By Andrew Luck-Baker

BBC Radio 4, Palau

Dense rafts and slicks of pink coalesced all around the boat

The annual mass spawning of corals on the Palau archipelago in the western Pacific has occurred right on cue. With Sunday night's full moon, coral polyps let forth a huge swathe of sperm and egg, to seed the next generation. The event was short-lived - only about 30 minutes - but so vast in its scale that it turned the sea water pink. Scientists from Palau, Australia and the UK are studying the practicality of collecting coral larvae to help restore damaged reefs elsewhere. Please turn on JavaScript. Media requires JavaScript to play. Advertisement As we got into the boat for our trip to Luke's reef, I admit I was not really expecting to see the mass spawning on the exact night of the full Moon. All the visiting scientists here thought it was more likely the reproductive extravaganza would happen the next evening or the following one - based on what had happened the last two years. The only person who seemed sure it would happen on cue was Steven Victor, the Palauan director of the Palau International Coral Reef Center. Local knowledge was spot on, as it turned out. Almost as soon as the boat engine switched off, we got a sense that something might be brewing. I was snorkelling in what looked like a reverse snow storm of orange and pink particles

Andrew Luck-Baker

Catching a spectacular moment There was a faint fishy whiff in the air, and then in the torchlight, one, then two orange particles - coral spawn - suspended in the water. Scanning from the other side of the boat, the excitement went up another notch - a steady stream of orange spat was rising to the surface in one small isolated patch. Scuba gear was flung on and the marine biologists were overboard. I paddled on the surface with snorkel, mask and diving torch, watching the scientists check the coral colonies on the reef bed five metres below. The minutes ticked by - lots of them. If our first stream of spawn was the warm up act, was the main attraction having a mighty tantrum and refusing to come on tonight? Apparently not. Sonia Bejarano, from the University of Exeter, UK, surfaced with an update. A great many of the branching table corals and stag horn corals - the chief reef builders - were close to spawning. The little egg and sperm bundles were visible in the open mouths of most of the individual coral polyps of each colony. Depending on the size of the colony, the number of tiny sea anemone-like polyps ranges from hundreds to thousands. At 8.29pm the mass spawning began. Across the reef, polyps contracted into their stony skeletons. Spawn particles popped out of their mouths. Because the egg and sperm bundles contain waxy yolk, they are buoyant and rise in the water column. Within minutes, I was snorkelling in what looked like a reverse snow storm of orange and pink particles. It became thicker and thicker as more and more colonies across the reef fired their latest shots at founding a new generation. The spawn just kept coming - the sea was becoming a pink soup. Pink was emerging as the dominant colour. Akin to taramosalata There was a rising pale rose particle for every cubic centimetre of seawater at least. Above water, the odour of spawn was also thick in the air - it smelt like taramosalata, the pink Greek dish made of fish roe. I spent most of the time in a state of amazement at the surface but I managed to get down a couple of times to the reef bed to see a colony close-up as it released its spawn. Profusions of pink blobs, each with a little tail of mucus, wafted from the antler branches of a stag horn colony. The reef fish were also excited. Earlier most of them were hidden, lurking in dark crevices and overhangs for safety away from night-time predators. But with the spawn bonanza, many threw caution to the winds and came out to feast. Back on the boat, Peter Mumby, also from Exeter, estimated it was about half hour from the time the first colony unleashed its spawn to the time the last one spewed forth to multiply. The time it all started was almost exactly the same as the moment the mass spawning began the previous year he'd visited Luke's reef to video the event. Somehow all the colonies are running with synchronised biological clocks. A "pink soup": Scooped from the sea In some way not really understood by scientists, they are entrained by factors such as light levels and durations from the Sun and the Moon over a year and over individual months. The result is they spawn within minutes of each other. That synchrony is vital if you reproduce as most corals do. The majority are hermaphrodites which release their eggs and sperm into the open water for fertilisation. The chances they will have any offspring would be terrible if they did not have tight coordination so that eggs of one colony meet sperm from another. By now, at the surface, the number of egg and sperm bundles was staggering. Dense rafts and slicks of pink were coalescing all around the boat for as far as we could see. The ripe smell of taramosalata hung in the air. And we could see the water below the floating spawn begin to cloud up. Seawater was penetrating every spawn particle - breaking apart separate compartments of eggs and sperm. The visibility was falling as billions of coral sperm were liberated for fertilisation. Andrew Luck-Baker is preparing a Frontiers programme to be broadcast on BBC Radio 4 on Monday 26 May. The programme will also be broadcast on Discovery on the BBC World Service.



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