''Keratella cochlearis'' is a rotifer. The

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...

ic animal occurs worldwide in freshwater and marine habitats.

Description

''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. Fish01

Distribution

''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.

Life cycle

''Keratella cochlearis'' is dioecious, with female specimens being larger than males. Typically, reproduction in this species is by

parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and developmen ...

. In this process, the female rotifer produces an unfertilised egg with a full set of

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

s. 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 Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

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 Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...

takes place. It is not known why this occurs at some times and not others. The female undergoes

meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...

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.

References

{{Taxonbar, from=Q150192 Brachionidae Taxa named by Philip Henry Gosse