Limnognathia Desmeti
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''Limnognathia'' is a genus of microscopic
acoelomate The coelom (or celom) is the main body cavity in many animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it r ...
freshwater animal that was discovered in
Disko Island Disko Island (, ) is a large island in Baffin Bay, off the west coast of Greenland. It has an area of ,Greenland Greenland is an autonomous territory in the Danish Realm, Kingdom of Denmark. It is by far the largest geographically of three constituent parts of the kingdom; the other two are metropolitan Denmark and the Faroe Islands. Citizens of Greenlan ...
, in 1994. Since then, it has also been found on the
Crozet Islands The Crozet Islands (; or, officially, ''Archipel Crozet'') are a sub-Antarctic archipelago of small islands in the southern Indian Ocean. They form one of the five administrative districts of the French Southern and Antarctic Lands. History ...
of
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...
as well as in the
British Isles The British Isles are an archipelago in the Atlantic Ocean, North Atlantic Ocean off the north-western coast of continental Europe, consisting of the islands of Great Britain, Ireland, the Isle of Man, the Inner Hebrides, Inner and Outer Hebr ...
and the Spanish
Pyrenees The Pyrenees are a mountain range straddling the border of France and Spain. They extend nearly from their union with the Cantabrian Mountains to Cap de Creus on the Mediterranean coast, reaching a maximum elevation of at the peak of Aneto. ...
, suggesting a worldwide distribution. There are two known species of ''Limnognathia'': ''L. maerski'', described in 2000, and ''L. desmeti'', described in 2025. ''Limnognathia'' is the only genus in the phylum Micrognathozoa ("small-jawed animal").


Description


Feeding

''Limnognathia'' mainly feeds on bacteria, blue-green algae, and
diatoms A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...
. It has very complex jaws, with fifteen separate elements; these elements are very small, ranging from 4 
μm The micrometre (Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a unit of length in the International System ...
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.


Anatomy

''Limnognathia'' has a large
ganglion A ganglion (: ganglia) is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system, this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system, there are ...
, 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 The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
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 ''Limnognathia'', 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.


Reproduction

All specimens of ''Limnognathia'' 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 The rotifers (, from Latin 'wheel' and 'bearing'), sometimes called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris ...
, where thick-walled eggs only form after fertilization by males. The youngest ''Limnognathia'' specimens collected may also have male organs, and it is now hypothesized that the animals hatch as males and then become females (
sequential hermaphroditism Sequential hermaphroditism (called dichogamy in botany) is one of the two types of hermaphroditism, the other type being simultaneous hermaphroditism. It occurs when the organism's sex changes at some point in its life. A sequential hermaphrodit ...
).


Taxonomy and phylogeny


Taxonomic status

''Limnognathia'' is nominally a
platyzoa The "Platyzoa" are a group of protostome unsegmented animals proposed by Thomas Cavalier-Smith in 1998. Cavalier-Smith included in Platyzoa the phylum Platyhelminthes (or flatworms), and a new phylum, the Acanthognatha, into which he gathered s ...
n, but has variously been assigned as a
class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...
or
subphylum In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum. The taxonomic rank of " subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists have also used th ...
in the clade Gnathifera or as a phylum in a Gnathifera
superphylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclat ...
, named Micrognathozoa. It is related to the
rotifer The rotifers (, from Latin 'wheel' and 'bearing'), sometimes called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic Coelom#Pseudocoelomates, pseudocoelomate animals. They were first describ ...
s and gnathostomulids, grouped together as the Gnathifera. Two species are described: ''Limnognathia maerski'' from Greenland and Spain, and ''Limnognathia desmeti'' from the Crozet Islands. There are no recognizable morphological differences between the two species, but genetic studies show that they are distinct species.


Species

* ''
Limnognathia maerski ''Limnognathia'' is a genus of microscopic acoelomate freshwater animal that was discovered in Disko Island, Greenland, in 1994. Since then, it has also been found on the Crozet Islands of Antarctica as well as in the British Isles and the Spani ...
'' (Kristensen & Funch, 2000) * '' Limnognathia desmeti'' (Worsaae & Møller, 2025)


Phylogeny

Cladogram 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 Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...
evolution.


References


External links

* — Comprehensive information on ''L. maerski'' * — article on ''L. maerski'' * {{Taxonbar, from1=Q18636218, from2=Q134876, from3=Q21447118, from4=Q10562273, from5=Q18636217 Animals described in 2000 Gnathifera (clade) Microscopic animals Platyzoa genera