by Piter Kehoma Boll
Some time ago I introduced a cool unicellular alga, the Sailor’s Eyeball, which can reach about 5 cm in diameter, being one of the largest unicellular organisms known to exist.
Today we’ll know one more creature of this type, only it is not an alga, but a testate amoeba more closely related to foraminifers. Named Gromia sphaerica, I will here call it the giant gromia.
The giant gromia was first found in the Arabian Sea at depths of more than 1100 m and was formally described in 2000. It lives lying on the substrate and is usually covered by a thin layer of sediment, appearing as small spheres scattered across the sea floor. The body is spherical or grape-shaped but hollow, with the interior filled with fecal material (called stercomata) or other fluids. This spherical cell is covered by a shell, or test, of organic material which shows several small perforations by which thin expansions of the cytoplasm, forming a kind of pseudopod, can be extended. The size of the test can reach up to 3 cm in diameter, being much larger than that of its best known relative, Gromia oviformis.
In 2008, another population of species was found in the waters around the Bahamas. Specimens there are not as spherical as in the population in the Arabican Sea and were seen associated with tracks that indicate that these organisms slowly move across the sediment. The tracks clearly resemble some fossil tracks from the Pre-Cambrian period, which are usually considered an indication of the early evolution of multicellular animals. However, this discovery of unicellular organisms being able to produce tracks similar to those associated with animals raises doubt about the time of origin of multicellular animals.
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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 II 47: 55–73.
Matz MV, Frank TM, Marshall NJ, Widder EA, Johnsen S (2008) Giant deep-sea protists produces bilaterian-like traces. Current Biology 18(23): 1849–1854. https://doi.org/10.1016/j.cub.2008.10.028