Hoilungia is a 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.[5]
Trichoplax adhaerens was discovered by the German zoologist Franz Eilhard Schulze in 1883. But its identification as to what kind of animal it was (systematic position) was not known.[6] Another German, Karl Gottlieb Grell, discovered the diversity of these animals and created a new phylum Placozoa, in 1971. Grell derived the name from the placula hypothesis, Otto Bütschli's notion on the origin of metazoans.[7]
The advent of molecular techniques allowed genetic analysis of placozoans. The first important report in 2004 by a team of zoologists at the Institute of Animal Ecology & Cell Biology in Hannover, Germany, led by Allen G. Collins and Bernd Schierwater, indicated that placozoans known under T. adhaerens could be genetically many species.[8] Schierwater, teaming up with Michael Eitel and Gert Wörheide from the Ludwig-Maximilians-Universität München, made further studies and found that the specimen H13 was a different placozoan animal for which they introduced the genus Hoilungia, and the species H. hongkongensis, in 2018.[1][3]
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]
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."[9] 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]