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Abstract

This comment presents acoustic and visual data showing deep seafloor
depression chains similar to those reported in Marsh et al. (R. Soc.
open sci. 5: 180286), though from a different deep-sea setting. Marsh et
al. present data collected during cruise JC120 from polymetallic nodule
rich sites within the Clarion-Clipperton Fracture Zone (CCFZ), at water
depths of between 3999 and 4258 m. Within this comment, we present data
collected with equivalent acoustic and imaging devices on-board the RV
Sonne (SO261-March/April 2018) from the Atacama Trench, approximately
4000 m depth, which shows comparable depression chains in the seafloor.
In contrast with the CCFZ observations, our study area was wholly free
of polymetallic nodules, an observation therefore weakening the 'ballast
collection' by deep-sea diving mammals formation hypothesis discussed in
their paper. We support their alternate hypothesis that if these
features are indeed generated by deep-diving megafauna, then they are
more likely the resultant traces of infauna feeding or marks made during
opportunistic capture of benthic fish/cephalopods. We observed these
potential prey fauna with lander and towed camera systems during the
cruise, with example images of these presented here. Both the SO261 and
JC120 cruises employed high-resolution sidescan systems at deployment
altitudes seldom used routinely until the last few years during
scientific deep-sea surveys. Given that both cruises found these
depression chains in contrasting physical regions of the East Pacific,
they may have a more ubiquitous distribution than at just these sites.
Thus, the impacts of cetacean foraging behaviour on deep seafloor
communities, and the potential relevance of these prey sources to
deep-diving species, should be considered.