At first glance, Haplozoon also do not appear unicellular – in fact they were originally classified as a possible transitional stage between protists and multicellular organisms.
[1] They have more recently been classified as compartmentalized syncytia – single cells with multiple nuclei that have been subdivided by internal membranes.
When they were first discovered, they were initially placed within Mesozoa, a group of highly reduced worm-like parasitic invertebrates.
Dogiel initially described Haplozoon as belonging to the Mesozoa, and established a new mesozoan class, the Catenata.
[4] A series of early protistologists continued to fine-tune the taxonomical position of Haplozoon for a number of years.
Within biology, the -cyte suffix historically denotes different cell types (osteocyte, lipocyte, erythrocyte).
Because Haplozoon were thought to be multicellular when originally described, the haplozoan compartments were named as if they are separate cells.
When it was determined that Haplozoon were not multicellular, and in fact a compartmentalized syncytium, the three compartments were renamed with the suffix -mere (trophomere, gonomere, sporomere), to accurately portray them as belonging to a single cell.
They are almost exclusively found in the gut of maldanid marine worms, with one study documenting a Haplozoon parasite infecting an appendicularian.
[11][12] Another possible feeding strategy is that the parasite absorbs nutrients that are released as the host worm digests food.
This feeding strategy is referred to as pinocytosis, a form of endocytosis where nutrients suspended in the fluid if the host's gut are absorbed through the cell membrane of the parasite.
The exterior of Haplozoon cells are covered by barbs that present as fine hair-like structures that might function in surface mediated nutrition similar to the microtriches of cestoda.
All three compartment types appear granular under light microscopy, due to starch granules in their cytoplasm.
It is thought that starch accumulation in the sporomeres provides a reserve of energy for a subsequent life stage to survive in a marine environment without a host.
However, H. axiothellae possesses a longitudinal row of ventral pores along their surface, and confocal scanning laser microscopy (CLSM) data have revealed two basal bodies embedded in the plasma membrane of each compartment.
Early transmission electron microscopy (TEM) briefly describes 3 layers of the cellular membrane.
[13] The divisions between gonomeres are described as outer continuous membranes and flattened vesicles but pressed so closely together that they are impossible to distinguish.
As of June 2023, molecular data exists for five Haplozoon species: H. axiothellae, H. praxillellae, H. ezoense, H. gracile, and H.
[8][9][14][10] Modern dinoflagellate phylogenies recognize 8 orders: Syndiniales, Gymnodiniales, Noctilucales, Suessiales, Peridiniales, Gonyaulacales, Prorocentrales, and Dinophysiales.
Subsequent phylogenetic analyses have confirmed that Blastodiniales is a polyphyletic group, whose existence is not supported by modern molecular data.
Therefore, Haplozoon remain in the polyphyletic order Blastodiniales, awaiting conclusive molecular data that confirms its true phylogenetic position.
In 2023, a collaborative group of Canadian and Japanese researchers used transcriptomics to finally resolve the phylogenic position of Haplozoon, placing them within the Peridiniales.