Haplozoon

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 purported 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.). In some cases, basic information such as the host species, or location of discovery, have not been recorded. The unfortunate result is that many purported species of Haplozoon are phantom species. Phantom species are those that have not been seen since their initial discovery, often due to an inadequate original description.

Only 5 species of Haplozoon have been sufficiently described by modern standards: Haplozoon axiothellae,[2][5][6] Haplozoon praxillellae,[7] Haplozoon ezoense[8], Haplozoon gracile[9], and Haplozoon pugnus[9].

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.[10][11]

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][7] H. ezoense, H. gracile, and H. pugnus were discovered on the coast of Japan.[8][9] There are currently no recorded observations from the tropics or the southern hemisphere, and consequently little is known about its biogeographical distribution.

Modern dinoflagellate phylogenies recognize 8 orders: Syndiniales, Gymnodiniales, Noctilucales, Suessiales, Peridiniales, Gonyaulacales, Prorocentrales, and Dinophysiales.[14] But this has not always been the case – early in the study of dinoflagellates, other orders were used such as Blastodiniales.[15] 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.[16] There is a history of former members of Blastodiniales being moved to new orders based on molecular data.[17][18]

As of June 2020, molecular data exists for five Haplozoon species: H. axiothellae, H. praxillellae, H. ezoense, H. gracile, and H. pugnus.[7][8][13][9] Beyond being dinoflagellates, 18S phylogenetic analyses have 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.

The following species are known to exist:[19]

• 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)[7]
• H. ezoense, Wakeman, 2018 (host: Praxillella pacifica)[8]
• H. gracile, Yamamoto, 2020 (host: cf. Petaloclymene sp.)[9]
• H. pugnus, Yamamoto, 2020 (host: Nicomache personata, Nicomache sp.)[9]