Morphometrics of the deep-sea sharks’ respiratory system New insights into the adaptations of sharks to the deep-sea

Deep-sea sharks are exposed to an array of extreme conditions, including low-oxygen levels, and gather multiple adaptations, notably the production of bioluminescence as a camouflage strategy. Here, we concentrate on the adaptation of their respiratory system to variation of oxygen availability and metabolism requirements. Indeed, contrary to previous belief, some deep-sea sharks have aerobic metabolic rates as high or even higher than benthic surface species despite the lower levels of oxygen of the deep-sea. Hence, we investigated here if their respiratory systems, and more especially their gills’ dimensions (lamellar frequency, filament length and number of lamellae), harbour adaptations to their high aerobic metabolic requirements in relatively low oxygen levels. Through morphological measurements of gills (via optical microscopy and scanning electron microscopy) of several deep-sea species and surface species, we showed that demersal deep-sea and demersal surface species of similar size have similar lamellar frequency and total filament length and thus total number of lamellae. That is, deep-sea species do not have specific adaptation of their total number of lamellae to compensate for the low oxygen levels. Moreover, we showed that there might not be any cost associated with being bioluminescent as there were no differences in the total number of lamellae between luminous and non-luminous species harbouring similar ecology. Though deep-sea sharks remain difficult to study, this work highlights the necessity of further analysis and comparison between deep-sea sharks and their surface relatives, to unveil their adaptations to the extreme environment that is the deep-sea.