Cryptomonads are a group of unicellular, biflagellate protists occurring in a variety of aquatic habitats. Several nonpigmented forms exist but the vast majority bear plastids containing chlorophylls a and c, and accessory pigments known as phycobiliproteins. Plastids of cryptomonads are derived from a secondary endosymbiotic event involving the engulfment of a unicellular red alga, its retention as an endosymbiont, and its conversion into a photosynthetic organelle. Unique features of cryptomonads include the cell covering termed a periplast, the surface architecture consisting of a furrow and/or gullet, extrusive organelles called ejectisomes, and the presence of a reduced eukaryotic genome known as the nucleomorph. Accurate identification of cryptomonad species using light microscopy is problematic owing to the range of morphological plasticity and the presence of two distinct morphotypes in certain species. In addition to traditional morphological characters, proper diagnosis of species cannot exclude molecular data.

Key Concepts:

• Accurate identification and estimation of cryptomonads from field samples requires counting cells in the living state.
  
• Proper characterisation of cryptomonad periplast plates requires use of quick-freezing and freeze-fracture techniques.
  
• Accumulating karyotypic data continues to provide insight into the genetic variability of nucleomorph genomes, which may prove systematically significant.
  
• Cryptomonad biliproteins all derive from an ancestral phycoerythrin, which has since evolved into at least seven spectral types.
  
• Morphological species concepts are problematic given the increasing awareness of morphological plasticity and dimorphisms among cryptomonads.
  
• Resolution of the large-scale phylogentic affinities of cryptomonads remains tenuous and additional molecular analyses are required.