1. University of Maryland, Center for Environmental Science, Horn Point Laboratory, PO Box 775, Cambridge, Maryland 21613, USA

Correspondence to: Correspondence and requests for material should be addressed to D.E.G. (e-mail: Email: gustafsn@hpl.umces.edu).

Abstract

Mesodinium rubrum (Lohmann 1908) Jankowski 1976 (= Myrionecta rubra)^{1, 2} is a common photosynthetic marine planktonic ciliate which can form coastal red-tides³. 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 interest⁴. 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 controversial⁷. 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.