Agglutinated foraminifera (superfamily Hormosinacea) across the Indian margin oxygen minimum zone (Arabian Sea)

We present a semi-quantitative survey of ‘live’ (stained) and dead hormosinacean foraminifera at six sites (500–2,000 m water depth; bottom-water oxygen concentrations 0.007–2.43 ml L−1) across the Indian margin oxygen minimum zone (OMZ). Abundance of stained and dead specimens was highest at 800 m followed by 1,100 m, lowest at 2,000 m (stained) and 500 m (dead). The peak at 800 m possibly represents a release from oxygen stress combined with a rich food supply (‘edge effect’). We recognised 31 species (27 Reophax, 2 Hormosinella, 1 Hormosina and 1 Nodosinella) among the 605 stained and dead specimens; the majority (21) are apparently undescribed. Species richness was low at 2,000 m; within the OMZ, it was maximal at 1,100 m and minimal at 500 m for both stained and dead populations. Three species (R. agglutinatus, R. aff. bilocularis and R. dentaliniformis) occurred across the entire depth range. However, most species were either confined to the 2,000-m site or to one or more sites within the OMZ. Multivariate analysis of assemblage composition revealed that the 2,000-m site was distinct from shallower sites. Within the OMZ, the 900- and 1,100-m sites were the most similar, and the 500-m site the most distinct. Stained:dead test ratios were maximal at 500–835 m, perhaps reflecting enhanced preservation of cytoplasm at very low oxygen concentrations. At least two Reophax species are common to the Indian and Pakistan margin OMZ; one of these may be confined to the core of the Arabian Sea OMZ.