based treatment followed a few weeks later with a stronger formaldehyde based treatment on the recommendation of my local aquatic centre.