Sack rotifer
Asplanchna
The sack rotifer is a transparent, sac-shaped planktonic organism notable for its significant size of up to 2.5 mm. It lacks a foot and moves through the water using a well-developed ciliary wreath (corona). As an active predator, it feeds on other rotifers, protozoa, and small crustaceans, often swallowing its prey whole.
Details
Identification
Sac-like, transparent body; absence of a foot; powerful, protrusible jaws (trophi); often visible embryos inside the body.
Social behavior
Mostly solitary in the pelagic zone, but can reach very high population densities under optimal conditions.
Diet
Predatory; feeds on smaller rotifers (e.g., Keratella), ciliates, flagellates, and algae.
Hunting strategy
Ambush tactics combined with active searching; prey is detected by tactile stimuli and seized with the jaws.
Spawning substrate
Pelagic; resting eggs sink into the sediment.
Overwintering
Formation of thick-shelled resting eggs (dauereier) that survive unfavorable conditions on the water bottom.
Ecology
Ecological role
Important top predator within the rotifer community; regulates plankton composition.
Natural predators
Planktivorous fish, predatory copepods, and water fleas such as Leptodora.
Competitor species
Other predatory zooplankton such as Chaoborus larvae.
Ecosystem service
Contributes to energy transfer from microorganisms to higher trophic levels (fish).
Threats
Chemical water pollution and significant changes in the food web caused by invasive species.
Scientific profile
Profile
Distinguishing features
Large, sac-like shape without a foot; possession of robust, grasping trophi (incudate type) for seizing prey; often exhibits pronounced polymorphism (saccate, cruciform, and campanulate morphs) induced by chemical cues from prey or predators (phenotypic plasticity).
Reproduction
Heterogony: alternation between obligate or facultative parthenogenesis during favorable conditions and sexual reproduction under stress or high population density.
Role in food web
Important top predator within the rotifer community; regulates the abundance of smaller zooplankters and serves as a link between the microbial loop and higher trophic levels.
Protection & threats
Status not on standard scale
Main threats
Climate change (alteration of thermal stratification), extreme eutrophication (oxygen depletion in the hypolimnion), and invasive species destabilizing the food web structure.