Important information about the health of the Great Barrier Reef

Reef Fish Reproduction

How do fish reproduce? There are 3 main ways that coral reef fish can maximize their effectiveness when breeding:

1) Broadcast spawning – this is the most common form exhibited on the reef, and includes most of the parrotfish, coral trouts, wrasses, etc. The fish will generally aggregate on the edges of the reef after dusk in large numbers, and most commonly when there is a bit of current running. Where as corals like to spawn when there is minimal current, coral reef fish prefer more flow, so that the fertilised eggs are swept away to the relative safety of the open water.

The females of the species are present in much larger numbers than the males, as eggs are larger than sperm, and one male can fertilise many females. The males will often have an elaborate courtship “dance”, trying to impress as many females as possible. When the females are responsive to the males charms, she will often start to mimic the dance, and will move up higher in the water column. When she is ready to release her eggs, the male will follow her in her dart to the surface. The eggs and sperm are released externally in large amounts to maximize fertilization. It’s interesting to note that many juvenile fish will do most of their worldly travels in the first few months of their lives.

2) Demersal spawning – This form of reproduction exhibits slightly more care, where the fish (mainly damsels and gobies) actually attach their eggs to the substrate, such as broken dead coral and old broken shells (an excellent reason not to take shells from the reef!). Usually the males will fertilise the eggs after they have been laid and attached. Blue-green chromis will actually change colour to yellow with black patches when they are about to reproduce, making sure that all of the population is aware of what is happening!

3) Parental care – This is where the eggs are fertilised, and the male (or female) take considerable more care of the brood, such as mouth brooders or pouch brooders (such as seahorses). The amount of young produced is generally much lower than broadcasting, but the chances of the young surviving to adulthood are a lot higher. Some sharks and rays go even further and have internal fertilization, which is the ultimate parental sacrifice!

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