Keeled rotifer
Notholca acuminata
The keeled rotifer belongs to the genus Notholca and is characterized by a rigid lorica (shell) featuring distinct longitudinal ridges or keels. It is a typical inhabitant of freshwater plankton, often reaching high densities in cold waters or during winter and spring seasons. These organisms move using a ciliated corona, which also functions as a feeding apparatus to create water currents for capturing particles. Like many rotifers, it exhibits cyclomorphosis, meaning its body shape can change in response to seasonal environmental factors such as temperature or predation pressure.
Details
Identification
Lorica with six spines on the anterior margin, longitudinal striations or keels on the dorsal plate, posterior end often pointed or with a short process.
Social behavior
Solitary living in the pelagic zone, but can form massive populations under optimal environmental conditions.
Diet
Feeds as a filter feeder on phytoplankton (especially small algae like cryptomonads), bacteria, and organic detritus.
Hunting strategy
Suspension feeder; uses the ciliated corona to create a water current that directs food particles to the mastax (jaws).
Spawning substrate
Pelagic; eggs are usually released freely into the water or carried briefly on the body.
Overwintering
Resting eggs (cysts) that can survive on the water body floor; however, often remains active in cold winter waters.
Ecology
Ecological role
Important primary consumer transferring energy from microalgae to higher trophic levels such as predatory zooplankton and fish larvae.
Natural predators
Predatory rotifers (e.g., Asplanchna), copepods, and planktivorous fish larvae.
Competitor species
Other filter-feeding rotifers such as Keratella species or small Daphnia.
Ecosystem service
Contributes to water self-purification by consuming bacteria and algae.
Threats
Eutrophication of water bodies, heavy pesticide pollution, and climate warming (being cold stenothermic).
Scientific profile
Profile
Distinguishing features
Anterior margin of the lorica with six spines, the median ones often being the longest. The surface of the lorica shows distinct longitudinal striations. A foot is completely absent. Cyclomorphosis (seasonal variation in shape) is common, especially regarding the length of the posterior spine.
Reproduction
Heterogonic; the life cycle is dominated by parthenogenetic (amictic) reproduction. Under stress conditions, a sexual phase occurs, leading to the production of males and resting eggs.
Role in food web
Important primary consumer in winter and spring plankton; transfers energy from primary production (algae) to higher trophic levels.
Protection & threats
Status not on standard scale
Main threats
Climate change (increasing water temperatures threaten its cold-stenothermic niche), severe eutrophication, and chemical pollution of water bodies.