Find of Oocardium in Belgium
The colonial, monotypic genus Oocardium (with O. stratum as only species) is quite an unusual desmid, both in cell shape and habitat.. Cells are more or less heart-shaped and enclosed by calcareous cylinders, see attached figure after West & West (1923). It is exclusively found in shallow, fast running waters in limestone areas, a habitat most inhospitable to other desmid species. According to West & West (l.c.), in Great Britain Oocardium would be very rare, but presumably it is often overlooked by desmidiologists because of its unusual habitat. Anyhow, Van Oye (1941) reports its common occurrence in the southeastern part of Belgium.
There are no records from The Netherlands, probably because specific environmental conditions required are lacking. However, last September Peter Coesel encountered it at Montauban, near Buzenol, SW of Arlon (Belgium). Montauban is known for its Gallo-Roman excavation (touristic attraction), with a source rising at that very site. Where the streamlet leaves the forest to flow down over the open, steep slope, the limestone substrate is covered by a calcareous crust of Oocardium. The find-site cannot be missed!
Plate 159 from West & West (1923) figuring Oocardium stratum.
1. Colony at natural size.
Fig. 2. Vertical section through a couple of calcareous tubes with cells at their open end.
Fig. 3. Some four tubes seen from above.
Fig. 4-6. Separate cells in lateral, apical and frontal view, respectively.
Fig. 7. Newly-divided cells.
Fig. 8. Cell, specifically stained for the presence of cell wall pores.