Ornithocercus is a genus of planktonic dinoflagellate that is known for its complex morphology that features considerable lists growing from its thecal plates, giving an attractive appearance.

[2] Discovered in 1883, this genus has a small number of species currently categorized but is widespread in tropical and sub-tropical oceans.

[3] The genus is marked by exosymbiotic bacteria gardens under its lists, the inter-organismal dynamics of which are a current field of research.

He specifically paired it with the genus Histioneis due to their morphological distinctiveness including what he described as a head-funnel and neck collar.

While beautiful figures were published in the early 20th century, often their scientific value was limited due to the incompleteness of morphological analyses.

A 2018 study found that species O. quadratus could be three separate morphospecies based on inferences from modern imaging techniques of morphology.

[2] Their theca have numerous pores which open flush to the surface of the plate on the outside but have a raised rim on the inside, their number being positively correlated with cell size.

The hypotheca of most species are covered in areolae (shallow depressions) which are deepened by secondary thickening which takes place in mature cells.

The typical flagellar propulsion of dinoflagellates would be resisted by their morphology and the differences in list development between sides could act as a keel.

The inhibition of rotation provided by their thecae would lead to increased water flow over parts of the cell which could enhance their feeding-current system.

[9] One study found that the area in contact with the highest volume of external medium due to water flow is also a region where there are markedly less barriers to nutrient transfer (two less membranes - including theca).

[20] One study of these cyanobacteria symbiotes found that the cell size ranged from 3.5-10 μm (thus approximately an order of magnitude larger than prevalent planktonic Synechococcus forms).

[21] A 2010 study found species to house both the described extracellular cyanobacteria as well as larger rod-shaped non-photosynthetic bacteria on their sulcal lists.

[19]  Recent studies called into question this conjecture as the cyanobacteria on two species of Ornithocercus were not found to produce the necessary enzyme nitrogenase.

[4] In one study, some of the numerous food vacuoles observed inside Ornithocercus species strongly resembled the ectosymbionts in size and colour but were too degraded by section preparation to confirm via transmission electron microscopy.

Therefore it was proposed that the cyanobacteria is not dependent on the host for critical functions such as metabolism, thus supporting the theory that Ornithocercus feeds on its “garden” of bacteria.

[14] Like other dinoflagellate genera, Ornithocercus has been found to show phased cell division (specific dividing times for species throughout the day).

[27] Recently, molecular phylogenetic studies have been undertaken which provide more accurate hypothetical frameworks for dinophysoid character evolution.

[28] A 2013 study summarized the state of potential Ornithocercus phylogeny and placed them in clade with Citharistes and Histioneis with which the ectosymbiontic feature is shared.