Turbellarian taxonomic database

Abstract / Notes

The basis of the previously established relationship between rainfall and fluctuations in the population of
Urceolaria mitra Von Sieb, epizoic on the freshwater triclad Polycelis tenuis Ijima, was examined. Abiotic
and biotic characteristics of 2 ponds were compared. The epizoic populations of Urceolaria on individual
triclad worms were not influenced by changes in the host population. Weekly samples over 1 year showed that
fluctuations in the bacterial population were largely determined by rainfall. When adjusted for a time lag
the fluctuations in bacteria and Urceolaria were significantly correlated for both ponds. The 2 Urceolaria
populations showed synchrony during most of the year; minor contrasts were attributed to differences in food
supply. In the ponds Urceolaria fed mainly on bacteria but also on flagellates, green algae and diatoms.
Attempts to obtain sterile cultures were largely unsuccessful but the peritrich can grow on bacterial food
alone. No predators or parasites of Urceolaria were observed. Competition for food by peritrichs may have
reduced the urceolarian population for a short time in the late spring. Changes in the small flagellate
population ( a possible alternative food), bacteria and Urceolaria were followed for 10 weeks. Closer
agreement was found between bacteria and Urceolaria than between flagellates and Urceolaria. Phases of
reproductive activity in Urceolaria generally produced an increase in the population. It is concluded that
fluctuations in Urceolaria are largely initiated by changes in the bacterial population which in turn are due
to rainfall. Temperature is a secondary factor only.

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