Diplonemidae is a family of biflagellated unicellular protists that may be among the more diverse and common groups of planktonic organisms in the ocean.

Primary studies done in the 1900s by Griessmann and Skuja had initially grouped Diplonemidae (or more specifically, the Diplonema and Rhynchopus taxa) with the euglenids.

[4] However, this conclusion was met with some controversy, as diplonemids lacked characteristic features shared by all euglenids, such as possession of pellicle strips and paraxonemal rods on their flagella.

[5] This was further supported through phylogenetic analysis, which discovered that Diplonemidae possess a functional splice leader RNA that is characteristic of the kinetoplastids.

[3] Diplonemidae display a rich diversity in marine and freshwater environments, with their relative abundance increasing with depth.

[2] There are potentially thousands of unknown marine Diplonemidae species, with this diversity highly stratified in accordance to depth.

Although molecular sequencing confirms the existence of these unnamed marine Diplonemidae, information regarding their morphology and lifestyle is absent.

[1] As marine Diplonemidae are the most abundant and genetically diverse protists (and potentially eukaryotes) in the sea, there are strong implications that they play a key role in aquatic ecosystems.

This is accomplished by either Uracil-insertion, nucleotide deanimation, or substitution, which eventually generates a fully mature and translatable transcript.

[14] As to date, this is one of the most-supported examples of lateral gene transfer from a bacterium to eukaryote and may have implications for diplonemid acquisition of biochemical abilities.

Although presently made up of less than a dozen named species, the existence of thousands of unknown diplonemid taxa has been suggested through environmental sequencing analyses.