A major advance in our knowledge of gromiids came in 1994 during a RRS Discovery cruise on the Oman margin of the Arabian Sea, when we discovered some large spherical organisms up to 3 cm or more in diameter and filled with sediment, living at depths below 1000 m. To cut a long story short, these ‘jellyballs’ turned out to be the first deep-water gromiids to be discovered. The crucial piece of evidence was the highly distinctive structure of the organic test wall, revealed in high voltage electron micrographs taken by our colleague Dr. Sam Bowser in Albany, New York. They were subsequently described as Gromia sphaerica (Gooday et al. 2000). An unusual feature of this species is the presence of multiple apertures scattered across the test surface.


Further opportunities to sample in the Arabian Sea came during cruises aborad the R.R.S. Charles Darwin on the Oman margin (2002) and on the Pakistan margin (2003). In both areas, we collected many gromiid-like organisms at depths between 1000 m and 2200 m. Gromia sphaerica was again present, but this time the assemblages were dominated by grape- and sausage-shaped morphotypes ranging in size from 0.5 to 2 cm and with a single aperture. They occurred in abundances of up to 45 individuals per m2. In most cases, the ‘grapes’ and ‘sausages’ lived with their apertures buried in the organic-rich surface sediment (Figures 1 and 2). We were still not absolutely sure that they were gromiids but confirmation came from analysis of SSU rDNA gene sequences in the laboratory of Dr. Jan Pawlowski (Geneva). The analysis was done using previously developed Gromia oviformis specific primers, and results showed that deep-sea gromiids and Gromia oviformis branch together, with 100 confidence, next to other protistan groups such as Haplosporidians, Plasmodiophorans, Cercozoans and Foraminiferans (Aranda da Silva et al. 2006). Within deep-sea gromiids, there are at least seven species, confirmed with SSU rDNA analysis. Four of these seven species have distinct morphological characteristics. However, the other three which have grape-like shapes could not be distinguished purely on the basis of morphology, showing the existence of cryptic speciation. An eighth species has been described on the basis of its morphology (Gooday and Bowser 2005).

Most single-celled eukaryotic organisms (protists) are small and invisible to the naked eye, so the idea of protists a centimetre or more in size is rather startling. Infact, giant protists are sometimes common in marine environments ranging from deep-sea to coastal waters. The real giants are the xenophyophores which can grow up to 10 or even 20 cm in size. These have been known since the 1880s and at least some of them are probably foraminiferans. Another group, not quite as big as the xenophyophores but still spectacular by protistan standards, are the gromiids. These form a sack-like, organic-walled shell (test) with a single, terminal aperture enclosing a cell body stuffed with large volumes of sediment and detrital material. Again, these were first described in the 1800s, but were believed until recently to be represented in marine environments by a single species, Gromia oviformis, which is confined to intertidal and sublittoral habitats such as the weed of coralline pools, on Cladophora, on the walls of rock crevices, undersurfaces of stones, holdfasts of kelp and the surface layer of sandy and muddy sediments and can be collected by divers or in some cases simply by snorkelling.

Figure 1. Gromiids visible on the surface of an undisturbed megacore collected at station 55767#1 from the Oman margin during CD 143.

Figure 2. Gromiids collected by the Agassiz trawl at Station 55837#1 (1800 m) CD 145 on the Pakistan margin.

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Giant deep-sea protists: gromiids

 

Figure 3. Ana and Andy sorting through gromiids on ice onboard the R.R.S. Charles Darwin in December 2002.

Gromiids seem to be widespread throughout the world, particular in areas of high productivity. Although only recognized recently in the deep-sea, we think that they probably have been found previously, mistaken for faecal pellets. They can be collected either with corers that collect undisturbed sediment samples or by trawls. They are then sorted under a binocular microscope on ice onboard the research ship as illustrated in Figure 3.

References:


[1] Aranda da Silva A, Pawlowski J, Gooday A (2006) High diversity of deep-sea Gromia from the Arabian Sea revealed by small subunit rDNA sequence analysis. Marine Biology 148: 769-777


[2] Gooday A, Bowser S (2005) The second Gromia species (testate amoeba) from the deep sea: its natural history and association with the Pakistan margin oxygen minimum zone. Protist 156: 113-126


[3] Gooday AJ, Bowser SS, Bett BJ, Smith CR (2000) A large testate protist, Gromia sphaerica sp nov (Order Filosea), from the bathyal Arabian Sea. Deep-Sea Research Part II 47: 55-73