External fertilization

External fertilization is a male organism’s sperm fertilizing a female organism’s egg outside of the female’s body.Internal fertilization, on the other hand, is the occurrence of internal insemination as the mode of combining sperm and egg. External fertilization occurs in water or a moist area because it gives the sperm external mobility to get to the egg. While in the water, the sperm and ova can shed simultaneously to fertilize the egg. The release of eggs and sperm into the water is known as spawning. When females spawn, they release a batch of eggs into a spot of their choice or just into the water, as in bottom dwelling or sessile species and all of the males start to release sperm that are in close proximity. Within vertebrates, it is the amphibians and fish that use external fertilization. When it comes to invertebrates, most are benthic, sessile, or benthic sessile combined animals such as coral, sea anemones, and tube dwelling polychaetes. The benthic zone is the lowest level of the ocean where organisms called benthos reside. An organism that is sessile does not have the ability to move or be mobile. Benthic marine plants such as algae also go through external fertilization to reproduce. Overall, environmental factors and the timing have a heavy influence over the success of external fertilization.

Sexual selection may not seem to occur during external fertilization, but there are ways it actually can. The two types of external fertilizers are nest builders and broadcast spawners. For female nest builders, the main choice is the location of where to lay her eggs. A female can choose a nest close to the male she wants to fertilize her eggs, but there is no guarantee that the preferred male will fertilize any of the eggs. Broadcast spawners have a very weak selection, due to the randomness of releasing gametes. To look into the effect of female choice on external fertilization, an in vitro sperm competition experiment was performed. The results concluded that there was a decreased importance of sperm number, but increased the importance of the sperm velocity, thus changing the outcome of sperm competition. The ovarian fluid also increased the paternity for the preferred male because they release fewer and faster sperm. The success of a male fertilizing an egg relies on the ability of a male’s sperm to outcompete other sperm that is looking to fertilize the same egg. Sperm chemotaxis is the use of chemical signals to give sperm the ability to navigate an egg and is a huge contributor to reproductive success.

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Wikipedia