Large organic-walled Protista (Gromia) in the Arabian Sea: Density, diversity, distribution and ecology

The genus Gromia includes large marine protists (‘gromiids’) with filose pseudopodia and sack-like organic tests. The first deep-water species were discovered in the 1990s on the Oman Margin of the Arabian Sea and subsequently found on the Pakistan Margin. We present a survey of gromiids in samples collected off Oman in 2002 and off Pakistan in 2003. In addition to the two species (Gromia sphaerica and Gromia pyriformis) already described from this area, at least eight undescribed gromiid species were present. Sausage shaped, grape shaped and spherical morphotypes were represented among this material. On the Oman Margin, gromiids occurred in densities up to several thousand individuals m−2 at 1400 and 1700 m but were much less common at 1100 and 2000 m. Apart from G. pyriformis, which was fairly common (several hundred individuals m−2) at 1000 m, gromiids were uncommon in core samples taken off Pakistan, with 11 indiv. m−2 at 1200 m and 19 indiv. m−2 at 1850 m. On both margins, these protists occurred at depths >1000 m where bottom-water oxygen concentrations exceeded ∼0.2 ml l−1 (=8.92 μM l−1) land sediments were fully bioturbated and oxidised. However, they were not observed at similar oxygen levels above the OMZ. Most gromiids lived on the sediment surface with their apertures facing down and their pseudopodia presumably deployed into the sediment to feed on surficial material and associated bacteria. We conclude that these large protists may play an important ecological role in the bathyal Arabian Sea, particularly in carbon cycling but also in structuring the surficial sediments. In addition, their tests, particularly those of G. sphaerica, provide substrates for attached Foraminifera.