''Limnognathia maerski'' is a microscopic freshwater animal, discovered living in warm springs on

Disko Island Disko Island ( kl, Qeqertarsuaq, da, Diskoøen) is a large island in Baffin Bay, off the west coast of Greenland. It has an area of ,Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland ...

, in 1994. Since then, it was also found in

Crozet Islands The Crozet Islands (french: Îles Crozet; 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 Antarcti ...

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...

.). With an average length of 100

micrometers The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...

(μm), it is one of the smallest animals known.

Description

Feeding

''L. maerski'' mainly feeds on bacteria, blue-green algae, and

diatoms A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...

. It has very complicated jaws, with fifteen separate elements. The parts of the jaw structure are connected by ligaments and muscles. The jaw parts are very small, ranging from 4

μm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...

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

''L. maerski'' has a large

ganglion A ganglion 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 both sympathe ...

, 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, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike proje ...

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.

Reproduction

All specimens of ''L. maerski'' that have been collected have had

female organs Female (symbol: ♀) is the sex of an organism that produces the large non-motile ova (egg cells), the type of gamete (sex cell) that fuses with the male gamete during sexual reproduction. A female has larger gametes than a male. Females ...

. 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 theorized that the animals hatch as males and then become females (

sequential hermaphroditism Sequential hermaphroditism (called dichogamy in botany) is a type of hermaphroditism that occurs in many fish, gastropods, and plants. Sequential hermaphroditism occurs when the individual changes its sex at some point in its life. In particular, ...

Taxonomy and phylogeny

Taxonomic status

''Limnognathia maerski'' is nominally a

platyzoa The paraphyletic "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 ...

n, but has variously been assigned as a

class Class 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 differently ...

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, named Micrognathozoa. It is related to the

rotifer The rotifers (, from the Latin , "wheel", and , "bearing"), commonly 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 H ...

s and gnathostomulids, grouped together as the Gnathifera.

Phylogeny

Cladogram showing the relationships of ''Limnognathia'': The Gnathifera is the sister group of the

spiralia The Spiralia are a morphologically diverse clade of protostome animals, including within their number the molluscs, annelids, platyhelminths and other taxa. The term ''Spiralia'' is applied to those phyla that exhibit canonical spiral cleavage ...

ns, and is crucial to understand because of its relationship to

animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...

evolution.

References

External links

* — Comprehensive information on ''L. maerski'' * — article on ''L. maerski'' * {{Taxonbar, from1=Q18636218, from2=Q134876, from3=Q21447118, from4=Q10562273, from5=Q18636217 Microscopic animals Platyzoa genera