By: Gobezai Abebe
Many species of invertebrates, fish and plants undergo a process which is rare and requires detailed research to understand (Allsop and West, 2003). The process that they undergo is known as sex change. Focusing specifically on hermaphroditic fish, sex change can occur in two directions. One direction that sex change can occur is the change from female to male which is known as protogyny (Allsop and West, 2003; Kuwamura et.al, 2002; Munday et.al, 1998). The other direction that sex change can occur is the change from male to female which is also known as protandry (Allsop and West, 2003; Kuwamura et. al, 2002; Munday et. al, 1998). The species use the process of sex change to help maximize the reproductive success of their colony (Warner, 1982). Sex change is favored when the reproductive success of females or males in a colony is unevenly distributed relative to size or age (Munday et.al, 1998; Oldfield, 2005). In colonies controlled by large male fish, protandry is favored in an attempt to have similar reproductive success in both sexes (Warner, 1982; Munday, 2002; Oldfield, 2005). The opposite directional sex change is true for colonies dominated by large female fish; protogyny is favored in an attempt to balance reproductive success in both sexes (Oldfield, 2005; Kazancioglu and Alonzo, 2009).The social structure amongst each colony also plays a major role in the direction sex change occurs (Munday et. al, 1998; Munday, 2002; Oldfield, 2005). A new phenomenon has been introduced after multiple studies dealing with sex changing hermaphroditic fish, bi-directional sex change. Bi-directional sex change is when a species is primarily either male or female then changes into a female or male and then returns back to it’s to original state as a female (Munday et.al, 1998; Munday, 2002). Bi-directional sex change should be anticipated if the maximum sex specific reproductive success of an organism changes more than once between sexes (Munday et.al, 1998).It also helps when gaining practical knowledge about how species evolve and adapt in their ecosystems over time to help them maximize their reproductive success and survival. Natural selection may favor species who are able to change sex rather than those who are not, according to the size advantage hypothesis (Hoffman et. al, 1985). This paper aims to present information that describes why sex change occurs in fish and how they use this process of sequential hermaphroditism to maximize their reproductive success and other life traits. This review is divided into sections which touch on the causes of sex change and the cost and benefits derived from this process. Ultimate Causes of Sequential Hermaphroditism
Many scientists believe that sex change is strongly favored when reproductive success is dependent on both the size and age of a species and the rate at which each sex reproduces in relation to size and age (Warner, 1982; Munday et.al, 1998; Chopelet et.al, 2009). Sex change is estimated to occur when a hermaphroditic fish is between 70 to 80 percent of its maximum body size (Allsop and West, 2003; Gardner et. al, 2005; Chopelet et. al, 2009). The age at sex change of hermaphroditic fish is not quantitatively defined (Warner, 1982). Allsop and West (2003) suggest that the age at sex change of hermaphroditic fish is related to the age when they reach maturity. Evidence has been found to support the idea that sex change in some species of fish (i.e. coral-dwelling gobies) is also dependent on the proportion of individuals of each gender present in each colony (Munday et.al 1998; Chopelet et.al 2009). In colonies of hermaphroditic fish where the reproductive success of males is high, but there a is large population of males in the colony and a smaller population of females, it would be beneficial for some males to change to females in order to maximize their reproductive success (Munday et.al 1998;Kuwamura...