Trioecy is maintained as a time-stable mating system in the pink sea urchin Toxopneustes roseus from the Mexican Pacific.

Trioecy is a sexual system that consists of the co-occurrence of females, males and hermaphrodites in a population and is common in plants; however, in animals it is uncommon and poorly understood. In echinoderms, trioecy had never been recorded until now. Frequencies of females, males, and hermaphrodites were evaluated and gametogenic development was histologically characterized in a population of Toxopneustes roseus inhabiting the Mexican Pacific. Trioecy in this population is functional and temporally stable, since the three sexes coexisted in each sampling month. The hermaphrodites presented similar gametogenic development as the females and males and participated during the spawning season, contributing to the population's reproductive process. Trioecy is considered an evolutionarily transitory state, and it is extremely difficult to explain its presence in a species. We hypothesize that continuous ocean warming represents a threat to the survival of this population of T. roseus, since its early developmental stages, which represent a population bottleneck, are more vulnerable to high temperatures than other sea urchins inhabiting the area, while its population density is significantly lower. These conditions generate a strongly stressed environment, which is the determining factor that maintains the stability of trioecy in the species in which it has been studied.

Effect of temperature increase on fertilization, embryonic development and larval survival of the sea urchin Toxopneustes roseus in the Mexican south Pacific.

Toxopneustes roseus performs a key role in the eastern tropical Pacific as a strategic herbivore and bioturbation promoter. We evaluated the effect of temperature on the fertilization success, embryonic development and larval survival of T. roseus under laboratory conditions, to understand how the increase in ocean temperature could affect it in a global warming. The highest percentage of fertilization occurred in gametes that were exposed to 30 °C, and a significant negative effect of 32 °C was evidenced by the lowest percentage. There was also a deleterious effect in embryos exposed to 32 °C, resulting in an abnormal development at all the time points. The highest percentage of larval survival occurred at 30 °C, while the lowest percentage occurred at 32 °C. The results suggest that T. roseus probably lives near its upper thermal limit, and future ocean warming could threaten the permanence of the species in the eastern tropical Pacific, or at least lead to contraction or fragmentation of its range limits. Therefore if sea temperature rises globally, it could cause the disappearance of these populations that are living at the edge of their thermal tolerance, but for other populations located in more temperate latitudes, it could propitiate favorable conditions for fertilization and survival of embryos and larvae.