Pyxidicula husvikensis (Beyens & Chardez, 1997) new comb.
Basionym: Microcorycia husvikensis Beyens & Chardez, 1997
Diagnosis: The upper test part is dome-shaped and has a yellowish tinting. A very delicate punctuation is visible at high magnification. The margin of the upper test part is divided into an external part and an internal part. The internal margin changes into a flexible, hyaline membrane, which spreads towards the periphery and forms a border around the upper test part. This membrane can fold up towards the centre as to reduce the diameter of pseudostoma. The upper test part of young individuals is hyaline and more pliable than with older ones. The cell body is located as a spherical or oval mass in the test and is connected to the upper test part through fine ectoplasmatic epipodia of varying number (two or three). Pseudopodia are lobate. Cytoplasm is finely granulate. It contains numerous inclusions (excreta and glycogen granules) and a contractile vacuole. The cell is mononuclear. The nucleus contains a central nucleolus. The nucleus has a diameter of 2.4-3,0 µm.In side view the shell looks basin-shaped, with a flattened or slightly bulged (swelled) top, forming thus a short conical cylinder, open at the base. The shell wall is composed of a yellowish organic layer. At great magnifications fine alveoli are visible. The shell membrane consists of two agglutinated layers, which are separated near the base. One of these layers forms a thin and hyaline extensible membrane, which is in a circular way displayed around the shell. It can also be transformed like a sail, or be folded to reduce the opening at the base. Amoeba fixed with formalin looks like a globular mass attached to the interior wall opposite the opening. The cytoplasm is finely granulated, grey, with an easily observed spherical nucleus containing a central nucleolus.
Encystment: Two types of cysts have been found. The resting cyst is formed inside the test. It is completely spherical in shape and has a diameter of 24 µm. Cytoplasm is dense and opaque. It is surrounded by a smooth membrane. Moreover, the cell can change into the state of pre-encystment. In pre-encysted state, the membrane is rough and irregular; it is attached to a test point. Pre-encysted cytoplasm has a diameter of about 28 pm . The lower test part is folded towards the peristome centre. The margin of the upper test part can also be folded inwards for reducing the test opening.
Type location: Husvik area. South Georgia (Sub-Antarctica).
Ecology: Slightly alkaline lakes and ponds, vegetated by moss (Beyens & Chardez, 1997), moss (Chardez, 1999). South Georgia (Beyens and Chardez, 1997), Netherlands and Austria (Siemensma, unpubl.).
Dimensions: Beyens and Chardez, 1997: shell diameter 32-34 µm, shell height 16-18 µm, nuclear diameter 2-2.4 µm, diameter of the contracted cytoplasm 18 µm, diameter of the extended membrane 38-40 µm (n=20). Chardez, 1999: Diameter upper shell part 32-35 µm, height 16-20 µm, diameter including the spread lower test part: 39-43 µm.
Remarks: M. husvikensis is a Pyxidicula species. Therefore I have transferred this species from Microcorycia to Pyxidicula