Bidirectional Sex Change in Fishes

Abstract

This chapter explains the distribution of bidirectional sex change among teleost fishes, the social context of bidirectional sex change in each mating system, and the adaptive significance. Of a total of 481 hermaphroditic fish species, 69 species have been confirmed to perform bidirectional sex change. Their mating systems are size-assortative monogamy, harem polygyny, or male-territory-visiting (MTV) polygamy. In size-assortative monogamous fish, bidirectional sex change occurs when a new same-sex pair is formed after movements between social groups following mate loss. Widowed individuals tend to prefer pairing with a nearby individual of the same sex rather than moving a longer distance to pair with an individual of the opposite sex. Therefore, bidirectional sex change can increase the reproductive value by reducing the risk of movement to seek a new mate. In haremic and MTV polygamous fish, the size-advantage model predicts protogyny (female-to-male sex change), but reversed (male-to-female) sex change occurs in some specific social situations. In haremic fish, males change back to females when they move to a mating group dominated by a larger male after all females have disappeared from their mating group. The widowed males need not conduct reversed sex change if new females or juveniles immigrate to their mating group or when they can take a female from a neighboring harem. Thus, the widowed males use reversed sex change as a last resort when they cannot gain new mating opportunities in any other way. Such a social context is predicted to be more likely to occur in low-density populations because the number of females in mating groups tends to become small and immigrations rarely occur. Some field studies have supported this low-density hypothesis for reversed sex change. In MTV polygamous fish, a small male, which has performed alternative mating tactics like group spawning, changes sex to perform pair spawning with a larger male. This sex change would be derived from the decrease in advantage of alternative tactics owing to decreased population density.