Host-Associated School Integration and Behaviour Among Juvenile Caranx crysos and Sarpa salpa in a Shallow Mediterranean Habitat.

Interspecific associations between fishes can generate complex behavioural interactions, yet their occurrence and functional significance in temperate marine systems remain poorly documented. Here we describe a previously unreported case of host-associated behaviour involving juvenile Caranx crysos and schools of juvenile Sarpa salpa in coastal Mediterranean habitats. Associations were recorded exclusively in C. crysos individuals ≤ 15 cm TL, suggesting a size- and context-dependent pattern of association with the host schools. Juvenile C. crysos integrated into S. salpa schools by adopting spatial positioning and behavioural patterns consistent with social mimicry. This association may provide antipredator advantages through group integration, while also allowing opportunistic foraging on benthic organisms and resuspended organic particles generated during host grazing activity. In rare cases, multiple individuals of C. crysos co-occurred within the same school, producing aggressive interactions and indicating intraspecific competition for access to the host aggregation. From the host perspective, S. salpa exhibited only brief and limited behavioural responses, with no evidence of persistent exclusion or disruption of schooling behaviour, suggesting a low-cost, largely tolerated interaction. Overall, our observations suggest the occurrence of a host-related behavioural strategy in juvenile C. crysos that combines group integration, commensalistic and opportunistic feeding and intraspecific competition, likely shaped by ontogenetic constraints. These results highlight a previously overlooked level of behavioural complexity in temperate fish assemblages and suggest that fine-scale interspecific interactions in Mediterranean coastal ecosystems may be more widespread than currently recognised.