Substrate preference of the Vulnerable seahorse Hippocampus patagonicus in Patagonia

Seahorses rely on available substrates within the ecosystem for both feeding and protection, using their prehensile tails to grasp different substrates while remaining camouflaged and foraging. Herein, we evaluated the selection and preference for grasping substrates by the Patagonian seahorse Hippocampus patagonicus in the coastal ecosystems of San Antonio Bay (Patagonia, Argentina). We carried out diving surveys to evaluate the availability and selection of biogenic substrates by seahorses and the abundance of prey in each type of substrate. We also conducted aquarium experiments to test selection by seahorses considering the presence of prey and 6 types of substrates. In the natural environment, seahorses showed a preference for the red alga Ceramium rubrum and the brown alga Dictyota dichotoma , the most abundant substrates in the habitat. However, these algae did not host the largest diversity or abundance of potential prey. The highest species richness of prey was observed in the sponge Hymeniacidon perlevis and the introduced ascidian Styela clava . In aquarium experiments where substrates were equally available, seahorses consistently selected the ascidian, regardless of prey presence, suggesting that the stability of the grasping structure may play a key role in substrate preference. The substrate selection behaviour of the Patagonian seahorse has important implications for individuals and populations, as these preferences are linked to essential resources for reproduction and survival. Given the large anthropogenic disturbances affecting the coastal ecosystems of San Antonio Bay, understanding these habitat requirements is crucial for developing conservation strategies to protect essential habitats of this Vulnerable species.