Earthworms are simultaneous hermaphrodites.
Simultaneous hermaphrodites are organisms that have male and female sexual organs. The most famous example of a simultaneous hermaphrodite is probably the snail. When snails mate, both partners exchange genetic material, with the sperm contained in “arrows” of calcium carbonate. Many other slug species are also simultaneous hermaphrodites, as are some fish.
Snails are simultaneous hermaphrodites.
The term “hermaphrodite” is often used generically to describe any type of organism that has sexual characteristics associated with both sexes. The term is derived from a character in Greek mythology who merged with someone of the opposite sex, creating an entirely new individual with both male and female characteristics. As a side note, it is considered offensive to describe humans with ambiguous genitalia or amorphous sexual characteristics as “hermaphrodites”. Most people prefer to use the term “intersex” to describe such individuals.
The clownfish is a sequential hermaphrodite.
This type of sexual development can be contrasted with gonochorism, in which the sexes are distinct and very different. Most mammals, including humans, are gonochorists. Some other animal species have evolved to demonstrate sequential hermaphroditism, in which the animal’s gender changes at some point in its life. Sequential hermaphrodites are usually fish; the clownfish is a notable example of a sequential hermaphrodite.
There are some distinct evolutionary advantages to being a simultaneous hermaphrodite. For example, some animals are capable of self-fertilization. These species can also reproduce faster, as both parties involved can generate eggs and sperm, thus distributing the species more widely. Damage to the sexual organs of the organism also does not always mean the end of sexual reproduction, because the organism has another set.
Pseudohermaphrodites, such as hyenas, are also viewed by some people as simultaneous hermaphrodites because they appear to have the physical sexual characteristics of both sexes and sometimes manifest behavioral traits that resemble both sexes. The case of hyenas is quite interesting; female hyenas develop a kind of false penis, and dominant females sometimes mimic the behavior of males to reinforce their position in the pack. However, hyenas are not true hermaphrodites because all of their sexual organs are female despite their outward appearance.
In some cases, these organisms have specific biological measures that prevent self-pollination, to increase biodiversity. Earthworms, for example, are simultaneous hermaphrodites, but must mate with other earthworms to produce fertilized eggs. Other concurrent hermaphrodites, such as banana slugs, are capable of self-fertilization but are biologically predisposed to seek a mate.