Psammonobiotus linearis Golemansky, 1971
Diagnosis: shell ovoid, in cross-section circular; neck short, strongly curved; fundus with or without a distinct projection, which can be short or longer, straight or bend; aperture circular, surrounded by a characteristic hyaline collar; shell composed of an organic material, embedded with amorphous, siliceous material, irregularly arranged; nucleus spherical.
Dimensions: Length 21-24 µm (Golemansky,1971, 1973), 26-32 µm, n=21 (Nicholls, 2004); my measurements 17-31 µm, incl. projection (n=17).
Ecology: sandy beaches, marine, brackish water and freshwater. Psammonobiotus linearis was originally described from the Black Sea and nearby waters, and later also from the Baltic Sea, the Bay of Biscay and recently from the Great Lakes in North America (Nicholls et al, 2004).
I found this species in sediment of a very shallow sandy shore of the Spiegelplas, a deep and large freshwater lake in the Netherlands, with a salinity of 200 mg Cl/l, which is normal for freshwater here. Its water is very clear and clean, only fed by rain and seepage. In the same sample also specimens of the rare Artodiscus saltans and Actinocoma ramosa has been found, together with Nadinella tenella and many more common testaceans.
In 2019 I found a single test in a shallow ditch in Gaasterland, the Netherlands, in a thin layer of organic sediment on a sandy bottom.
Remarks: This is a highly distinctive species, the only one of the nine known Psammonobiotus species with a posterior ‘tail’, which is usually well developed with the exception of just a couple of specimens that lack a typical tail and whose aboral extremity of the test can best be described as ‘pointed’.
The discovery of P. linearis in the Spiegelplas is surprisingly, given its widespread occurrence at marine/brackish water beach locations. P. linearis is known as an euryhaline psammobiont. It is a so-called sand-dwelling testate rhizopod. Its collar, which is a flared disc-like structure, may facilitate attachment of the organism to a sand grain. Marine associations of sand-dwelling rhizopods comprise uniquely specialized species that can be referred to as obligate psammobionts (exclusively sand-dwellers), which are believed to inhabit the littoral zone sands of seas, but not of freshwater lakes. In particular, Psammonobiotus communis Golemansky, is well known from beach sands in virtually all oceans including the Antarctic, where it is often a component of a very rich community of a score or more of species representing several exclusively marine genera of testate rhizopods (Nicholls et al, 2004). According to Golemansky (pers. comm.) this species is also a freshwater species.