Xenophyophores - the giants of the protozoan world
compiled by Dave Walker, UK
10cm to 25cm single-celled 'monsters' live in the depths of our oceans!
How would you answer the question 'What is the world's largest protozoan?'. In freshwater, Stentor can grow to ca. 3mm and Pelomyxa can grow to ca. 5mm, although I've never had the fortune as an amateur pond-dipper to find a specimen of the latter. My concept of the largest is of the order of a few millimetres (see footnote 1), but a news item in 'New Scientist' recently (ref. 1) rapidly changed my view by a factor of twenty - yes, we are talking about 10 cm single-celled 'monsters'!
Apparently UK scientists have discovered 1km down in the North Atlantic what is believed to be the largest specimen of a single-celled organism ever found. It is a globular Xenophyophore 10 cm across of the species Syringammina fragillissima. (See footnote 2). The news item is brief but ref. 2 has a good section on the phylum with illustrations. If a single dimension is taken as the criterion for the largest, apparently even Syringammina would be dwarfed by a 1mm thick disc-shaped Stannophyllum specimen which was found to be 25 cm in diameter.
It seems a little puzzling at first why such a fascinating group is so little known about outside of the specialist literature, as extremes of the animal and plant world usually capture a wider public imagination. It's perhaps not so surprising, as the 'giants' of the freshwater protozoan world like Stentor can be found with ease in most ponds and studied by anyone with a modest microscope. However, Xenophyophores are benthic deep sea organisms i.e. they live on the sea bed where specialist collecting apparatus is needed to recover them. Scientists still have much to learn about the organisms that live in the ocean depths, and this includes the Xenophyophores where various aspects of their life cycle and ecology are not fully understood.
As a group though, these organisms are by no means a recent discovery and they have a fascinating history. The specimen (we hope to show above!) and others were recovered in late 19th century trawlings of the ocean depths. In1889 they were identified incorrectly as a type of sponge by Haeckel, and in 1907 as giant testate amebas by F.E. Schulze (who also gave the group their current name). They are now assigned their own Phylum Xenophyophora in the Kingdom Protoctista.
References 2 and 3 give details on what is known of their structure and life cycle, but here are a few fascinating features.
Different species occur in a variety of shapes and sizes, some species resemble bath sponges, others are flattened discs or with four branches forming a tetrahedra.
They seem to feed by using pseudopodia to engulf their particulate food (i.e. in a similar way to Amoeba).
They are easy to damage when collected from the ocean depths and cannot be cultured, so their life histories remain largely a mystery.
Xenophyophores are known to be quite numerous on the sea floors despite the little we know about them.
They are multinucleate masses of cytoplasm and enclosed by a branched tube system.
The many nuclei can be 2-10 mm in diameter and are evenly distributed through the cytoplasm.
Fecal matter is accumulated outside the tube system in dark strings of pellets.
There are 42 known species of Xenophyophores in 13 genera.
As a final comment, this recent discovery of probably the largest protozoan specimen found to date, illustrates that the ocean depths are still very much an undiscovered frontier for scientists to explore. Ever more sophisticated deep sea submersibles aid this exploration, so who knows what remains to be found. But novel life forms are still being discovered much closer to home than the ocean depths. Do you remember the microscopic marine life form that caused such world-wide excitement when discovered by Danish researchers on the lips of the Norway lobster? If not read an illustrated Micscape article on Symbion pandora the organism that caused the excitement; it was so unusual it was assigned a new phylum.
1) Apparently a number of foraminifers and marine amoeba-like organisms can grow to 10 mm or more but the majority are smaller than 5 mm. Return to article.
2) Although Syrangammina fragillissima was known from the Faroe-Shetland Channel since 1883, at 850-1016m. The previous largest known specimen from the area was 38mm in largest dimension. Return to article.
The author would like to thank Dr. Ole S. Tendal of the Invertebrate Department, Zoological Museum, Copenhagen, Denmark for providing useful comments on and additional information for this article. Dr. Tendal's research interests include the systematics, zoogeography and general biology of Xenophyophora. (Any errors in the article are those of the author).
References and Further Reading
1) New Scientist, 24 Oct, 1998, p.23.
2) 'Five Kingdoms - An Illustrated Guide to the Phyla of Life on Earth' by L. Margulis and K. V. Schwartz, 3rd edition, Freeman, New York, 1998, pp. 130-133.
3) 'Xenophyophores' by O. S. Tendal in 'Handbook of Protoctista', (Ed. L. Margulis, et al), 1989, pp. 135-138. (Thanks to Richard Howey for providing this reference).
Note: Web searches disappointingly revealed very little on the Web for this phylum, if you know of a good on-line resource please let us know.
Update October 2003: Thanks to Phil Clarke who has pointed out that the Southampton Oceanography Centre has some fascinating resources and illustrations on the Xenophyophores.
Published in November 1998 Micscape Magazine.
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