Ultrastructure and autecology of the marine, heterotrophic flagellate Leucocryptos marina (Braarud) Butcher 1967 (Katablepharidaceae/Kathablepharidae), with a discussion of the genera Leucocryptos and Katablepharis/Kathablepharis.

New ultrastructural data on the flagellate Leucocryptos marina (Braarud) Butcher 1967 (= Bodo marina Braarud 1935 = Chilomonas marina Halldal 1953), the type species of Leucocryptos Butcher 1967, show that this is not a cryptomonad as previously supposed. L. marina has tubulocristate mitochondria, a surface sheath, three new types of extrusomes, a cytoskeleton composed of pellicular microtubular bundles, and a cytostome associated with: 1) vesicles, 2) a cytopharyngeal cylinder of microtubules and 3) the cytoskeleton. Katablepharis/Kathablepharis ovalis Skuja 1948, another heterotrophic flagellate, is similar in ultrastructure. K. hyalums Skuja 1939 resembles these species, but lacks a cytostome. The status of Katablepharis/Kathablepharis and Leucocryptos is discussed, and the family Katablepharidaceae (ICBN) or Kathablepharidae (ICZN) is redefined. Leucocryptos marina is a phagotrophic nanoplankton predator. New data on its occurrence and abundance and a summary of previous records are presented. L. marina may be found during all seasons, but it attains highest numbers in coastal waters with high phytoplankton production. Autecological experiments and field data show that the flagellate is euryhaline and eurythermal and with a cosmopolitan distribution.