Sex-specific dynamics of reproductive investment by a Mediterranean hermaphroditic fish in a warming sea

Sex-specific patterns of reproductive allocation remain poorly documented in wild vertebrate populations under rapidly changing environmental conditions. Here, we quantify variation in reproductive investment in a wild population of a simultaneous hermaphrodite, the painted comber (Serranus scriba). Three successive sampling periods spanned a pronounced and anomalously warm spawning season in the Mediterranean Sea, during which temperatures increased rapidly and coincided with multiple marine heatwave events. Across these periods, gonadosomatic indexes declined markedly in both sexual components, with a significantly steeper decline in female allocation. This reduction was accompanied by lower female relative fecundity and smaller oocytes, despite histological evidence of sustained reproductive activity, whereas sperm concentration and motility showed no detectable seasonal change. The latero-ventral blue spot became less saturated and darker over the warming spawning season and was closely associated with female reproductive investment. This study provides the first reproductive phenology data for S. scriba in the southern Tyrrhenian Sea, using individuals that respond at the same time in the male and female functions (simultaneous hermaphrodites), and suggests a potential sex-specific sensitivity to thermal stress. These findings establish a baseline for populations reproducing under contemporary ocean warming conditions and highlight the need for multi-year studies to evaluate the consistency and underlying mechanisms of these sex-specific responses.