In 1981, Australian researchers on the Great Barrier Reef stumbled across an amazing natural phenomenon. All of a sudden, millions of tiny cells began spewing out of the living corals. They poured into the water column like champagne bubbles, floating to the surface and forming a thick, pink 'slick'.
The researchers had witnessed coral spawning, an annual event, which occurs in all coral reefs around the world. Today we know that many corals living on the Great Barrier Reef spawn about four to five days after the full moon in October or November and sometimes in December.
Why do corals spawn after a full moon, and why do the east and west coast corals spawn at such different times of the year, despite the fact that these reefs have many species in common?
There are three triggers that set off spawning in corals, according to coral reef expert Associate Professor Peter Harrison from Southern Cross University.
- Firstly there must be a gradual rise in sea temperature, triggering the eggs and sperm to mature inside the adult coral.
- Second, the lunar cycle coordinates the exact moment at which the corals release their precious cargo into the water.
- The third stimulus is related to the diurnal cycle - coral need the darkness of night to do the wild thing!
It's the tide, baby!
According to coral researcher Associate Professor Bette Willis from James Cook University, it's not actually the moonlight, which gives them the final nudge, but what the tides are doing at this time of the lunar cycle. When the moon is full, there is quite a big difference between high and low tide. Then about five nights later a neap tide occurs. In this kind of tide, there is a small difference between the heights of the high and low tides.
Why is this important for coral? A small tidal difference means the water flow is reduced, and this means weaker currents. This is important for coral spawn, because calm waters enable the eggs and sperm to hang around and mix, rather than become too quickly dispersed by water currents.
When corals spawn, they produce what is known as a spawning slick. These can be metres wide and kilometres long, and often have a pink or brown tinge. Slicks have even been detected from space by satellite imagery!
Slicks have been observed by fishermen for years and even occur in the mythology of some maritime communities. For example, coral slicks have been associated with a particular Asian legend of a princess who was captured by a dragon and taken beneath the sea. The coral spawn supposedly marks the trail of her menstrual blood.
In another story, local fishermen in the East China Sea avoid catching a normally popular reef fish for about three weeks of the year. During this time, they say, the fish tastes horrible and becomes inedible. This period, it turns out, coincides with when the corals spawn. The fish are probably eating the eggs in the morning. Coral eggs contain a lot of lipids and also chemicals that deter predators. Indeed, researchers have noticed that coral spawning slicks sometimes have a sickly sweet or oily odour, and can even smell like turpentine.
Girl meets boy
Most corals are hermaphrodite, where both eggs and sperm will develop within the same polyp. In the branching corals the eggs take about nine months to develop, and sperm about 3-4 months. They mature at the same time and when the water temperature and tides are just right, eggs and sperm are ejected rapidly from the polyps in massive numbers. The sperm are free-swimming while the lipid-filled eggs float quickly to the surface. When an egg is fertilised, a tiny embryo develops within 24 hours. The tiny baby coral is ready to settle after about five days, and drops down to the bottom. If it is lucky, it will find a good spot to grow for many decades.