Sexual selection and reproductive success in hermaphroditic seabasses

Mating behavior in simultaneously hermaphroditic seabasses has been often cited as an example of cooperation among unrelated conspecifics. The predominant mating behavior in this group involves egg trading, where individuals reciprocally fertilize parcels of eggs from a partner. Egg trading has been suggested as a good example of a tit-for-tat cooperative mating strategy. Although simultaneous hermaphroditic fishes are often held up as strong examples of cooperation in mating behavior, a closer examination reveals significant sexual selection and sexual conflict between male and female roles among individuals. In the 7 species where data exist, there is a significant increase in male reproductive success with individual size, and in all but 1 species success through male function increases faster than reproductive success through female function. Despite this male-size advantage in simultaneous hermaphrodites, most species maintain their hermaphroditism for their entire life, and the increased male allocation while engaging in biased forms of reciprocation appear to increase the evolutionary stability of hermaphroditism in these species. Thus, egg-trading behavior is probably more complicated than was initially recognized, with individuals releasing different numbers of eggs in spawns, spawning at different rates as males and females, and partitioning male effort between pair and alternative mating tactics. The departures from equal reciprocity can probably be best understood by including aspects of traditional mating-system theory, with individuals increasing male mating success through a variety of behavioral tactics.