We all know about the imperilled rainforests, but the world's coral reefs are also disappearing faster than you can say "So long, and thanks for all the fish". Artists Margaret and Christine Wertheim from LA's Institute of Figuring bring this wonder of nature to the Hayward Gallery via the medium of crocheting (a craft a bit like knitting, with looping stitches and a single hooked needle). The exhibition mixes together handicrafts, mathematics and ecology......