| Summary: | Halocyprid ostracods are small abundant crustaceans in marine mesozooplankton communities. They feed on the detrital fluxes sinking from the surface into the deep ocean and so play an important ecological role in the food webs and carbon fluxes. During the northeast monsoon of 1997, plankton samples were collected from the Gulf of Oman, using a multiple rectangular midwater trawl. A day and night series of bathymetrically stratified plankton samples was taken to a depth of 2000 m. Ostracods were analysed from these samples. The subsurface samples contained two size groups of Euconchoecia that could not be assigned to any previously described species. Two new species, Euconchoecia omanensis and E. hormuzensis are described. Below 1500 m a species was collected that resembled Paraconchoecia mamillata Müller, 1906 found in the Atlantic. However, both differ significantly from Paraconchoecia spinifera, the type species of the genus Paraconchoecia. Consequently a new genus, Mamilloecia, is established to accommodate the Atlantic species as Mamilloecia mamillata and the new Oman species as Mamilloecia indica. Ostracods from the 1600 m sample have been assigned to a new genus and species as Huxleyoecia muscatensis. At 2000 m ostracods were identified as Mollicia minki Poulsen, 1973. However, the generic name was preoccupied by Mollicia Marples, 1964 a genus of jumping spider. So an alternative name Mollicoecia is proposed to replace Mollicia Poulsen, 1973. The Oman species of Mollicoecia minki Poulsen 1973, new combination, is comprehensively re-described. 3 Ostracod abundance was greatest above 200 m and sharply declined from 400 – 1000 m coinciding with the extreme oxygen minimum zone. The greatest species richness was below this oxygen minimum zone at 1400 m. These data contrast with those from 30ºN 23ºW in the Atlantic Ocean, where there is no extreme oxygen minimum and ostracod abundance peaked at 200 – 300 m and species richness peaked at 700 m.
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