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The resource use of rock pool Corixids Arctocorisa carinata (Sahlb.) and Callicorixa producta (Reut.) was compared in single-species and mixed populations established in the natural environment. The experimental recruitment consisted of daily transfer of newly moulted first-stage larvae to the study pools. The changes in numbers of larvae and the increase of the biomass corresponded closely to the changes in natural populations. The growth of populations was limited by elimination of the youngest individuals. The maximal biomasses attained per unit of pool surface area were same in all cases. This indicates that the species have completely overlapping resources and use them with nearly equal efficiency. The strong interference by A. carinata is an additional reason for the unstability of the competitive interaction.

Coexistence of Callicorixa producta and Arctocorisa carinata in an archipelago of rock pools appears to be facilitated by division of the environment on the time axis. To test this, a hypothesis is built on r-K theory. As A. carinata is twice the size of C. producta it is expected that the former species is more K-selected living in more permanent pools where food resources are limiting and competition strong. The smaller C. producta is expected to prefer newly refilled pools with abundant food, and thus to be more r-selected. Gathered test data on 17 life history and population ecological features agree with the prediction. It is also shown that the both rock-pool corixids are more r-selected than other species of the family.