Haplozoon (/hæploʊ’zoʊən/) are unicellular endo-parasites, primarily infecting maldanid polychaetes. They belong to Dinoflagellata but differ from typical dinoflagellates. Most dinoflagellates are free-living and possess two flagella. Instead, Haplozoon belong to a 5% minority of parasitic dinoflagellates that are not free-living. Additionally, the Haplozoon trophont stage is particularly unique due to an apparent lack of flagella. The presence of flagella or remnant structures is the subject of ongoing research.

Quick Facts Haplozoon, Scientific classification ...

Haplozoon
Light micrograph of Haplozoon axiothellae in its trophont stage
Scientific classification
Domain:	Eukaryota
Clade:	Diaphoretickes
Clade:	SAR
Clade:	Alveolata
Phylum:	Myzozoa
Superclass:	Dinoflagellata
Class:	Dinophyceae
Order:	Haplozoonales Poche
Family:	Haplozoonaceae Poche
Genus:	Haplozoon Dogiel

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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.^[2] Their life cycle is also largely unknown; while there is a well-observed adult trophont stage, understanding of other life stages is speculative at best.

There is a single published case of Haplozoon infecting an appendicularian.^[3] Otherwise, they are almost exclusively documented as infecting Maldanidae, and the extent to which they are an appendicularian parasite has not been investigated.

Haplozoon derives from two Greek words: haploos meaning “single, or simple” and zoion meaning “animal”. When they were first discovered, they were initially placed within Mesozoa, a group of highly reduced worm-like parasitic invertebrates. Therefore, the genus name Haplozoon means “simple animal”.

The type species Haplozoon armatum was discovered by Russian zoologist Valentin Dogiel in 1906. 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.^[1] The following year, the French biologist Édouard Chatton indicated similarities between Haplozoon and Blastodinium in 1907. Later, in 1911, the German naturalist Franz Poche created a new protozoan class – the Haplozooidea. And again in 1920, Chatton created a new family (Haplozoonidae) for the genus, of the order Gymnodinida (modern syn. Gymnodiniales). By the 1970s Haplozoon had been moved from the Gymnodiniales to the order of Blastodiniales.^[5]

Our modern understanding of Haplozoon is limited. There were a series of species discoveries in the early 1900s, however these publications lack the modern techniques that would be mandatory today (e.g. transmission electron microscopy, scanning electron microscope, molecular sequencing, etc.). Only 5 species of Haplozoon have been sufficiently described by modern standards: Haplozoon axiothellae,^[2][5][6][7] Haplozoon praxillellae,^[8] Haplozoon ezoense,^[9] Haplozoon gracile,^[10] and Haplozoon pugnus.^[10]

In some cases, insufficient information has resulted nomen nudum. This is the case for H. inerme^[5], and H. hirsutum. In many cases, species of Haplozoon are also phantom species. Phantom species are those that were perhaps adequately described at first, but have not been seen since their initial discovery.

The three types of haplozoan compartments were originally named with the suffix -cyte (trophocyte, gonocyte, and sporocyte). 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.^[7]

Haplozoon are obligate parasites. They are almost exclusively found in the gut of maldanid marine worms, with one study documenting a Haplozoon parasite infecting an appendicularian.^[3] Each trophont has a stylet, which it uses to pierce the gut lining of the host worm. It is unclear if the stylet is used only for anchoring the parasite, or if it is involved in feeding as well. If Haplozoon do use their stylet to feed, this would be referred to as myzocytosis, and commonly referred to as “cellular vampirism”. This is an established feeding method among other alveolate parasites.^[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.^[2]

With few exceptions, most descriptions of Haplozoon are from European coastlines. Haplozoon clymenellae is from the Atlantic coast of North America,^[1] while H. axiothellae, and H. praxillellae have been found on the Pacific coast of Washington, US and British Columbia, Canada.^[2][5][8] H. ezoense, H. gracile, and H. pugnus were discovered on the coast of Japan.^[9][10] There are currently no recorded observations from the tropics or the southern hemisphere, and consequently little is known about its biogeographical distribution.

As of June 2023, molecular data exists for five Haplozoon species: H. axiothellae, H. praxillellae, H. ezoense, H. gracile, and H. pugnus.^{[8][9][14][10]} Modern dinoflagellate phylogenies recognize 8 orders: Syndiniales, Gymnodiniales, Noctilucales, Suessiales, Peridiniales, Gonyaulacales, Prorocentrales, and Dinophysiales.^[15] But this has not always been the case – early in the study of dinoflagellates, other orders were used such as Blastodiniales.^[16]

Originally, Blastodiniales was a group for all parasitic dinoflagellates. Through the years, Blastodiniales has fallen out of favour and use. Subsequent phylogenetic analyses have confirmed that Blastodiniales is a polyphyletic group, whose existence is not supported by modern molecular data.^[17] There is a history of former members of Blastodiniales being moved to new orders based on molecular data.^[18][19]

Beyond being dinoflagellates, 18S phylogenetic analyses failed to resolve the position of Haplozoon. 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.^[20]

The following species are known to exist:^[21]

• H. armatum, Dogiel, 1906 (host: Travisia forbesi)
• H. lineare, Dogiel, 1907 (host: Clymene lumbricalis)
• H. delicatulum, Dogiel, 1910 (Host: maldanid, gen sp?)
• H. ariciae, Dogiel, 1910 (host: Aricia norvegica)
• H. macrostylum, Dogiel, 1910 (host: maldanid, gen sp?)
• H. obscurum, Dogiel, 1910 (host: Terebellides strömii)
• H. clymenellae, Calkins, 1915 (host: Clymenella torquata)^[1]
• H. clymenidis, Dogiel & Mikelsson, 1923 (host: Euclymene palermitana)
• H. tuberculatum, Dogiel & Mikelsson 1923 (host: maldanid gen sp?)
• H. villosum, Dogiel & Mikelsson 1923 (host: Polyphysia crassa, formerly Eumenia crassa)
• H. dogielii, Shumway, 1924 (host: Leiochone clypeata)
• H. inerme, Cachon 1964 (host: Appendicularia sicula)
• H. axiothellae, Siebert, 1973 (host: Axiothellae rubrocincta)^[2][5][6]
• H. praxillellae, Rueckert and Leander, 2008 (host: Praxillella pacifica)^[8]
• H. ezoense, Wakeman, 2018 (host: Praxillella pacifica)^[9]
• H.gracile Yamamoto, 2020 (host: cf. Petaloclymene sp.)^[10]
• H.pugnus Yamamoto, 2020 (host: Nicomache personata, Nicomache sp.)^[10]