What are some disadvantages of external fertilization?
External fertilization requires the presence of water to carry the sperm to the egg. That means animals with external fertilization must return to water to lay their eggs and fertilize them there. That in turn limits the distribution of the animals to areas with water only. Further, if the current is too strong, it may carry the sperms away from the eggs, leaving many eggs unfertilized. External fertilization also makes it impossible to have shelled eggs, because the shell would block the entrance of the sperm.
