The rotifer Keratella cochlearis is a common freshwater species of plankton with a shell-like protective outer cuticl. It can produce a spiked form for enhanced protection against predators. Females are larger than males in these species and often reproduce by parthenogenesis.

Courtesy of <link>Pierluigi Colangeli.

The rotifer Brachionus angularis is a freshwater species with an interesting reproductive cycle which feeds on different strains of algae.

Courtesy of Pier Luigi Colangeli.