High diversity of deep-sea Gromia from the Arabian Sea revealed by small subunit rDNA sequence analysis

Abstract

Gromia is a large marine protist with filose pseudopodia and ovoid test, common in coastal intertidal and sublittoral waters. Although deep-water Gromia-like morphospecies were discovered in the 1990s, their relations to the shallow water species have never been established. Moreover, very little is known about the diversity within Gromia, reflecting the fact that these morphologically relatively simple protists have few characters useful for species identification. Consequently, we have analysed the SSU rDNA and ITS rDNA genes to examine gromiid diversity in two different areas located on the Oman and Pakistan margins of the Arabian Sea (water depths 1,000-2,000 m). In total, 27 deep-sea gromiid sequences of the SSU rDNA gene and six sequences of the ITS rDNA region were obtained. Our data confirm that Gromia-like protists from the bathyal deep sea are related to the shallow-water gromiids. Within Arabian Sea Gromia, we identified seven distinctive lineages, five of which form a monophyletic group branching as a sister group to shallow-water species. Six lineages of Arabian Sea Gromia can be defined morphologically, while one lineage includes specimens that look identical to specimens from two other lineages. This indicates that each Gromia lineage represents probably a separate species and suggests that deep-sea gromiid diversity is higher than indicated by their simple morphology.