Cryptophyte algae are robbed of their organelles by the marine ciliate Mesodinium rubrum

Mesodinium rubrum (Lohmann 1908) Jankowski 1976 (= Myrionecta rubra)(1,2) is a common photosynthetic marine planktonic ciliate which can form coastal red-tides(3). It may represent a 'species complex'(4,5) and since Darwin's voyage on the Beagle, it has been of great cytological, physiological and evolutionary interest(4). It is considered to be functionally a phytoplankter because it was thought to have lost the capacity to feed and possesses a highly modified algal endosymbiont(5,6). Whether M. rubrum is the result of a permanent endosymbiosis or a transient association between a ciliate and an alga is controversial(7). We conducted 'feeding' experiments to determine how exposure to a cryptophyte alga affects M. rubrum. Here we show that although M. rubrum lacks a cytostome (oral cavity)(8), it ingests cryptophytes and steals their organelles, and may not maintain a permanent endosymbiont. M. rubrum does not fall into recognized cellular or functional categories, but may be a chimaera partially supported by organelle robbery.