Living cells of the ciliate Mesodinium rubrum were observed and photographed during summer red-water events in the Southampton Water estuary, southern England. These blooms were characterized by increases in maximum cell diameter and range of cell size, a pronounced morphological variation of the ‘oral’ hemisphere, and the presence of free-swimming ‘fragments’ of larger cells.

Red-water occurrences caused by the planktonic, phototrophic ciliate Mesodinium rubrum (Lohmann) Hamburger & Buddenbrock, have been characterized by some of the highest recorded levels of chlorophyll and primary productivity for natural populations of marine phytoplankton (Lindholm, 1985; Taylor, 1982). However, as these events are rather unpredictable and all attempts to culture M. rubrum have failed, descriptions of the morphology of living cells have been rather limited. A study into the impact of red-tides caused by M. rubrum in the Southampton Water estuary presented a useful opportunity to observe morphological variation in living cells. Water samples were taken with a ‘Van-Dorn’ sampler, and transported in polyethylene bottles in a cool box. Living cells were observed and photographed at 400x magnification using the phase-contrast optics of an Olympus BH2 photomicroscope. Autofluorescence of living cells was examined using a Zeiss fluoresence microscope.