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The trophic interactions of the marine rotifer Synchaeta cecilia were investigated by determining its feeding and growth rates on a wide variety of marine phytoplankton and by determining its susceptibility to predation by the calanoid copepod, Acartia tonsa. Reproduction of S. cecilia was sustained in four-day feeding trails by 13 of 37 algal species tested. Growth-supporting species included species of Cryptophyceae, Dinophyceae, Chlorophyceae and Haptophyceae in sizes from 4 to 47 pm. Within these taxa, other species in the acceptable size range failed to support growth. No species of Cyanophyceae, Bacillariophyceae, or Chrysophyceae supported growth of the rotifer. S. cecilia can be maintained on unialgal cultures of Cryptophyceae but growth is enhanced by a combination of two or three species; a mixture of Chroomonassalina (Cryptophyceae), Heterocapsa pygmaea (Dinophyceae), and Isochrysis galbana (Haptophyceae) has sustained laboratory stocks of S. cecilia for over four years. The expected response of S. cecilia to food quantity was observed: as food concentration was increased from 58 to 1154 pg C 1-

The area of the filtering setae of thoracic limbs 3 and 4 of two Daphnia species increases approximately as the square of the body length.In contrast, the rate of increase in the filtering rate relative to body length attains maximum exponential values of 2.5-3.0. The filtering area and the filtering rates of Duphnia rosea become increasingly greater than those of Daphnia magna as body length increases, but the difference in the calculated rate of flow of water through the filtering setae remains nearly constant at all body lengths.

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