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The paper presents the morphometric characteristics of the histophagous ciliate Mesanophrysfound in the hemolymph of the isopod Saduria entomonfrom the Baltic Sea. The ciliate is compared with another, very similar one, found free-living on the coast of Vancouver Island, British Columbia, as well as with all literature data on the genus Mesanophrys. The ranges of all morphometric characters among these ciliates and all Mesanophrysspecies described so far strongly overlap. It seems, therefore, that the available morphological data, in the absence of other criteria, do not justify distinguishing separate species. The two ciliates presented here, as well as all nominal Mesanophrysspecies, should be referred to M. maggii(Cattaneo, 1888) unless new evidence concerning host specificity, life cycle or biochemical dissimilarity, for example, indicates a taxonomic difference.

Phenotypic transformation of C. kleinioccurred when the prey coexisted with the predatory ciliate Lembadion bullinumfor at least 24 hours, and was completed after 36–48 h. Net-separated predator medium caused visible shape changes in the prey. The induced phenotype was about 60% wider and 20% shorter than the typical one and had an almost spherical shape. The results showed that the induced morphological changes are a defense against L. bullinum, which is a raptorial feeder. In the presence of 100 predators, on an average 132 of 150 induced phenotypes survived for 2 h, but in the same conditions only 33 of 150 normal phenotypes did. The predator needed significantly more time and had to repeat attacks many times to catch and engulf the morphologically transformed prey. Only 16% of the attacks ended in ingestion of the transformed prey in less than 15 min, while in 54% of the cases the typical C. kleiniform was ingested during the first predator attack, which occurred in the first 2 min. The growth rate of induced C. kleiniphenotypes was on an average about 25% less than that of typical ones, indicating the substantial cost of the defense mechanism. Other predacious ciliates such as Dileptus margaritifer, Homalozoon vermiculareand Stylonychia mytilusdid not cause any morphological changes in C. kleini.