Acclimation capacity of two-banded seabream juveniles (Diplodus vulgaris) to port conditions: implications for the role of ports as nursery habitats

orts are highly anthropized coastal environments characterized by elevated pollution and habitat alteration, posing significant challenges for marine organisms. However, the effects of port conditions on juvenile coastal fish using ports as nurseries remain poorly understood. Here, we combined field sampling and in situ caging experiments to investigate the genetic structure (RAD-seq), physiological condition (growth, lipid content, CYP1A-dependent biotransformation activity), and transcriptomic responses (RNA-seq) of juvenile two-banded sea bream (Diplodus vulgaris) exposed to port conditions. Juveniles from a port site were compared with individuals from two nearby non-port sites (T0), before fish from all populations were caged within the port for one month (T1). The site furthest from the port was slightly genetically differentiated. Port juveniles were larger and fatter at T0 than fish from non-port populations, and all populations showed differential regulation of lipid metabolism, xenobiotic detoxification, hypoxia response, and circadian rhythm pathways between T0 and T1. After one month of caging, weight differences persisted while muscle lipid content sharply declined in all populations. Only juveniles from the site furthest from the port significantly grew between T0 and T1. Port juveniles had a different transcriptional response to caging than non-port juveniles, with a higher expression of genes associated with energy metabolism, while non-port individuals overexpressed genes in detoxification, lipid metabolism, and oncogenic pathways. While we admit limitations related to caging conditions, our results suggest that port conditions strongly influence juvenile fish physiology and transcriptional levels, potentially promoting short-term acclimation strategies that could have long-term consequences for health and survival.