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The largest continental shelf Oxygen Minimum Zone (OMZ) in the world is formed along the Indian western shelf in the eastern Arabian Sea during the Southwest Monsoon [(SWM); June–September], which is a natural pollution event associated with the coastal upwelling. This study examines the composition, abundance, and distribution of copepods during the Northeast Monsoon [(NEM); November to February] and SWM in 50 m depth zones along the Indian western shelf in the eastern Arabian Sea. The NEM was characterised by warm, stratified, and low-salinity waters in the southeast Arabian Sea and cold, high-salinity, and well-mixed waters in the northeastern Arabian Sea. During the SWM, cold and Dissolved Oxygen (DO) deficient waters (<22 μM/0.5 ml L−1), which are the signs of coastal upwelling, were evident all along the study zone, but with more intensity off Kochi, Mangalore, and Goa in the south than off Mumbai and Okha in the north. The zooplankton total biomass and abundance showed seasonality with a general decrease during the SWM (av. 3.68 ± 1.29 ml m−3 and av. 5711 ± 3096 Ind. m−3, respectively) compared to the NEM (av. 7.37 ± 2.17 ml m−3 and av. 14,473 ± 4966 Ind. m−3, respectively). At the same time, the abundance of Polychaeta and Siphonophora showed an increase during the SWM (av. 1187 ± 1055 Ind. m−3 and av. 169 ± 119 Ind. m−3, respectively), probably a result of the DO deficient waters associated with upwelling. Two striking seasonal features in Copepoda community were evident in this study: (a) a compositional shift from Cyclopoida dominant during the NEM to Calanoida dominant during the SWM, and (b) the coastal OMZ along the Indian western shelf during the SWM was dominated by Calanoida, which include oceanic OMZ species such as Pleuromamma indica , Lucicutia flavicornis , L.paraclausii , Eucalanus elongatus , Subeucalanus pileatus , S.subcrassus , and Clausocalanus furcatus. This forms a clear imprint for the extension of the oceanic OMZ into nearshore waters during the SWM due to coastal upwelling. • Considered Copepoda as an indicator of coastal hypoxia along the Indian western shelf. • A compositional shift of Copepoda was evident during different seasons. • Many Copepoda of oceanic OMZ were found in the coastal hypoxic zones. • The study shows the biological continuity of the coastal and oceanic OMZ. [ABSTRACT FROM AUTHOR]

• Presents the copepods in the OMZs of the Arabian Sea (AS) and the Bay of Bengal (BoB). • Summarises the oceanography of a thicker OMZ in the AS than in the BoB. • Lucicutia grandis is an indicator of the lower boundary of OMZ in the AS. • Calanoides carinatus is a diapausing species of OMZ in the AS. • A severe lack of information exists on OMZ copepods in the BoB. 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 shape the copepod communities in these regions and the OMZs they harbour is completely unknown. [ABSTRACT FROM AUTHOR]