What are the different methods of external fertilization?

External fertilisation methods include broadcast spawning, egg scattering, nest building and direct transfer.

Broadcast spawning is a common method of external fertilisation in aquatic animals such as fish, molluscs, and corals. In this method, both males and females release their gametes into the water simultaneously. The eggs and sperm then meet in the water to fertilise. This method is advantageous in large water bodies as it increases the chances of fertilisation and genetic diversity. However, it also has a high risk of gamete wastage and predation.

Egg scattering is another method used by many species of fish. The female scatters her eggs in different locations, and the male follows behind to fertilise them. This method reduces the risk of predation as the eggs are spread out, but it also means that not all eggs may be fertilised.

Nest building is a method used by some species of fish and amphibians. The male builds a nest and the female lays her eggs in it. The male then fertilises the eggs in the nest. This method provides some protection for the eggs and increases the chances of successful fertilisation. However, it requires a lot of energy and time from the male.

Direct transfer is a method used by some amphibians. The male deposits a spermatophore, a packet of sperm, in the environment. The female then picks it up and inserts it into her cloaca, where fertilisation takes place. This method ensures that all the sperm reaches the eggs, but it requires precise timing and coordination between the male and female.

Each of these methods has its own advantages and disadvantages, and the choice of method often depends on the specific environmental conditions and life history traits of the species.