046 | First report of interspecific aggregation and behavioural mimicry between Caranx crysos and Sarpa salpa

Marcello Mezzasalma, Valentina Basile, Elvira Brunelli | Department of Biology, Ecology and Earth Science, University of Calabria, Rende, Cosenza, Italy.

Interspecific aggregations are a widespread phenomenon in marine environments, documented across numerous taxa and behavioural contexts, and are particularly frequent among juvenile fishes. They may confer a variety of functional benefits and are commonly associated with enhanced predator avoidance, increased foraging efficiency and feeding opportunities, habitat-driven interactions, and spawning and migratory activities. However, such interactions remain poorly documented in temperate environments, particularly in the Mediterranean Sea. This study presents the first documentation of a particular case of interspecific aggregation between juvenile of Caranx crysos and schools of Sarpa salpa. This peculiar behaviour was first observed in shallow waters of a Mediterranean coastal habitat in the southern Tyrrhenian Sea (Ogliastro Marina, SA, Italy), characterized by a mixed rocky and sandy seabed. The initial observation was followed by underwater field surveys conducted over two consecutive summer seasons (2024 and 2025) at the same locality. Two observers independently performed random transects lasting 15 minutes each, for a total of 10 hours of observation and 40 transects per season (20 hours and 80 transects in total). Each observer recorded the following data on an underwater PVC slate: (i) the number of schools of S. salpa; (ii) the number of individuals within each school; (iii) the presence and number of juvenile C. crysos individuals within each S. salpa school; (iv) the observed behaviours. Overall, field observations consistently document four types of recurrent behavioural interactions which were classified as: behavioural mimicry, opportunistic foraging, host reactions by S. salpa and intraspecific interactions among juvenile C. crysos. In particular, juveniles of C. crysos (usually solitary individuals) positioned themselves within schools of S. salpa displaying foraging-like movements that mirrored those of S. salpa. During these interactions, C. crysoss individuals, while not effectively grazing like S. salpa, were occasionally observed feeding on resuspended particulate food matter. In turn, some individuals of the host species school were sporadically documented mobbing the juvenile C. crysos. On rare occasions, multiple C. crysos individuals were found within the same S. salpa school, often exhibiting aggressive intraspecific behaviour. We interpret the observed interaction as a case of facultative commensalism involving C. crysos, which likely benefits from increased protection from predators by associating with host-species schools and from the opportunistic exploitation of food resources. The host species does not appear to be harmed by its interaction with C. crysos; however, the occasional mobbing behaviour observed suggests a mechanism of “tolerance with friction.” Lastly, aggressive behaviours among C. crysos conspecifics within the same S. salpa school appear to represent a clear example of intraspecific competition. These findings reveal notable behavioural complexity during the early life stages of C. crysos and contribute to a better understanding of the species’ natural history and adaptive strategies. Our results also highlight that the behavioural, trophic, and ecological structural richness of Mediterranean communities remains understudied. Further research should focus on quantifying the ecological costs and benefits of such interspecific associations and assessing their prevalence across different habitats.