So, today we are back onto corals (Anthozoa), after a little detour yesterday, and starting out at the beginning, with the life-cycle. Different types of corals within the reef structure will be covered later, this post is just going to be about how they spawn.
Most corals reproduce sexually, and the majority of them are hermaphrodite (meaning they have both male and female reproductive organs). Most corals reproduce once a year, and the results are very spectacular!
The corals which are the same species within a reef have synchronized spawning. This means that all the corals of one species release their sperm and/or eggs at exactly the same time! It is currently thought that environmental triggers such as temperature, day length, or the lunar cycle. An experiment carried out by NOAA (the National Oceanic and Atmospheric Administration) on the Flower Garden Reef in the Gulf of Mexico in 2008 showed that the although the day of spawning is determined by the lunar cycle, the exact time of spawning appears to be dependent on the time of sunset. (A link to the paper is here) Other scientists have looked at how the corals are able to time the release of sperm and eggs, and have found that corals have cells which respond to light (photoreceptors).
They have found that some corals have light sensing cells (photoreceptors) of a variety known as cryptochromes, which respond to blue light. These produce a protein in response to the amount of light available. In the study (information in this link here) they found that the corals produce a lot of this protein when it is very light, and less when it is dark. They do produce the protein at night, but a lot more of it when there is a full moon, as shown in the image below (the graph to the right is the amount of the protein produced during a new moon, or a full moon).
Why would corals respond to light, and how does this tie into their breeding cycle?
The cells which respond to light are found on the outer most layer of the coral (the ectoderm), and may be sensors for when there is too much UV light, allowing the corals to protect themselves against it, much the same way as we go brown in response to an increase in UV (Not red, that is when we burn, and the skin is damaged, but going brown is due to an increase in a chemical called melanin).
In relation to the spawn cycle of corals, most corals appear to spawn 6-10 days after a full moon, this is when the lowest tides occur (neap tides). Neap tides also occur in the same period after a new moon, but it appears that most corals time their spawn for the neap tides after a full moon. The reason for the spawning events to occur during this tidal period is that the currents and waves are weakest during this period, and so the eggs and sperm are less likely to get spread across the ocean before they mix.
Ok, now onto the much cooler (and less technical) bit!
I mentioned at the beginning that many corals are hermaphrodites, having both male and female reproductive organs. During spawning, these corals release a bundle of eggs, surrounded by a sperm package. This does not fertilise the eggs, but keeps the bundle held together until it reaches the surface, where it breaks apart so that the sperm and eggs can mix with others from the same reef!
There is so much sperm and eggs released by the corals on a reef that it can form a sheen, like an oil slick on the surface, and can actually be smelled!
Because it looks much cooler when you see it on video, I will leave you with a short clip from the same reef as the image above, and one slightly longer one with David Attenborough.