A review of typhloplanid flatworm ecology, with emphasis on pelagic species

Abstract

The study of microturbellaria is not popular, and hence, the taxonomy and ecology of these animals are poorly known. In temperate zone ponds and lakes, the number of species may be up to 50+ per water body. The Typhloplanidae, the subject of this review, live in the littoral–benthic zone, but pelagic occurrences have been accumulating since the first case was reported in 1952. Four species are currently known to be occasionally pelagic; all live in warm–temperate or tropical lakes. Typically, one species per lake is found, but in one lake in Brazil, 2 species co-occur. One of these feeds on the other and on zooplankton, with a preference for Cladocera. Flatworms seem to trade the littoral zone for the pelagic if predation pressure in the littoral exceeds that in the open water, and if the pelagic offers better food. Most Typhloplanidae feed on arthropods, often with a preference for cladocerans, and show parallels with the cladocerans and rotifers in that 2 types of eggs (subitaneous and resting) are produced. In some flatworms, however, resting egg formation does not require amphimixis. There is also great variation between species in both the number of clutches and their composition, and within species, latitudinal differences may be important. At the cold extreme, species produce only one clutch of resting eggs; at the tropical extreme, species produce many subitaneous clutches, although resting eggs continue to be produced as well. The factors driving the life cycles of the pelagic species are discussed and include familiar variables such as temperature (circulation patterns), food, and predation. Flatworms also produce mucus and toxins that add complexity to their ecology; toxins are used for catching prey, and possibly for defense against predators (but not fish). If they become too abundant for their food supply, flatworms also seem to switch to resting egg production or block reproduction.