Zooplankton of the Great Lakes


Kingdom - Animalia
Phylum - Rotifera
Class - Monogononta
Order - Ploimida
Family - brachionidae
Genus - Keratella
Species - cochlearis


Keratella, like all rotifers are pseudocoelomates. This is a paedomorphic condition in which the blastocoel remains even once the animal has reached maturity. It is thought that this condition is secondarily derived (Brusca and Brusca, 1990).

Keratella is a footless rotifer which posesses a protective covering, or lorica. Keratella cochlearis occur in two body forms. The tecta form exists without the posterior spine displayed by the typica form. Generally, spines are present on those Keratella which are born in the presence of predators such as cladocerans, Asplanchna and cyclopoid copepods. The typica form can be born from a tecta mother and vice versa (Gilbert and MacIsaac, 1989).

Distribution/ Habitat

Keratella cochlearis is a nearly ubiquitous species, found in almost any body of water imaginable. Indeed, memberes of the genus Keratella are found throughout the entire world. Because Keratella is able to survive with low amounts of food, it is often found in food-poor environments which cannot sustain larger zooplankton. The small size of Keratella, however, means that its populations can be greatly impacted by the presence of predators (Thorpe, 2001).


The ciliated corona is the source of locomotion for Keratella,creating the appearance of spinning which gave rotifers their name. However, the main function of the corona is to provide the current necessary for highly efficient filter feeding. The cilia of the corona direct food particles into the buccal funnel and down into the mastax where they are crushed by the trophi. This feeding mechanism allows Keratella to eat a variety of shapes and sizes of food, primarily consisting of phytoplankton such as Cryptomonas and Chlamydomonas (Bogdan and Gilbert, 1982).

Growth and Reproduction

Reproduction of rotifers is generally achieved parthenogenetically. For this to occur, the female produces a 2n egg via mitosis which will then hatch into a 2n clone of the mother. A noticable feature of the cloning of Keratella is the fact that the offspring may vary in body form from that of their mother.

Sexual reproduction occurs periodically in response to unknown stimuli. In sexual reproduction the female undergoes meiosis and produces 1n eggs, some of which hatch into 1n males. These males then undergo mitosis to create 1n sperm which will fertilize a 1n egg. The fertilized egg will hatch into a 2n female and sexual reproduction will be complete.

Keratella cochlearis

Figure 1. The lorica and posterior spine of this typica form of Keratella cochlearis offer a measure of protection against predators.

Figure 2. The ciliated corona directs food into the gut tract.

Figure 3. This Keratella is carrying an egg.

Works Cited

Bogdan, Kenneth G. and John J. Gilbert. Seasonal patterns of feeding by natural populations of Keratella, Polyarthra, and Bosmina: Clearance rater, selectivities and contributions to community grazing. Limnology and Oceanography v27(5) 1982. pp 918-934.

Brusca, R.C. and Brusca, G.J. 1990. Invertebrates. Sinauer Associates, Inc. Sunderland, Massachusettes, USA. pp 431-633.

Gilbert, John J. and MacIsaac, Hugh J. The susceptibility of Keratella cochlearis to interference from small cladocerans. Freshwater Biology v22 (1989) pp 333-339.

Thorp, J.H. and Corich, A.P. 2001. Ecology and classification of North American freshwater invertebrates. Academic Press, Orlando, Florida.