Ingestion of cryptophyte cells by the marine photosynthetic ciliate Mesodinium rubrum

We investigated the mechanism of capturing and ingesting cryptophyte cells by a laboratory strain of the marine photosynthetic ciliate Mesodinium rubrum Lohmann 1908 (= Myrionecta rubra Jankowski 1976), a cosmopolitan red tide species. When offered cryptophytes as food, M. rubrum, originally grown photosynthetically for 2 wk, used its bifurcated oral tentacles to instantly seize prey cells when encountered. Immediately after capturing a prey cell, M rubrum swam in a zigzag pattern (30 to 60 mum long linear paths) for >4 s, without showing the large jumps (with ca. 2000 mum long linear paths) that were usually observed when the predator was not feeding. M rubrum with a cryptophyte attached to its tentacles became motionless while the prey cell was moved to the oral surface of the predator, a process that took < 10 s. Engulfment of a captured prey cell by M. rubrum occurred through a cytostome-like structure and took ca. 15 s. Once engulfed, the prey was slowly delivered to the posterior end of the ciliate over a period of ca. 63 s. The whole feeding process lasted approximately 92 s. With increasing mean prey concentration, specific growth rates of M rubrum feeding on the cryptophyte increased, with saturation at a mean prey concentration of 44 cells ml(-1). The maximum specific growth rate (mixotrophic growth) of M rubrum feeding on the cryptophyte was 0.521 d(-1), under continuous illumination of 60 muE m(-2) s(-1), while its growth rate (phototrophic growth) under the same light conditions without added prey was 0.357 d(-1). The ingestion rate of M rubrum feeding on cryptophytes increased continuously with increasing prey concentration. The maximum ingestion rate was 8.9 cryptophytes ciliate(-1) d(-1). M rubrum may sometimes exert considerable impact on prey populations.