The impact of diver-induced stress on the ventilation patterns of Mediterranean seahorses: A field-based approach

Seahorses (Hippocampus spp.) are among the most popular marine species for wildlife tourism, being highly sought by divers and underwater photographers. Due to their unique morphology and behaviour, seahorses are particularly vulnerable to the visual and acoustic disturbances caused by underwater photographers, who often use flash photography to compensate for the seahorses' cryptic appearance. The present study utilises Opercular Beat Rate (OBR) as a non-invasive method to evaluate the stress induced by underwater tourism activities on Mediterranean seahorses, specifically the long-snouted Hippocampus guttulatus and the short-snouted H. hippocampus. In-situ experiments were conducted on both species, which are commonly found at dive sites throughout the Mar Piccolo of Taranto (MPT) in Apulia, Italy. The OBR was assessed across all four seasons and in both sexes, under both natural and stress-inducing conditions. The results indicate that stress from underwater photography affects both Mediterranean seahorse species, as evidenced by acute alterations in their ventilation rates. Unlike the behaviour exhibited by other seahorse species, the opercular beat rate of Mediterranean seahorses decreases during periods of stress. In H. guttulatus, this decrease may serve as a defensive mechanism, leading to instances where ventilation rates cease completely, resembling thanatosis.