Limnognathia

Limnognathia
Schematic drawing of Limnognathia maerski
Microscopic on L. maerski
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria
Clade:	Nephrozoa
(unranked):	Protostomia
(unranked):	Spiralia
Clade:	Gnathifera
Phylum:	Micrognathozoa
Order:	Limnognathida
Family:	Limnognathiidae
Genus:	Limnognathia
Species:	L. maerski
Binomial name
Limnognathia maerski Kristensen & Funch, 2000

Limnognathia maerski is a microscopic acoelomate freshwater animal, discovered living in cold springs on Disko Island, Greenland, in 1994.^[1] Since then, it has also been found on the Crozet Islands of Antarctica^[2] as well as in the British Isles,^[3] suggesting a worldwide distribution, although there are likely different species yet to be described.

With an average length of 100 micrometers (μm), it is one of the smallest known animals.

Etymology of Micrognathozoa: From the Greek Micros (= very small) Gnathos (= jaw) and Zoon (= animal)

L. maerski is the only species that belongs to the Micrognathozoa, a relatively new phylum of animals that was only described in 2000.^[4][5]

L. maerski mainly feeds on bacteria, blue-green algae, and diatoms. It has very complex jaws, with fifteen separate elements; these elements are very small, ranging from 4 μm to 14 μm. The animal can extend part of its jaw structure outside its mouth while eating. It also extends much of its jaw structure outside its mouth when it is regurgitating indigestible items.

L. maerski has a large ganglion, or 'brain', in its head, and paired nerve cords extending ventrally (along the lower side of the body) towards the tail. Stiff sensory bristles made up of one to three cilia are scattered about the body. These bristles are similar to ones found on gnathostomulids, but up to three cilia may arise from a single cell in L. maerski, while gnathostomulids never have more than one cilium per cell.

Flexible cilia are arranged in a horseshoe-shaped area on the forehead, and in spots on the sides of the head and in two rows on the underside of the body. The cilia on the forehead create a current that moves food particles towards the mouth. The other cilia move the animal.

All specimens of L. maerski that have been collected have had female organs. They lay two kinds of eggs: thin-walled eggs that hatch quickly, and thick-walled eggs that are believed to be resistant to freezing, and thus capable of over-wintering and hatching in the spring. The same pattern is known from rotifers, where thick-walled eggs only form after fertilization by males. The youngest L. maerski specimens collected may also have male organs, and it is now hypothesized that the animals hatch as males and then become females (sequential hermaphroditism).

Limnognathia maerski is nominally a platyzoan, but has variously been assigned as a class or subphylum in the clade Gnathifera or as a phylum in a Gnathifera superphylum, named Micrognathozoa. It is related to the rotifers and gnathostomulids, grouped together as the Gnathifera.^[6][7]

Cladogram^[8] showing the relationships of Limnognathia:

The Gnathifera is the sister group to the rest of the spiralians and is crucial to understand because of its relationship to animal evolution.

• "Limnognathia introduction". zmuc.dk. Københavns Universitet (University of Copenhagen). Archived from the original on 2 December 2019. Retrieved 10 December 2021. — Comprehensive information on L. maerski

• "Jaws: New animal discovered". Science Now. Natural History Museum. California Academy of Sciences. c. 2006. Archived from the original on 23 March 2007. — article on L. maerski

• Bekkouche, Nicolas; Worsaae, Katrine (1 October 2016). "Nervous system and ciliary structures of Micrognathozoa (Gnathifera): Evolutionary insight from an early branch in Spiralia". Royal Society Open Science. 3 (10): 160289. doi:10.1098/rsos.160289. PMC 5098970. PMID 27853545.