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21 pages, 4 figures, 6 tables, supplementary information https://doi.org/10.1007/s11160-021-09681-9.-- Data availability statement: All the data used in the present study is contained in the online Supplementary information ; The distribution of hermaphroditism in fishes has traditionally been mainly explained by its dependence on biotic factors. However, correlates with major abiotic factors have not been investigated on a quantitative basis and at a global scale. Here, we determined the incidence of hermaphroditism in fish at the family and species level, tested the hypothesis that evolutionary relationships account for the poor presence of hermaphroditism in freshwater species, and tested the association of sexual systems with latitude, habitat type and depth. Functional hermaphroditism is reported in 8 orders, 34 families and 370 species of fishes, all teleosts. Sequential hermaphroditism predominates over simultaneous hermaphroditism at a ratio ~ 5:1 and protogyny (female-to-male sex change) predominates ~ 6:1 over protandry (male-to-female). We found 12 hermaphroditic species that can live in freshwater. However, seven of these species are from four primarily marine families while there are only five species from two mostly freshwater families. Protogynous and bi-directional sex changers have a tighter association with reef-associated tropical and subtropical habitats when compared to protandrous species, which tend to be more plastic in terms of distribution requirements. Finally, simultaneous hermaphrodite species live both in the deep sea and shallow waters in similar proportions. This study can be the basis for further research in specific groups for different purposes, including ecological and evolutionary issues as well as conservation and management of exploited species. Understanding the environmental correlates can help to forecast changes in the distribution or phenology of hermaphrodites in a global change scenario ; We also acknowledge the funding of the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000928-S) and grant PID2019-108888RB-I00. Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature ; Peer reviewed

12 pages, 5 figures, 3 tables, supplemental material https://doi.org/10.1038/s41598-020-60376-w.-- The datasets supporting this article have been uploaded as part of the supplementary material ; The Sparids are an ideal group of fishes in which to study the evolution of sexual systems since they exhibit a great sexual diversity, from gonochorism (separate sexes) to protandrous (male-first) and protogynous (female-first) sequential hermaphroditism (sex change). According to the size-advantage model (SAM), selection should favour sex change when the second sex achieves greater reproductive success at a larger body size than the first sex. Using phylogenetic comparative methods and a sample of 68 sparid species, we show that protogyny and protandry evolve from gonochorism but evolutionary transitions between these two forms of sequential hermaphroditism are unlikely to happen. Using male gonadosomatic index (GSI) as a measure of investment in gametes and proxy for sperm competition, we find that, while gonochoristic and protogynous species support the predictions of SAM, protandrous species do not, as they exhibit higher GSI values than expected even after considering mating systems and spawning modes. We suggest that small males of protandrous species have to invest disproportionally more in sperm production than predicted not only when spawning in aggregations, with high levels of sperm competition, but also when spawning in pairs due to the need to fertilize highly fecund females, much larger than themselves. We propose that this compensatory mechanism, together with Bateman's principles in sequential hermaphrodites, should be formally incorporated in the SAM ; This project was supported by Spanish Government grant AGL2016-78710-R to FP and the Santander Universities Travel Award Grant 2016/2017 to CB and SP that facilitated the collaboration among the three institutions involved in this study. SP was supported by an AGL2016-78710-R contract