Predation success of declining long-snouted seahorse (Hippocampus guttulatus) population under habitat shifts

Seahorse populations are facing significant declines worldwide, often associated to illegal trade and habitat loss or fragmentation. Emergent habitat shifts towards invasive seaweeds, facilitated by global warming, can also be at play, having still unknown effects on seahorse ecology. Particularly, indirect impacts on seahorse populations via altered predation success remains largely unexplored. Here, we conducted mesocosm experiments to investigate differences in the prey capture success of the long-snouted seahorse (Hippocampus guttulatus) among native habitats (i.e. seagrass meadows and unvegetated sediments) and non-native Caulerpa prolifera meadows when offered different preys (amphipods and shrimps), at low or high prey availability, and under low or high habitat complexity. Great plasticity of H. guttulatus to all habitats was revealed, as habitat type did not significantly affect the capture success. Instead, capture success depends on prey identity and availability regardless of the habitat type, as well as on habitat complexity. Seahorse capture success was higher under high prey availability, as well as when predating on amphipods that were offered together with shrimps. High habitat complexity of both vegetated habitats enhanced by double the capture success, whereas an opposite effect of similar magnitude was found for more complex unvegetated habitats. This indicates that vegetation complexity, regardless their native or invasive nature, reduces prey awareness to predator, whereas small structures in unvegetated habitats provide refuge for preys but not seahorse camouflage. We found no evidence that shifts from native habitats to invasive C. prolifera negatively impact H. guttulatus capture success, but further research on other aspects of its feeding ecology is still necessary.