Variations in the efficiency of ciliate extrusomal toxins against a common ciliate predator, the catenulid Stenostomum sphagnetorum

Species of ciliates are able to defend against predators by discharging toxins contained in their extrusomes (cortical extrusive organelles). This chemical defence was comparatively investigated in four species of ciliates, i.e., Climacostomum virens, Loxodes striatus, Spirostomum teres, and Stentor coeruleus against their common predator, i.e., the turbellarian Stenostomum sphagnetorum. The results showed that S. sphagnetorum ingests wild‐type specimens of all the four ciliate species. However, the wild‐type specimens of L. striatus and S. teres were immediately regurgitated apparently unharmed by S. sphagnetorum, while specimens artificially deprived of their extrusomes were normally eaten and ingested. In addition to reinforcing previous evidence that ciliates rely on extrusomal toxins for defence against their natural predators, these results were taken as a general indication that a successful defence of ciliates against their predators depends also on a different sensitivity of the predator to their extrusomal toxins.