Structural drivers of biodiversity on shipwrecks and natural hard-bottom reefs in the mesophotic zone

Hard-bottom habitats at mesophotic depths have the potential to serve as refuges from climate change, but they are understudied. Anthropogenic structures, including shipwrecks, provide critical habitats for hard-bottom-obligate invertebrates and structure-oriented fishes. A key question in marine ecology and the emerging interdisciplinary field of Maritime Heritage Ecology is how the structure of a shipwreck influences the biological community inhabiting it. To answer this question, we analyzed video recordings from 4 shipwrecks and 3 naturally occurring hard-bottom reefs in the mesophotic zone in the northern Gulf of Mexico/Gulf of America. We tested the influences of habitat size, distance between sites, and type (shipwreck or natural reef) on species richness, community composition, and functional composition of invertebrates and fishes. For both shipwrecks and natural hard-bottom reefs, our results showed a significant influence of habitat size on species richness for invertebrates but not for fishes. All factors had significant relationships with community composition and functional composition. Larger shipwrecks provided vertical relief and overhangs, which were rare on natural reefs and supported pelagic predators and sessile invertebrates, respectively. Shipwrecks hosted the non-native sun coral Tubastraea coccinea and regal demoiselle Neopomacentrus cyanomos, likely because of the microhabitats they provided. Our study demonstrates the important role that shipwrecks play in supporting biodiversity in the mesophotic zone but also highlights the possibility that they could facilitate the spread of non-native species.