Sequential hermaphroditism (called dichogamy in botany) is one of the two types of hermaphroditism, the other type being simultaneous hermaphroditism. It occurs when the organism's sex changes at some point in its life. In particular, a sequential hermaphrodite produces eggs (female gametes) and sperm (male gametes) at different stages in life. Sequential hermaphroditism occurs in many fish, gastropods, and plants. Species that can undergo these changes do so as a normal event within their reproductive cycle, usually cued by either social structure or the achievement of a certain age or size. In some species of fish, sequential hermaphroditism is much more common than simultaneous hermaphroditism.
In animals, the different types of change are male to female (protandry or protandrous hermaphroditism), female to male (protogyny or protogynous hermaphroditism), and bidirectional (serial or bidirectional hermaphroditism). Both protogynous and protandrous hermaphroditism allow the organism to switch between functional male and functional female. Bidirectional hermaphrodites have the capacity for sex change in either direction between male and female or female and male, potentially repeatedly during their lifetime. These various types of sequential hermaphroditism may indicate that there is no advantage based on the original sex of an individual organism. Those that change gonadal sex can have both female and male germ cells in the gonads or can change from one complete gonadal type to the other during their last life stage.
In plants, individual flowers are called dichogamous if their function has the two sexes separated in time, although the plant as a whole may have functionally male and functionally female flowers open at any one moment. A flower is protogynous if its function is first female, then male, and protandrous if its function is male then female. It used to be thought that this reduced inbreeding, but it may be a more general mechanism for reducing pollen-pistil interference.
Zoology
Teleost fishes are the only vertebrate lineage where sequential hermaphroditism occurs.
Protandry
In general, protandrous hermaphrodites are animals that develop as males, but can later reproduce as females. However, protandry features a spectrum of different forms, which are characterized by the overlap between male and female reproductive function throughout an organism's lifetime:
Protandrous sequential hermaphroditism: Early reproduction as a pure male and later reproduction as a pure female. Protandrous hermaphroditism with overlap: Early reproduction as a pure male and later reproduction as a pure female with an intervening overlap between both male and female reproduction. Protandrous simultaneous hermaphroditism: Early pure male reproduction and later reproduction in both sexes.
Furthermore, there are also species that reproduce as both sexes throughout their lifespans (i.e simultaneous hermaphrodites), but shift their reproductive resources from male to female over time.
Protandrous examples
Protandry occurs in a widespread range of animal phyla. In fact, protandrous hermaphroditism occurs in many fish, mollusks, and crustaceans, but is completely absent in terrestrial vertebrates.
Protandrous fishes include teleost species in the families Pomacentridae, Sparidae, and Gobiidae. A common example of a protandrous species are clownfish, which have a very structured society. In the Amphiprion percula species, there are zero to four individuals excluded from breeding and a breeding pair living in a sea anemone. Dominance is based on size, the female being the largest and the reproductive male being the second largest. The rest of the group is made up of progressively smaller males that do not breed and have no functioning gonads. If the female dies, in many cases, the reproductive male gains weight and becomes the female for that group. The largest non-breeding male then sexually matures and becomes the reproductive male for the group.
Other protandrous fishes can be found in the classes clupeiformes, siluriformes, stomiiformes. Since these groups are distantly related and have many intermediate relatives that are not protandrous, it strongly suggests that protandry evolved multiple times.
Phylogenies support this assumption because ancestral states differ for each family. For example, the ancestral state of the family Pomacentridae was gonochoristic (single-sexed), indicating that protandry evolved within the family. Therefore, because other families also contain protandrous species, protandry likely has evolved multiple times.
Other examples of protandrous animals include: