LOCUS TYPICUS: London
Marine environment: Camargue, S France (De Ridder, 1960a & 1961); Bay of Bothnia in Bothnian Sea (Sandström, 1979); Rovigno (Wulfert 1942a); Lakes C Sweden (Pejler, 1965 & 1972); Sea, brackish and freshwater pools in S Finland (Björklund, 1972a); Coast of S Finland (Eriksen, 1969); Fjord Himmerfjärden, N Baltic Sea, Sweden (Johansson, 1983); Barther Bodden, S Baltic Sea, Germany (Vietinghoff et al., 1984); Neva Bay, Gulf of Finland, R.F. (Telesh, 1986); Brackish water bay near Porvoo, S Finland (Järnefelt, 1964); Brackish waters in the Lorraine Orientale, Fr. (De Ridder, 1969a); Brackish waters near Bruges, Belgium (De Ridder & Verheye, 1980); Poland (Adamkiewicz-Choynacka & Rozanska, 1990); Scandinavian coastal waters (Thane-Fenchel, 1968); Black Sea (Rudescu, 1961); Gulf of Bothnia, G. of Finland, G. of Riga (Berzinš, 1960); Poland (Adamkiewicz-Choynacka & Rozanska, 1990); Germany (Althaus 1957a); Belgium (De Pauw 1969; De Ridder 1957a, b, 1981); British Channel (De Ridder 1959); Lorraine orientale – France (De Ridder 1968); Delta del Ebro (Fores et al. 1986); rock-pools in theTvaminne archipelago, southern Finland (Godske Bjorklund 1972b); Germany (Hauer 1925 1957); Finland (Lagus & Lindholm 2000); Baltic Sea (Meiners et al. 2002); Germany (Remane 1929; Rentz 1940; Schwarz 1955/1956; Schwarz 1959, 1962); Chech Republica (Sladecek 1955); Southern Baltic Bay (Thiel 1996); Northern Baltic Sea (Viitasalo et al. 1995); Netherland (Wibaut-Isebree Moens 1954); Germany (Ahlrichs 2003).
Continental waters: everywhere
Cosmopolitan in freshwater, brackish and marine habitats. This is probably the mostwidely distributed and common rotifer in nature. It is also a truly planktonic species.
Like all monogononts, K. cochlearis punctuates parthenogenesis with the production of males and resting eggs in particular condition. The cycle may end in the reduced forms by the intervention of a sexual phase, or the changes may again occur in reverse order and complete the cycle in that manner. In Keratella cyclomorphosis is most noticeable in the variation in length of the posterior spine (or spines) and is best exemplified in this species. Ahlstrom (1943) found in K. cochlearis a form with a long posterior spine (this species has a single, median posterior spine) in the collections during the winter months; there is a gradual reduction in size until July or August at which time a form with a short posterior spine or the tecta form without posterior spines dominates. During the remainder of the year there is a gradual reversal until the form with a long posterior spine again dominates. The present species is not only one of the commonest rotifers but also one of the most variable in shape.
The taxonomy of K. cochlearis is particularly confused. Validation of the numerous proposed subspecies and infrasubspecific variants awaits a thorough revision of the taxon.
Lorica spoon-shaped; with six spines in front; the medial pair curving strongly forwards: posterior extremity attenuated into a long slender spine, inclined forwards. From Ahlstrom (1943): Lorica terminates in a stout median posterior spine which usually varies in length from as long as the body proper to onethird as long but which may be even more reduced or completely wanting. Lorica oval shaped in dorsal view, depth of the dorsal plate is a little more than half the width; the greatest width of the lorica is slightly behind the middle of the body, although the lorica tapers but slightly anteriorward; the lorica is about two-thirds as wide as long (width varies from 0.56-0.71 of length of body exclusive of spines). Anterior dorsal margin with six spines: medians longest, curve ventrally, intermediates usually slightly divergent, somewhat shorter than lateral spines which are convergent at their tips and which arise at a slight angle toward the ventral. The lorica has the usual pattern of minute interlacing areolations on both plates. The ventral plate is usually pustulate on the upper third only, although this feature is somewhat variable. The dorsal plate is often somewhat pustulate, the small pustules being at the corners of areolations and scattered over the entire plate. The foundation pattern of the dorsum is characterized by a mediam line extending longitudinally from behind the median frontal area to the base of the posterior spine. Often there is but one pair of fully enclosed carinal plaques: the antero-carinal polygons (hexagons). Not infrequently, however, the postero-carinal polygons are fully formed; when this is the case there are two very small unenclosed carinal areas posterior to the two enclosed pairs of carinal plaques. There are two pairs of enclosed lateral polygons to the sides of the anterocarinal polygons; a third pair may be present to the sides of the postero-carinal plaques.
This species is easily cultured and it has been used as a model in order to quantify variation in size at different conditions.
Keratella cochlearis has an oval lorica, a shell-like protective outer cuticle. At the anterior end are three pairs of spines. The central pair curve towards the ventral surface, the next pair diverge slightly and the outer pair converge. There is a single red eye There is also a central funnel-shaped mouth and on either side of this are rings of cilia which twirl and help waft food particles into the mouth. They are also used for locomotion. There are two forms of this rotifer; some individuals have a long spine at the posterior end and others do not. Neither form has a foot.
Keratella cochlearis is found worldwide in marine, brackish and freshwater habitats. Any body of standing water is likely to contain rotifers and Keratella cochlearis is probably the most common and widespread species in the world.
Keratella cochlearis is dioecious, with female specimens being larger than males. Typically, reproduction in this species is by parthenogenesis. In this process, the female rotifer produces an unfertilised egg with a full set of chromosomes. This is carried around by the mother and hatches into a miniature adult. Whether the offspring has a posterior spine or not seems to depend on the number of predators in the body of water where the rotifer lives. If predation rates are high, spined "typica" forms are produced, but if low, spineless forms known as "tecta" develop. Tecta females can produce typica offspring and vice versa. It has also been found that different forms tend to predominate at different times of year. In the winter, most individuals have a long spine at the posterior end. As the spring advanced, the spine shortens and by July it is very short or non-existent. During the remaining months of the year it gradually lengthens once more. These changes take place over several generations as each individual rotifer has a lifespan of just a few days or weeks.
Periodically sexual reproduction takes place. It is not known why this occurs at some times and not others. The female undergoes meiosis and produces eggs with half the usual number of chromosomes. Some of these develop into male rotifers. Each of these is able to inseminate another of these eggs to restore the full complement of chromosomes in the offspring.