Influence of seagrass meadows on predator-prey habitat segregation in an Adriatic lagoon

Seagrass meadows have been hypothesized to protect mobile macrofauna from predation by providing structural complexity that interferes with predator movement and vision. However, if ambush predators are common in seagrass, they may exclude many prey species, resulting in lower total abundance and diversity in seagrass relative to neighboring bare substrates, and segregation of predators and their prey into different habitats. We tested these abundance and diversity predictions using visual censuses of fish and invertebrates employing georeferenced videography to quantify habitat and pinpoint every observation, and using bait stations and drop nets over 38 km of scuba transects in all 4 seasons and by day and night in an enclosed estuary of the northeast Adriatic (Novigrad Sea, Croatia). The primary fish predators at our study site were the ambush or stalk-attack predators, grass goby Zosterisessor ophiocephalus and European eel Anguilla anguilla, and we found that these species strongly preferred seagrass habitat. In contrast, most other fish, including small juveniles, avoided seagrass and preferred bare sediment to vegetated habitats. Species strongly preferring seagrass were primarily sessile invertebrates not preyed on by the goby or eel. Both total macrofaunal density and total taxon richness were greatest on bare sediment and lowest in dense seagrass habitats. The 3-dimensional aspect of habitat structure was as important in delineating the communities as the presence or absence of habitat structure. These results are consistent with the hypothesis that ambush predators reduce the value of seagrass habitat for their prey relative to neighboring bare sediment or rock; that structured habitat might carry a higher mortality risk for many prey species, and that neighboring unvegetated habitat, including bare sediment, can be at least as important as seagrass in harboring biodiversity.