Coral Spawning -- BackgroundLarge coral reefs are made up of different types of coral, each making their own unique structures, such as branches or round boulders. Each branch or boulder is made up of hundreds to thousands of individual coral polyps – little animals that build the stony structures they live on – bit by bit. Such groups of coral polyps all existing interconnected are referred to as coral colonies. Each coral colony is started by an individual coral polyp that literally grows copies of itself, a process known as budding. The new coral polyps are alike in every way and even have the same genetic make up. They look the same as the original coral polyp, and have the same characteristics, just like identical twins.

So how do completely new coral colonies get started? That first coral polyp has to come from somewhere!

Reef-building corals reproduce in many different ways, but in order to make a completely new and genetically different offspring, all corals must go through three basic steps. First, gametes (sperm and egg cells) are produced. Then, the gametes must be released by the parent corals. Finally, the gametes must combine in the process of fertilization and form a planula, or "baby coral." It is this newly formed planula that is the first building block needed to start a completely new coral colony. Let’s look at how the process works.

A single coral colony may be hermaphroditic, which means they have organs of both sexes and can produce both sperm and eggs. Although they are animals, these corals can be compared to flowers which have both male and female parts. Other coral colonies are gonochoric, which means that a single coral colony and all of its identical coral polyps are of a single sex, either all male or all female.

After the gametes are produced, the next step can occur in two ways, brooding, or broadcast spawning. In brooding, only male gametes are released into the water. These sperm cells are taken in by female coral animals containing egg cells. Fertilization occurs inside the female coral, and a small planula develops inside it. This planula is released through the mouth of the female coral and drifts or crawls away to settle elsewhere and grow into a new colony.

In broadcast spawning, which is much more common (Szmant 1986), both male and female gametes are released into the water at the same time, and fertilization occurs when they meet in the water column or at the surface. These gametes often float up in little packets which are greedily sought by predators as the perfect meal. At first, this method of reproduction may seem inefficient, since many of the gametes will not encounter another gamete and simply die or be eaten by predators in the water, but there are many important advantages in this method.

The corals spawn synchronously (at the same time). So many gametes may be released into the water that they can form slicks on the surface which can even be seen from a low flying airplane. With so many gametes released at once, predators can only eat a small portion before they are full. If the gametes were released over a longer time period, a single predatory fish might hang out by one coral colony and eat every single one of the gametes it releases. Also, with such high numbers, there is a higher chance that male and female gametes of the same species will meet and fertilization will take place. This increases the overall success of reproduction.

There are other important evolutionary advantages of this method as well. With all corals on a reef releasing their gametes synchronously, there is a greater possibility that many different genetic combinations of a single coral species may form. This genetic diversity is an advantage when environmental conditions change and corals are stressed by extreme temperatures, diseases, or other harsh conditions. With more variety, there is a better chance that at least some of the new corals have gene combinations that will help them survive. There are other advantages in this method as well. Ocean currents sometimes carry gametes a long distance, increasing genetic diversity even more, and sometimes establishing coral colonies in news areas, establishing new reefs.

You might ask how the corals can time the release of their gametes so closely. Scientists are still puzzled by some of the details, but we now know that it is closely related to the lunar cycle. For example, branching corals in the Florida keys predictably spawn every year 3-5 days after the full moon in August at 10:00-10:30 pm (~2 hrs after sunset), whereas massive corals release their gametes 6-8 days after the full moon in August around 11 pm (~3 hrs after sunset) (Szmant 2003). Scuba divers often gather to dive during these night-time events to watch the packets of gametes float to the surface, and view the array of predators that gather on the reefs to feed on them. As scientists study the behavior of corals more closely, we may learn more about the factors which control this amazing natural process.

Reference

Szmant AM (1986) Reproductive ecology of Caribbean reef corals. Coral Reefs 5: 43-54.

Szmant AM (2003) Coral-list listserver message archive. http://coral.aoml.noaa.gov/pipermail/coral-list-old/2003-February/006155.html