Habitats and fish communities at mesophotic depths in the Mexican Pacific

Aim: Mesophotic ecosystems, found at the limit of light penetration in the
ocean, are rich in biodiversity and harbor unique ecological communities.
However, they remain among the least studied habitat zones on earth due to
high costs and technological limitations. Here, we characterize mesophotic
communities in two marine reserves across a range of habitat types,
depths, and temperatures using submersible technologies, with the goal of
understanding the processes that structure these communities across
biogeographic regions. Location: The Bay of La Paz and the Revillagigedo
Archipelago, Mexico. Taxa: Fish and algal species. Methods: We used a
small and inexpensive Remotely Operated Vehicle (ROV) to conduct
roving-swim surveys of major habitat types in depths from 20–100 m. With
the resulting binary data of fish species presence, we used generalized
linear mixed models and canonical correspondence analysis and to determine
whether biogenic habitat, depth, and/or temperature best explained species
richness and community structure across reef and non-reef substrate.
Results: We identified 81 species, including new depth records for nine
fish species and a new geographic record for one fish species. Our surveys
included large undocumented rhodolith beds (free-living coralline algae)
and mesophotic algal communities, in addition to diverse communities of
soft corals and sponges. Fish species richness was positively associated
with rocky substrate and warmer water, and reef fish communities differed
significantly by depth, temperature, and biogenic habitat. Main
conclusion: Our results highlight the importance of biogenic habitat in
structuring communities across gradients of depth and temperature. We also
demonstrate the effectiveness of a small and economical ROV for conducting
mesophotic surveys in remote regions. Our methods and results provide a
framework that can be used to greatly increase the biogeographic and
taxonomic scope of mesophotic research, especially for readily
identifiable taxa such as fish.