Broadcast spawners generally spawn during annual mass spawning events in one or more consecutive months (spawning videos: elkhorn coral, Porites, Acropora). For well-studied coral species, we can predict the timing almost to the minute. The annual cycling of water temperature sets the month, the moon cycle determines the day, and sunset triggers the spawning that usually occurs within a few hours after dusk.
Most corals are hermaphrodites—each polyp is both male and female—and form egg-sperm bundles a few hours before spawning. The bundles are released simultaneously within a few minutes and drift to the water surface. After about half an hour they break apart and become ready for fertilization. Having buoyant gametes that gather at the water surface in a defined volume of water ensures a high enough sperm concentration for successful fertilization.
For several days, the embryo drifts in the ocean as plankton while developing into a larva. It starts to divide and forms a spherical blastula within hours. Gastrulation then shapes a simple concave sac, called the gastrovascular cavity, which consists of two cell layers. Soon after cilia have developed on the larvas surface, it becomes mobile, and actively swims down to the sea floor to start searching for a suitable place to settle.
By carefully collecting released eggs and sperm from a few colonies and fertilizing these gametes, it is possible to culture millions of coral larvae. In areas where spawning is limited, in vitro fertilization may give these corals a helping hand and increase their reproductive success.
We use two ways to collect gametes from spawning corals. If the target species forms large colonies, we collect the spawn on the reef by placing specially designed nets above the respective colonies. The egg-sperm bundles are trapped in the net on their way up to the water surface, and concentrate in a sample bottle at the top of the net. The bottle containing the gametes is promptly brought ashore for in vitro fertilization.
Specimens that form smaller colonies can be transferred from the reef to laboratory culture tanks with running seawater a few days prior to the spawning event. If this procedure is carried out carefully, the corals will not be harmed and will spawn at the same time as their counterparts do on the reef. The egg-sperm bundles can easily be collected from the water surface. Afterwards, the corals are safely returned to the reef and attached thereto.
Fertilization may be performed with selected individuals, such as crosses of certain genotypes, or with a batch culture, originating from multiple parental colonies. By maintaining sperm concentration in the ideal range, a fertilization success over 90 % can be reached (Fertilization: When Egg Meets Sperm).
Raising larvae requires excellent water conditions, as all larval stages are very sensitive. Remaining sperm and unfertilized eggs decompose, releasing waste products that quickly reduce water quality. Hence, the larvae need to be transferred frequently to fresh seawater. In order to simplify this procedure, SECORE has developed a specially designed device: the SECORE Kreisel (Evolution of SECORE Kreisels, video: setting up kreisels). The kreisels’ shape and constant water flow creates smooth currents and maintains water quality to rear the fragile embryos without any harm. Several research groups are successfully using this method.