The visibility was about 5m – average to good. I sank beside an algae-encrusted rope hanging from a scruffy marker buoy in the Skomer Marine Conservation Zone (MCZ) off the coast of Pembrokeshire, south-west Wales. Some 15m down, the rope divided at a large knot – one branch disappeared under a car-sized boulder, the other into a cleft in a rock ridge. So far, so familiar. I installed these ropes when I established a monitoring site here in 1982, and I have returned regularly over the years to follow the progress of the reserve’s marine wildlife. On this particular visit I was with Kate, a former colleague in the MCZ team, to help with this vital research. A couple of metres deeper, a V-shaped cliff dropped away into the greenish-black depths and – yes – my old friend was still there. Though it looked like a 0.5m tall, peach-coloured, leafless tree, it was a pink sea fan Eunicella verrucosa – a warm-water-loving soft coral close to the northern limit of its distribution.

Newly settled sea fans grow at an annual rate of 1cm or more, but as they mature this drops to 3mm or less. Several of the nearly 120 fans that are now monitored every year in the MCZ are over 0.5m high and wide, implying they are at least a century old. This one was the biggest of all and had grown only slightly in the 30 years I had known it, which represented perhaps a quarter of its life. That dive occurred in 2013. When Kate and I returned in July 2015, the buoy had gone, though it had been lost and replaced many times before and we knew the way by then. I swam a few metres over the diff edge, only to find that ‘m/ sea fan had disappeared too. We looked at each other with wide eyes, gesticulated wildly and swam around madly in a desperate search. We surfaced babbling to swear and speculate about the disappearance.

The sea fan wasn’t vulnerable to many direct threats. Could a diver have kicked it off? It’s unlikely. Skomer’s North Wall is only for experienced divers, who make sure that they leave the seabed as they find it. Perhaps it was snagged by the rope on a string of lobster pots, and torn free? This is more plausible – lobster potting is legal there, and we have seen pot lines tight up against sea fans several times over the years, and have carefully moved the ropes away. Some fans that have been dislodged have even been reattached using epoxy resin. It’s difficult not to bond with wildlife you see for decades. Even so, I was surprised at just how devastated I felt. The fan’s longevity and tenacity meant something personal. I returned to the site a few weeks later and visited another handsome sea fan living under an overhang close inshore, where ifs protected from fishing gear. I’ve only known this one for a couple of decades, so ours is a comparatively recent relationship, but with the loss of my old friend I look forward to following its future.