Coral colonies can be dioecious or hermaphrodite. Dioecious means that the coral species are composed of either male or female polyps. Hermaphrodite means that the coral species are polyps with both sexes in each organism. Gonads are reproductive organs that produce gametes. The gametes contain genetic code. The gonads are made in the mesentery, in masoglaea.
By hermaphroditic corals gamets are thrown in egg-and-sperm bundles. These three photographs below show coral colonies just before spawning takes place. All colonies are hermaphrodite. You can see orange egg-and-sperm bundles. These bundles are so called, because they contain both egg and sperm. Before fertilization bundles burns, so egg don't unit with sperm from the same coral.
First photograph from left: Platygyra sinensis; photograph by Ed Lovell; John Veron Corals of Australia and Indo-Pacific. Second photograph: Platygyra, photograph by Petter Harrison; John Veron Corals of Australia and Indo-Pacific. Third photograph: Goniastrea; hotograph by Betty Wills; John Veron Corals of Australia and Indo-Pacific
By dioecious corals there could be, that only sperm is thrown or both eggs and sperm are thrown.
When eggs are big female polyps do not throw them into the water. Male polyps throw sperm from the mouth into the water. In the water, sperm enter females polyps through the mouth. Fertilization takes place in the female polyp body. Zygotes grow in parent polyps and leave through the mouth. The initial growth stage takes place in the mesoglaea of the parent polyp’s mesentery. In some coral species, all development takes place in the coelenteron up until the formation of the parent polyp.
When eggs are small, polyps will also throw eggs as well as the sperm through the mouth. Fertilization takes place in the water where zygotes grow.
Photographs above shows dioecoius colonies. Photograph on the left - female colony of Goniopora throwing eggs into the water; photograph by Betty Wills; John Veron Corals of Australia and Indo-Pacific, on the right - male colony of Goniopora throwing sperm that you can see in the middle of the photo as an smudged cloud; photograph by John Bul; John Veron Corals of Australia and Indo-Pacific.
The embryonic development is relatively easy. Like other eumetazoans, by corals striation takes place (eumetazoans are animals that have tissues). Striation is a process in which an egg dividies into descendant cells, called blastomeres. First, the egg divides into two blastomers. Every blastomer then divides into two new blastomers so that the embryo now has 4 cells. Every cell continues to divide into two new cells until the embryo has a few dozens cells total. At this stage it is called a blastula. Blastula is a bubble with only one cell layer.
This photo shows early stages of Acropora coral. Here can you see some blastula, planula larvas and some unfertilized eggs; photograph by Jamie Oliver; John Veron Corals of Australia and Indo-Pacific.
Blastula develop into ciliated larva in process called gastrulation. Gastrulation is the metamorphosis of an embryo with one cell layer into an embryo with two cell layers. An embryo with two cell layers is a gastrula. On one side of the blastula are cells that move separately from the inside larva. It moves the cell ends when whole cavity is filled with cells. The larva now has two body layers: an ectoderm and gastroderm. After this, some cells degenerate to form the coelenteron. A larva with two body layers and a primordial coelenteron is called planula.
The photo shows later development of an Acropora. Larva changes shape and becomes ciliated; photograph by Petter Harrison; John Veron Corals of Australia and Indo-Pacific.
The planula is not round like the blastula but is oval and covered with cilia. Using cilia, the planula swim freely for a time. It then settles on the sea floor and fixes itself to a substrate. The primordial coelenteron then forms into a fully grown coelenteron. On the opposite side of the substrate where the larva is fixed is the mouth. Tentacles from along the border of the mouth. The planula is now a polyp. The coral polyp will grow, divide, and reproduce.
This photo shows young polyp. It settled down on a suitable substrate. Now it grows and produces skeleton.photograph by Carden Wallace; John Veron Corals of Australia and Indo-Pacific.