For most species, reproduction is simple: boy meets girl and the rest is history. But mangrove rivulus (Kryptolebias marmoratus) cut directly to the chase: ‘This fish has the unique ability of “selfing” [where the fish reproduce sexually with themselves]’, says Mark Garcia from the University of Alabama, USA. The majority of the fish in rivulus populations are hermaphrodite – maintaining both male and female reproductive tissues in a single ovotestis – with only a small percentage of males making up the numbers. But, this fish also has another talent. When the temperature rises, they can change sex from hermaphrodite to male and do away with their ovarian tissue – although they only embark on this radical alternative when the costs of maintaining a hermaphrodite lifestyle exceed those of being male. So far, the only factor that is known to trigger the fish's transition is high temperature. ‘We were interested in understanding the differences between the sexes so that we can better predict the point at which it would be favourable for them to transition from hermaphrodite to male’, says Garcia.
To find out more, Garcia and his colleagues collected 40 hermaphrodites and 40 males from the labs’ 4400-strong rivulus colony. Monitoring the fish's sex hormone levels and several physical characteristics (including the size of their gonads and the amount of fat they possessed), in addition to measuring their metabolic rate, aggression levels and how daring the fish were, the team found that the hermaphrodites were more aggressive, but took fewer risks than the males. In addition, the hermaphrodites’ metabolic rates were higher than those of the males and they invested more energy in maintaining their large ovotestis. In contrast, the males carried more fat, were generally larger than the hermaphrodites and had higher levels of the estradiol and 11-ketotestosterone sex hormones.
The researchers suspect that the hermaphrodites invest more energy in maintaining their reproductive organs than the males, and they suggest that the hermaphrodites transition to become male when the cost of maintaining their expensive ovotestis exceeds the benefits of selfing. In addition, they speculate that increased competition for resources could be a factor that increases the fish's costs when there are large numbers of hermaphrodites in the population. However, they admit that they are unsure why the males’ estradiol levels are so high, as the hormone can be involved in feminising some species, but they add that it can also be essential for the maintenance of testicular tissue in some circumstances.
The team says, ‘Our findings provide an excellent opportunity to explore why exposure to high temperatures during adulthood mediates the transition between sexes’, and Garcia hopes to learn more about the impact of the fish's metabolic budget on their decision to change sex.
