1. Sperm and Egg Production:
- Corals are colonial organisms, and individual coral polyps within the colony produce specialised reproductive structures called gonads.
- Female coral polyps produce eggs, which are usually released into the water column.
- Male coral polyps produce sperm, which are also released into the water column.
2. Synchronised Spawning:
- Many coral species exhibit synchronised spawning, which is a remarkable reproductive strategy.
- During synchronised spawning events, large numbers of coral colonies within a specific area release their gametes (eggs and sperm) into the water simultaneously.
- This coordinated release increases the chances of fertilisation and successful reproduction.
3. External Fertilisation:
- Fertilisation in corals is an external process that takes place in the water column.
- The released eggs and sperm mix in the water, allowing the sperm to come into contact with and fertilise the eggs.
- During this process, the sperm penetrates the egg's protective layer, leading to the formation of a zygote.
4. Zygote Development:
- After fertilisation, the zygote undergoes rapid cell division, forming a hollow ball of cells known as a blastula.
- The blastula then transforms into a gastrula, a more complex stage in which the embryo develops different layers of tissues.
5. Planula Larva Formation:
- The developing embryo eventually forms a free-swimming larva known as a planula.
- Planula larvae are tiny, ciliated organisms capable of dispersing and finding suitable new locations to settle and grow into new coral colonies.
6. Settlement and Metamorphosis:
- Planula larvae swim or drift in the water column for varying periods, depending on the coral species.
- When a planula larva finds a suitable substrate (e.g., a hard surface like a rock), it settles and undergoes metamorphosis.
- During metamorphosis, the planula larva transforms into a tiny coral polyp, which begins secreting a limestone skeleton.
7. Colonial Growth:
- The newly settled coral polyp grows and reproduces asexually, forming a colony through budding or asexual fragmentation.
- Over time, the coral colony expands and contributes to the growth of the coral reef.
Synchronised spawning and external fertilisation in corals ensure genetic diversity and enhance the chances of successful reproduction. These processes are essential for the survival and resilience of coral reef ecosystems, which provide habitats and sustenance for a wide range of marine organisms.