Micractinium pusillum is closely related to Chlorella sp., but in contrast to the latter M. pusillum is is able to produce long bristles as shown here. There are also forms of M. pusillum with no bristles; then it is very difficult to distinguish both species from each other on a morphological basis. Scientists have shown that the growth of the bristles and also the formation of colonies can be induced by the presence of a substance (presumably a polypeptide) that is produced by the monogonont rotifer Brachionus calyciflorus, which is a predator of Micractinium pusillum.