Oxygen minimum zone copepods in the Arabian Sea and the Bay of Bengal: Their adaptations and status

Abstract

The Arabian Sea and the Bay of Bengal are cul-de-sacs of the northern Indian Ocean, and they contain more than half of the world's Oxygen Minimum Zones (OMZs). The current study reviews the vast and advancing literature on the oceanographic settings that lead to distinct OMZs in the Arabian Sea and the Bay of Bengal and links them with the copepods thriving there, their status, and likely adaptations.The Arabian Sea has a thicker perennial subsurface OMZ (∼1000 m) than the Bay of Bengal (∼500 m), which is linked to high plankton production via upwelling and winter convection in the former and river influx and mesoscale eddies in the latter. Studies world over show that OMZs adversely affect the zooplankton community as their core always sustains reduced zooplankton biomass. Exclusive studies on copepods in the perennial OMZ in the northern Indian Ocean have been limited to the Arabian Sea so far, which showed that the calanoid copepod Lucicutia grandis is an indicator species of the OMZ lower boundary, whereas Calanoides natalis is a diapausing species in the OMZ. Studies also evidenced that many calanoids (Pleuromamma indica, Lucicutia longicornis, Rhincalanus nasutus, Paracalanus aculeatus, Eucalanus attenuatus, Euchaeta rimana, Subeucalanus subcrassus), cyclopoids (Oithona nana, Oncaea conifera, Oncaea subtilis, Saphirina), Harpacticoids (Microsetella sp., Aegisthus mucronatus) and Mormonilloids (Mormonilla minor) living in the perennial OMZ are either vertical migrators or having a patchy distribution between the epipelagic to the deeper OMZ stratum. These OMZ copepods are believed to have distinctive growth and reproductive traits that allow them to exist in the OMZs. Their high enzyme activity allows them to carry out vertical migration, their high lipid reserves allow them to stay alive in starving conditions, and their slow lifestyle reduces their energy consumption in deeper OMZs. Unlike the perennial OMZs in the Arabian Sea, copepods in the seasonal OMZs in the coastal upwelling zones are almost unexplored, except for a recent attempt that demonstrated that cyclopoids have better survival strategies there. The Arabian Sea and the Bay of Bengal around India have a strong seasonal exchange of their water masses, but how they influence and shapethe copepod communities in these regions and the OMZs they harbour is completely unknown.