In external fertilisation, males and females release their gametes into the surrounding water and the gametes fuse outside the body. Most bony fish, amphibians such as frogs, and many marine invertebrates such as sea urchins and corals reproduce this way.Because the gametes are released into open water, many problems must be overcome:
• gametes are diluted, washed away or eaten, so very large numbers of gametes are released.
• males and females release gametes at the same time and place. This is called synchronised spawning and is triggered by environmental cues such as water temperature, day length or the lunar cycle, or by courtship behaviour (for example a male frog clasping a female while she lays eggs).
• sperm swim towards the egg in response to chemicals released by the egg (chemotaxis).
• species-specific proteins on the egg surface only bind sperm of the same species, so eggs are not fertilised by sperm of a different species.External fertilisation must take place in water, because sperm need water to swim and gametes would dry out in air. There is usually no parental care, so most zygotes and young do not survive.Its advantages are that no physical contact between the parents is needed and huge numbers of offspring can be produced and dispersed widely by currents.
• gametes are diluted, washed away or eaten, so very large numbers of gametes are released.
• males and females release gametes at the same time and place. This is called synchronised spawning and is triggered by environmental cues such as water temperature, day length or the lunar cycle, or by courtship behaviour (for example a male frog clasping a female while she lays eggs).
• sperm swim towards the egg in response to chemicals released by the egg (chemotaxis).
• species-specific proteins on the egg surface only bind sperm of the same species, so eggs are not fertilised by sperm of a different species.External fertilisation must take place in water, because sperm need water to swim and gametes would dry out in air. There is usually no parental care, so most zygotes and young do not survive.Its advantages are that no physical contact between the parents is needed and huge numbers of offspring can be produced and dispersed widely by currents.