Hyalosphenia gigantea De Graaf, 1952
Diagnose: The theca is perfectly hyaline as in all other species of the genus. The colour is a very light yellow. The form of the theca is oval with the greatest width about halfway; slightly compressed laterally. The pseudostome is oval too. There are no lateral pores as in Hyalosphenia papilio.
The living animal fills only about half of the theca and is attached to it by very thin plasma-threads, epipodes. There are 3-6 pulsating vacuoles, especially situated in the hind part of the animal. A nucleus has not been observed. The plasma is rather clear and without zoochlorelles. The pseudopodes are loboform. When moving, seldom more than one lobopodium is developed. Movement rather quick, in any case quicker than that of Hyalosphenia papilio and H. elegans. The animal is able to retire very quickly into the theca when disturbed.
Dimensions: Length 204-272 µm; greatest width 64-96 µm; thickness 43-45 µm; pseudostome 52-57 µm; length/width 2.3-3.4.
Ecology: Freshwater, Loenerveense polder, the Netherlands.
De Graaf: “Living specimens were found on 27 Aug. 1951 in the open water on Dec. 18, 1951 in the bottom mud. Whether the species in benthonic or planktonic cannot be said, as it was found with equal frequency and abundance in both biotopes. The animal is certainly eurytherm, being found at temperatures of 19°C as well as less than 5°C.
The water of the ’’Loenerveense Polder” is alkaline throughout the year with very little variation in the pH which is nearly always 8,0. The CL~ content of the water varies between 37.5 and 48.5 p.p.m., the bicarbonate between 86 and 118 p.p.m., the KMnO4-consumption between 22 and 30 p.p.m., and the O2 content between 8.4 and 15.1 p.p.m. This environment is totally different from that in which the other species of the genus dwell. H. papilio, H. elegans, H. subflava and others live almost exclusively in Sphagnum, and always in waters with a more or less strongly acid reaction.
They are acidophilous. H. gigantea however appears to be an alkaliphilous species, as it seems unlikely that such a characteristic animal should have been overlooked in the Sphagnum vegetation, which have been studied very intensively.
The bottom mud in which the species was found, consists of very finely divided peat material. This material originates from the time that the lake was excavated in order to sell the peat as fuel. The remaining peat on the bottom has long since ceased to have an acid reaction. On windy days this material is easily suspended in the water, as it is very loose. Moreover there is little vegetation on the bottom. Because of these characteristics the bottom mud is not a very suitable biotope for microorganisms, as they are easily buried under the peat material, which settles down again after a windy period. Consequently only few living organisms are found on the bottom. The absence of non-motile algae is not surprising.
Testate amoebae accompanying H. gigantea in the mud are: Paulinella chromatophora, Cyphoderia ampulla, Difflugia acuminata, D. oblonga, D. oblonga lacustris, Centropyxis aculeata and Trinema lineare. These are the common testate amoebae in alkaline waters. In the plankton sample P. chromatophora and C. ampulla were found together with H. gigantea. Both these species can very well have a planktonic mode of life and were regularly present in the net samples in greater abundance and frequency than on the bottom.
After this paper was written, H. gigantea has been found again at the same sampling station on several occasions.. Living specimens were observed in plankton catches from June 16, -July 28, and August 11, 1952 only. From this it seems likely that the species is most abundant in early summer.
All the species found confirm the diagnosis as given in this paper, and justify the description of H. gigantea as a new species.”