Tinned Psammon

Psammonobiotus communis, copyright Hugh MacIsaac.

In several previous posts on this site, I have discussed representatives of the remarkable group of organisms that are the Foraminifera. However, forams are not the only group of unicellular amoeboids to encase themselves in a shell. Today, I want to consider another such group, the Psammonobiotidae.

Psammonobiotids are a group of testate amoeboids forming part of (as their name suggests) the psammon, the community of organisms inhabiting the interstitial spaces between sand grains along the edge of the sea. Until the 1960s and '70s, most authors who encountered amoebae tests in marine samples assumed that they were the remains of freshwater organisms washed downstream (Golemansky 2008). Eventually, though, it was realised that there is quite a diversity of amoeboids that not only tolerate salty conditions, they prefer it. The Psammonobiotidae was recognised in the 1970s for a number such organisms. They produce a proteinaceous test without regular scales, the test structure being amorphous or composed of irregular plates. The test is generally more or less flattend to help the organism fit into the narrow spaces between grains. An aperture at one end of the test allows the organism access to the outside world; in many cases, this aperture may be bent to one side to allow the test to lie close to its substrate.

Campascus minutus, from Microworld.

Many psammonobiotids inhabit the supralittoral zone, just above the high tide mark. Groundwater in this region forms the contact zone between fresh water flowing out from under the land and salt water coming in from the sea. As a result, psammonobiotids and other inhabitants of this region need to be able to handle constantly shifting salinity levels. Many interstitial amoeboids can handle variations from 2% salinity in merely brackish waters to 37% in warm tropical seas (Golemansky 2008). Some normally marine psammonobiotids have even been recorded from entirely freshwater streams (Golemansky & Todorov 2007) though I personally suspect misidentifications may be involved.

The relationships of psammonobiotids to other testate amoeboids requires research (Adl et al. 2012). They possess filose rather than lobose pseudopodia, indicating relationships with other testate amoeboid groups in the Cercozoa. A leading possibility is a relation to the Euglyphida, which resemble psammonobiotids in many features but have tests with distinct scales. I haven't found any references to any psammonobiotids being covered by molecular analyses which may reveal where they really come from.

REFERENCES

Adl, S. M., A. G. B. Simpson, C. E. Lane, J. Lukeš, D. Bass, S. S. Bowser, M. W. Brown, F. Burki, M. Dunthorn, V. Hampl, A. Heiss, M. Hoppenrath, E. Lara, E. Le Gall, D. H. Lynn, H. McManus, E. A. D. Mitchell, S. E. Mozley-Stanridge, L. W. Parfrey, J. Pawlowski, S. Rueckert, L. Shadwick, C. L. Schoch, A. Smirnov & F. W. Spiegel. 2012. The revised classification of eukaryotes. Journal of Eukaryotic Microbiology 59 (5): 429-493.

Golemansky, V. 2008. Origin, phylogenetic relations, and adaptations of the marine interstitial testate amoebae (Rhizopoda: Lobosea, Filosea, and Granuloreticulsea). In: Makarov, S. E., & R. N. Dimitrijević. Advances in Arachnology and Developmental Biology. Papers dedicated to Prof. Dr. Božidar Ćurčić pp. 87–100. Inst. Zool, Belgrade; BAS, Sofia; Fac. Life Sci., Vienna; SASA, Belgrade & UNESCO MAB Serbia.

Golemansky, V., & M. Todorov. 2007. Taxonomic review of the genus Centropyxiella (Rhizopoda: Filosea) with data on its biology and geographical distribution. Acta Zoologica Bulgarica 59 (3): 227–240.