Diplonemids are unicellular, eukaryovorous protists found in the marine and freshwater environment. The group is classified within the phylum Eurlenozoa alongside the kinetoplastids (kinetoplastida) and euglenids (euglenida) (see below for the visual representation of their phylogeny). Diplonemids are relatively unknown, therefore, the aim is to provide a brief, comprehensive introduction to the current science of the diplonemids, addressing the classic diplonemid, Hemistasia and planktonic diplonemids whilst reavealing the first known photographs and suggest how this reveals flaws in the existing knowledge.

Taxonomic illustration of the Kingdom Eurlerozoa. Diplonemids is comprised of planktonic dirlonemids, the classic Diplonrma and Rhynchopus and a newly categorised planktonic diplonemids.  Adapted from Cell.

Diplonemids are heterotrophs (therefore possess a feeding apparatus). They measure approximately 20 μm in total length and possess a characteristic bi-flagellate measuring 3–4 μm in length which is used to propel them through the water. Diplonemids were discovered in 2000 and in a recent global environmental survey found that they are among the most abundant and diverse lineages within the oceans, primarily residing in deep waters. Although nothing is known of the morphology; behaviour; or basic biology of many species, their genetic contents has fascinated scientists which has resulted in them being extensively studied. They possess an unusual mitochondrial genome which divides into more than one hundred chromosomes that are generally monomeric and possess thousands of circular DNA molecules as seen in the Diplononema papillatum.

Classic diplonemids

Irrespective of the amount of genomic knowledge, due to the technical challenges associated with the identification that is, a highly specialised field requiring devotion. Until recently, just two genera were identified in the diplonemids; the Diplonrma and Rhynchopus. They can be genetically distinguished by the analysis of the 18S rRNA (sub-unit) and widely known at the ‘classic diplonemids‘, since they are predominantly benthic and found in depths of 250–3,000 m, early understandings lead scientist to the assumption that all diplonemids reside in deeper waters.
Rhynchopus species have been the subject of study in terms of their feeding strategies. Parasitic and free-living forms appear and it is thought that parasitic forms have evolved from free-living forms. A study revealed that the parasitic Rhynchopus species can be found in the infected blood in the leg of a Norway lobster Nephrops norvegicus and is also present in the gills of a non specified animal. Free-living predatory forms can penetrate and digest the cell of diatoms – for example Coscinodiscus concinnus – through the process of phagotrophy


A sister group to the classic diplonemids; the Hemistasia e.g. Hemistasia phaeocysticola, are widely distributed but are a widely overlooked predator or parasite of dinoflagellates and haptophytes, as well as metazoans, in particular, the copepods. They were originally classified within the kinetoplastidshowever, were later repositioned in the phylogeny as a sister clade.

The study of diplonemids steadily came to a halt, this was until a recent study by Gawryluk et al (November, 2016) explored diplonemids giving them a new light. 

The study by Gawryluk et al (2016) – Revealing the flaws

Since no morphological data existed to help in the identification, multiple displacement amplification (MDAwas performed on all isolated cells collected, revealing 10 new marine diplonemids from a range of depths (50 m–160 m). Thus, the assumption of diplonemids residing at depths can be flawed, planktonic and deep sea individuals can now be seen. The ecology of planktonic diplonemids has been the subject of few studies, little is known whether they are free-living, commensals or parasites however phototrophy has been excluded. The classic diplonemids are colourless and approximately 20 μm in length with a oblong morphology. Since we could only differentiate between diplonemids genetically, we can finally see the corresponding morphological expression thus, with no further a due, here are the 10 newly discovered diplonemids below whom are in their size (∼7–25 μm) and shape (see figure below).


The first visual evidence of diplonemids identification; top row from left Cell; 4sb, 47, 27, 13, 9sb. Bottom row from left cell; 21sb, 1sb, 3, 37, 21. Scale bars represent 10 μm. Taken and adapted from Credit.

The identification of the new planktonic diplonemids has flawed the concept that they resided at depths associated with the classic diplonemids; that they are generally around 20 μm in length based on their morphology. In additoon to the recent major changes to the Euglenozoan higher-level taxonomy as suggested by Cavalier Smith in October 2016 may need further modification to account for the 10 new species.

Future study

For future work to be undertaken, scientists will need to create a mode of keeping the diplonemids alive in the lab, which according to Keeling “will be a lot easier since we know what they look like”. The basic biology desperately needs to be understood before even considering the possible utilisation in other industries for example, medical and to understand its roles in ecosystems. It is remarkable that we are studying possible extra-terrestrial life forms but we still do not know the basics of the very abundant predators and parasites of our very own water masses.

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