Myrionecta rubra (or Mesodinium rubrum) is a species of single-celled organisms belonging to a group of ciliated protozoans. It constitutes a plankton community and is found throughout the year, most abundantly in spring and fall, in coastal areas. Although discovered in 1908, its scientific importance came into light in the late 1960s when it attracted scientists by the recurrent red colouration it caused by forming massive blooms, that cause red tides in the oceans. In 2009, a new species of Gram-negative bacteria called Maritalea myrionectae was discovered from a cell culture of M. rubra.