Sac rotifer
Asplanchna spp.
Sac rotifers of the genus Asplanchna are large, predatory rotifers characterized by their transparent, sac-like body and the absence of a foot. They swim in the pelagic zone of lakes and ponds, using powerful trophi to consume other planktonic organisms. A notable feature is their phenotypic plasticity, allowing them to develop different body shapes in response to predators or food availability.
Details
Identification
Large, transparent, sac-like body; absence of a foot; prominent, powerful jaws (incudate trophi); often with visible embryos inside.
Social behavior
Solitary living, but can occur in high densities in plankton during population blooms.
Diet
Predatory diet; feeds on other rotifers (e.g., Keratella), small crustaceans, ciliates, and occasionally larger algae.
Hunting strategy
Contact predator that recognizes prey upon touch with sensory organs on the head and seizes it with eversible trophi.
Spawning substrate
Eggs are usually released directly into the open water or carried within the body until hatching.
Overwintering
Overwintering occurs through the production of thick-walled resting eggs that lie dormant on the water body floor.
Ecology
Ecological role
Important regulator of the zooplankton community; acts as a link between small consumers and higher trophic levels.
Natural predators
Planktivorous fish, predatory copepods, and aquatic insect larvae (e.g., Chaoborus).
Competitor species
Other predatory zooplankton such as predatory copepods or the rotifer genus Ploesoma.
Ecosystem service
Contributes to the control of population densities of other zooplankton and supports nutrient cycling in standing waters.
Threats
Pesticide pollution, water acidification, and drastic food web changes caused by invasive species.
Scientific profile
Profile
Distinguishing features
Large, hyaline appearance; absence of a foot; possession of incudate trophi (jaws) that can be protruded to seize prey; embryos or resting eggs are often visible inside the body.
Reproduction
Heterogony (cyclical parthenogenesis); rapid asexual reproduction by amictic females, followed by a sexual phase with mictic females and dwarf males during environmental stress.
Role in food web
Important top predator within the rotifer community; significantly influences the population structure of smaller zooplankton (top-down control).
Protection & threats
Status not on standard scale
Main threats
Anthropogenic water pollution, pesticide input (insecticides), extreme acidification, and habitat loss due to siltation or drainage of small water bodies.