Influence of salinity on the larval development and juvenile fitness of the estuarine shrimp Palaemon longirostris

Salinity fronts structure the nursery habitat of the estuarine shrimp Palaemon longirostris . This species inhabits estuaries in Western Europe and North Africa. Using a salinity gradient representative of estuarine conditions (10, 15, 20, 25, and 30 PSU), we evaluated the effects of salinity on larval history traits (including larval survival, developmental pathways, duration of larval development, metamorphosis success, larval weight gain and juvenile weight). Our results demonstrate that increasing salinity towards marine levels shortened larval development time and improved metamorphosis success, with the best results observed at 25 PSU. Juvenile weight and weight gain were significantly higher at elevated salinities, indicating a positive relationship between salinity and larval fitness. Conversely, low salinities were associated with reduced survival and performance, indicating limited osmoregulatory capacity in response to osmotic stress. These findings emphasize the important role of salinity in determining the phenotypic plasticity and survival of P. longirostris larvae. Together, these results highlight an operational optimum of around 25 PSU in the laboratory and sharp declines in performance at the extremes of the estuarine gradient. By clearly demonstrating for the first time an optimum high salinity level for the larval development of P. longirostris , this study provides original insights into the life cycle and population dynamics of this species. • We tracked Palaemon longirostris larvae along a 10–30 PSU gradient. • Optimal larval performance at 25 PSU; 10–15 PSU increased mortality and duration. • Juvenile mass and daily mass gain show no compensatory plasticity at low salinity. • A rare seventh instar at 10 PSU suggests stress plasticity without growth benefits. • Salinity thresholds support larval export to coastal waters and nursery assessment.