The taxonomy of K quadrata is particularly confused. Validation of the numerous proposed subspecies and infrasubspecific variants awaits a thorough revision of the taxon.
Marine: Camargue, S France; Gulf of Bothnia; Balearic Archipelago, Spain; Bay of Bothnia, Bothnian Sea; Minorca, Spain; Venice Lagoon; Brackish waters in the Lorraine Orientale, Fr; Brackish waters near Bruges, Belgium; Black Sea; C Sweden; Fjord Himmerfjörden, N Baltic Sea, Sweden; Barther Bodden, S Baltic Sea, Germany; Gulf of Finland; Sea & rock-pools in S Finland; Vistula lagoon, Poland; Coast of S Finland; Senegal; Germany; Belgium; Lakes C Sweden; Belgium; British Channel; Lorraine orientale – France; Delta del Ebro; rock-pools in theTvaminne archipelago, southern Finland; Germany; Baltic Sea; Germany; Chech Republica; Southern Baltic Bay; Northern Baltic Sea; Netherland; Finland; Hungary.
Continental: Cosmopolitan.
The usual sex of Keratella encountered in nature is female, and the usual method of reproduction is parthenogenetic. Only at certain seasons or under certain environmental conditions are males produced which fertilize eggs from certain females that have undergone a meiotic reduction division, the fertilized egg becoming a hard-shelled resting egg. Dicycly in this species has never been established. Wesenberg-Lund notes that the sexual period has not been observed in the pelagic region of larger lakes, and suggests that in such situations the species may be acyclic; also K. quadrata from larger lakes has a hyaline lorica and long, often well developed postero-spines. Wesenberg-Lund suggests that seasonal variations are not only dependent upon variation in external conditions but also depend upon an internal cycle, especially the distance of the generation from a sexual period. However, it is hard to understand how one reconciles the internal cycle with what is observed in the pelagic region of larger lakes, for forms found in that situation must often be many generations removed from a sexual phase, yet they continue to show no evidence of reduction in size. Widely distributed in fresh water habitats in arctic and temperate regions but often absent from the aestival plankton. Probably absent from truly tropical waters. This is a limnetic species, forming part of the deep water plankton of lakes, but occurs as well in a great variety of situations. Keratella quadrata is limited in its distribution by temperature and possibly also by hydrogen ion concentration (it is an alkaline water species); it is apparently confined to Arctic or temperate waters and to the winter plankton of waters having a relatively high summer temperature.
Lorica terminates in two widely separated subequal posterior spines, usually divergent or bowed, varying in length from about as long as the body proper to very short or entirely wanting. The lorica is much compressed dorso-ventrally. The maximum width is about two-thirds of the way back along the body from the anterior margin. The lorica narrows very slightly posteriorward, so that the width of the lorica at the point of origin of the posterior spine (posterior breadth) is but slightly less than the maximum breadth. The lorica narrows to a greater extent anteriorward, so that the width at the base of the anterior spines is less than the posterior breadth. Anterior dorsal margin with six spines: medians longest and stoutest, curved ventralward; laterals usually somewhat longer than intermediates, intermediates are slightly divergent, laterals are slightly divergent or, as often, somewhat convergent. The lateral and intermediate spines are commonly rather wide in the basal two-thirds of the length and taper abruptly in the distal third. The lorica has a pattern of minute interlacing areolations. The dorsal plate is usually pustulate, the small pustules being at the corners of the areolations; the ventral plate usually lacks pustules. However, in some localities the lorica has a glassy appearance and is not pustulate on either plate. The foundation pattern of the dorsum is characterized by a series of median plaques. Behind the anterior median area are three median facets, the anterior two of which are enclosed hexagons, and the posterior plaque is usually not closed posteriorly. To the sides of the median plaques are three pairs of lateral polygons and three pairs of smaller-sized, triangular-shaped marginal plaques.
MALE.-The male of Keratella quadrata was first described by Plate, 1886, p. 64. Marks and Wesche, 1903, described the male of Anuraea brevispina, p. 509, fig. 2. An excellent figure and discussion of the male are given in Kriatzschmar, 1908, p. 629, P1. xiv, figs. 1-3, and in Wesenberg-Lund, 1923, p. 245, P1. xiii, figs. 1, 2. The body is broad, conical and widest in the middle. The lorica is feebly developed, without facets or thorns of any kind; there is a quadrangular ventral plate, and the dorsal plate has a mesial division. The alimentary canal is apparently lacking. The brain is very large, with a red eye spot; there are two lateral antennae in the posterior portion of the body, and there may be an anterior dorsal antenna. The testis is large, globular, with two sorts of spermatozoa; the penis is well developed, protruded and has a row of cilia at its distal end. Total length, 70-100 µm; length of penis, 20 µm.
