Hypoxic guts in small benthic copepods

Copepod guts host intensified microbial activity, forming oxygen depleted or even anoxic microenvironments in otherwise oxic realms. This may have important implications for element cycling and biogeochemical processing. However, current studies have only targeted relatively large zooplankton specimens ignoring ubiquitous, omnipresent small (<1 mm) copepod species. Using oxygen sensitive nanobeads and a phosphorescence imaging approach, we imaged the internal O2 distribution of small living copepods; Tachidius discipes and Nitocra spinipes with benthic lifestyles. The internal hypoxic O2 distribution was strongly dependent on position and content within the gut and by the ambient O2 levels experienced by the animals. Anoxia was only rarely encountered, but extrapolations and measurements at lower ambient O2 levels strongly suggest that even small copepods will have anoxic interiors when exposed to O2 depleted environments such as surface sediments, detrital enriched material patchesor pelagic anoxic minimum zones.