Hoilungia
| Hoilungia | |
|---|---|
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| Microscopic image of Hoilungia hongkongensis. Scale bar is 0.2 mm. | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Placozoa |
| Class: | Uniplacotomia |
| Order: | Hoilungea |
| Family: | Hoilungidae |
| Genus: | Hoilungia Eitel, Schierwater & Wörheide, 2018 |
| Species: | H. hongkongensis
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| Binomial name | |
| Hoilungia hongkongensis Eitel, Schierwater & Wörheide, 2018
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Hoilungia is genus that contains one of the simplest animals and belongs to the phylum Placozoa.[1][2] Described in 2018, it has only one named species, H. hongkongensis, although there are possible other species.[3] The animal superficially resembles another placozoan, Trichoplax adhaerens, but genetically distinct from it as mitochondrial DNA analysis revealed.[1]
Hoilungia was discovered in brackish water from mangrove swamps in Hong Kong.[4] These organisms are generally found in the biofilm surfaces in tropical and subtropical environments. Phylogenetically, they are placed closest to cnidarians. They are diploblastic animals and are believed to have dorso-ventral polarity along top and bottom body layers. Their body is overtly similar to oral-aboral axis of cnidarians. They feed from lower tissue layer which has various peptidergic gland cells. They feed on algae, bacteria, yeast and other byproducts of biofilms. They reproduce asexually through binary fission and there is also some evidence of sexual reproduction.[5] The H. hongkongensis genome adds support to the phylogenetic placement of the Placozoa in the animal tree of life
Discovery
Over a long time, only a single species of placozoan clades, Trichoplax adhaerens, was described but lately, it has been found that the relatively newer species, Hoilungia hongkongensis is a result of reproductive isolation mechanism of speciation.[3]
Etymology
The genus name is derived from the phrase "hoi lung", which means "sea dragon" in Cantonese. The species name is after Hong Kong from where it was discovered.[1]
Structure
Hoilungia do not have well-defined body plan much like amoebas, unicellular eukaryotes. As Andrew Masterson reported, "[as other placozoans] they are as close as it is possible to get to being simply a little living blob."[6] An individual body measures about 0.55 mm in diameter.[3] There are no body parts; as Eitel described: "There's no mouth, there's no back, no nerve cells, nothing."[1]
As do other placozoans, Hoilungia has only three anatomical parts as tissue layers inside its body: the upper, intermediate (middle) and lower epithelia. The upper epithelium is the thinnest portion and essentially comprises flat cells with their cell body hanging underneath the surface, and each cell having a cilium. The middle layer is the thickest made up of numerous fiber cells, which contain mitochondrial complexes, vacuoles and endosymbiotic bacteria in the endoplasmic reticulum. The lower epithelium consists of numerous monociliated cylinder cells along with a few endocrine-like gland cells and lipophil cells. Each lipophil cell contains numerous middle-sized granules, one of which is a secretory granule.[3]
The body axes of Hoilungia and Trichoplax are overtly similar to the oral–aboral axis of cnidarians,[5] animals from another phylum with which they are most closely related.[7] Structurally, they can not be distinguished from other placozoans, so that identification is purely on genetic (mitochondrial DNA) differences.[3]
Evolutionary history
Hoilungia and Trichoplax are considered one of the earliest branching animal lineages, and have relatively simple morphologies their complexity of NO-cGMP-mediated signaling is greater to those in vertebrates. This evidence has been found in their DNA by experimentation using ultra-sensitive capillary electrophoresis assays.[3]
References
- ^ a b c d Wood, Charlie (2018-10-06). "Simplest Animal Reveals Hidden Diversity". Scientific American. Retrieved 2023-06-02 – via Quanta Magazine.
- ^ World Placozoa Database. "Hoilungia hongkongensis Eitel, Schierwater & Wörheide, 2018". World Register of Marine Species. Retrieved 18 August 2020.
- ^ a b c d e f Eitel, Michael; Francis, Warren R.; Varoqueaux, Frédérique; Daraspe, Jean; Osigus, Hans-Jürgen; Krebs, Stefan; Vargas, Sergio; Blum, Helmut; et al. (2018). "Comparative genomics and the nature of placozoan species". PLoS Biology. 16 (7): e2005359. doi:10.1371/journal.pbio.2005359. PMC 6067683. PMID 30063702.
{{cite journal}}: CS1 maint: unflagged free DOI (link) - ^ Wood C (6 October 2018). "World's simplest animal reveals hidden diversity". Quanta Magazine.
- ^ a b DuBuc TQ, Ryan JF, Martindale MQ (May 2019). ""Dorsal-Ventral" Genes Are Part of an Ancient Axial Patterning System: Evidence from Trichoplax adhaerens (Placozoa)". Molecular Biology and Evolution. 36 (5): 966–973. doi:10.1093/molbev/msz025. PMC 6501881. PMID 30726986.
- ^ Masterson, Andrew (2018-08-01). "Simple organisms not so simple, after all". Cosmos Magazine. Retrieved 2023-06-02.
- ^ Laumer, Christopher E.; Fernández, Rosa; Lemer, Sarah; Combosch, David; Kocot, Kevin M.; Riesgo, Ana; Andrade, Sónia C. S.; Sterrer, Wolfgang; Sørensen, Martin V.; Giribet, Gonzalo (2019-07-10). "Revisiting metazoan phylogeny with genomic sampling of all phyla". Proceedings. Biological Sciences. 286 (1906): 20190831. doi:10.1098/rspb.2019.0831. ISSN 1471-2954. PMC 6650721. PMID 31288696.
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