Mesopelagic fish reference specimen photographic library

Mesopelagic fishes are a crucial component of the world’s oceans in terms
of their abundance, biomass, and ecosystem function. These fishes are
important contributors to the biological carbon pump via their feeding and
behaviors, whereby they facilitate the transfer of carbon from shallow
waters to the deep sea. Several species undertake diel vertical migration,
feeding in shallower waters at night and moving to deeper waters during
the day. This process actively expedites the downward flux of carbon.
However, carbon budgets and climate models require accurate information
regarding the depth distributions and migration patterns of these fishes,
and environmental DNA (eDNA) analyses can provide this information. Here,
we utilize eDNA approaches, generating taxonomically-informative COI and
12S reference barcodes for 80 species of mesopelagic fishes, which can be
used to for species-level identification of eDNA sequences. Using these,
along with a publicly available barcode database, we compare results from
eDNA analysis with traditional net sampling and explore the ability of
eDNA techniques to detect diel vertical migration in fishes from samples
collected in Northwest Atlantic Slope Water. We found that eDNA and net
samples often resulted in different species identifications, demonstrating
that eDNA can detect species that would otherwise be missed with
traditional methods. In our eDNA samples, we also detected more species
(12) in our shallowest depth category (0–100 m) from nighttime samples
than from daytime samples (3). This is consistent with increased diversity
in shallow waters at night due to diel vertical migration. Based on the
variability observed in sample duplicates, we suggest that future
mesopelagic eDNA studies incorporate larger sample volumes and scaled-up
sampling efforts. We also note the potential for eDNA analysis to address
ecological questions such as predator-prey relationships and
identification of foraging hotspots, yielding insights into carbon flow
through the ocean’s midwaters.