Kinorhynchans are segmented, limbless animals. Their body has a head, neck, and a trunk. The trunk has eleven segments. They do not have external cilia, but have a number of spines along the body, and up to seven circles of spines around the head. These spines are used for movement. They can pull back the head and push forward; then they can grip the substrate with the spines, and draw up the body.
The body wall is a thin layer with a tough cuticle. It is moulted several times during growth. This layer is a syncytium, with nuclei, but no cell membranes.
The spines are are hollow and covered by cuticle. The head is completely retractable, when is covered by a set of neck plates.
The nervous system is a ventral nerve cord, with one ganglion in each segment, and an anterior nerve ring surrounding the pharynx. This is the common arrangement for invertebrates. Some species have simple ocelli (eye-spots) on the head, and all species have tiny bristles on the body as touch receptors.
There are two sexes that look alike. A pair of gonads are in the mid-region of the trunk, and open to pores in the final segment. In most species, the sperm duct has two or three spiny structures that presumably help in copulation. The larvae are free-living.
Their closest relatives may be the phyla Loricifera and Priapulida. Together they are the Scalidophora.
Phylum Kinorhyncha
Loricifera
Loricifera is a recently discovered ecdysozoan phylum of marine animals. The phylum includes the first animals known to exist in a completely anoxic environment (no oxygen at all). The phylum was described in 1983 by Reinhardt Kristensen.
They are a phylum of very small animals which live in sediment. There are now twenty-two described species, in eight genera. In addition, there are about 100 more species which have been collected, but not yet described. Their size ranges from 100 µm to about 1 mm.
Their habitat is in the spaces between marine gravel to which they attach themselves. They are among the most recently discovered groups of Metazoans. They attach themselves quite firmly to the gravel, and this explains why they remained undiscovered for so long. The first specimen was collected in the 1970s, and later described in 1983. They are found at all depths of water, in different sediment types, and in all latitudes. They have not yet been found in the fossil record.
These animals have a protective outer case called a lorica. Protected by the lorica, the animals have a head, mouth and digestive system. Development is usually direct, though there are larvae in some species. The animals have two sexes as adults. There are some complex and flexible life cycles with different forms of parthenogenetic reproduction.
Their closest relatives are thought to be the Kinorhyncha and Priapulida, with whom they share some characteristics. Together they make up the clade Scalidophora.
Three species of Loricifera have been found in the sediments at the bottom of the L'Atalante basin in the Mediterranean Sea. The 'surface' of this basin is 3,500 meters down. They are the first multicellular organisms known to spend their entire lives in an oxygen-free environment. It is thought they are able to do this because they rely on organelles which, unlike normal mitochondria, do not use oxygen for their energy cycle. They are able to do this because they use hydrogenosomes to get their energy rather than mitochondria.
The newly reported animals complete their life cycle in the total absence of light and oxygen, and they are less than a millimetre in size. They were collected from the deep basin where they live in a nearly salt-saturated brine. This brine, because of its density, does not mix with the waters above. As a result, the brine is completely anoxic, and high in ammonia. Also, due to the action of sulphate reducers, it contains sulphide at a high concentration. In the brine, the abundance of cells is not high. Extremophiles predominate, including members of the deep-sea hydrothermal vent community of archaea and bacteria. Eukaryotes are also found in l'Atalante, including ciliates (45%), dinoflagellates (21%) and choanoflagellata (10%).
This article incorporates CC-BY-2.0 text from references
Saturation (chemistry)
The term saturation in chemistry is used in several ways:
Sulphide
A sulphide (sulfide in US English) is an anion of sulphur in its lowest oxidation number of -2. Sulphide is also a term for thioethers, a common type of compound which are known for their bad odours (example: HS = hydrogen sulphide).
The dianion S exists only in strongly alkaline aqueous solutions.
Sulphate
In chemistry, a sulphate (sulfate in US English) is a salt of sulphuric acid. The sulphate ion is a group of atoms with the formula SO and two negative charges. It consists of a central sulphur atom surrounded by four equivalent oxygen atoms. It occurs when sulfuric acid is deprotonated twice. If it is only deprotonated once, the hydrogen sulfate ion is made.
L'Atalante basin
The L'Atalante basin is a very salty 'lake' at the bottom of the Mediterranean Sea.
It is about 190 km (~120 miles) west of the island of Crete. It is called a 'lake' because, though it is under more than two miles of water, it is almost completely separate from the sea above it. Its huge salt content prevents the waters mixing. Sometimes these lakes are called "brine pools". They are found in many oceans; there is one in the Gulf of Mexico.
L'Atalante is only one of several such lakes underneath the normal waters of the Mediterranean. When the Strait of Gibraltar was closed, about six million years ago, the Mediterranean dried up and became a huge salt lake, rather like the Dead Sea and the Great Salt Lake are today. Then it dried out completely, leaving an enormous amount of salt. When the Straight of Gibraltar opened again about five million years ago, water from the Atlantic Ocean rushed in to form the Mediterranean we see today. Even today, the Atlantic feeds the Mediterranean with water, because the evaporation from the 'Med' is greater than is provided by its rivers. With its huge supply of salt underneath, and the constant evaporation, the Med is a high salt sea compared with the Atlantic.
The L'Atalanta lake and its neighbours the Urania and Discovery lakes are about 35,000 years old. They were formed by salt deposits dissolving out of the Mediterranean Ridge and collecting in deep depressions at the bottom of the Med. L'Atalante is the smallest of the three. Its surface begins at about below sea level.
L'Atalante's salinity (salt content) is near saturation, at 36.5%, about eight times more than seawater. This prevents the water in the brine pool from mixing with the 'normal' seawater above. Without mixing, oxygen from above can't get into the brine pool. The basin's water is completely anoxic: it has no oxygen.
Between the normal water and the brine is a short (1.5 metres) boundary zone called a halocline. The salt content rises sharply there, and oxygen reduces towards zero. This halocline between the seawater above and the brine below has many bacterial and archaeal cells: they are chemoautotrophs, which feed on ammonia from the brine but cannot function without some oxygen. Certain methane oxidizers and haloarchaea are found only in the halocline.
Below, in the brine, the number of cells is much lower than in the halocline. Extremophiles are most common, including some of the deep-sea hydrothermal vent archaea and bacteria. Eukaryotes are also found in l'Atalante, mostly ciliates (45%), dinoflagellates (21%) and choanoflagellata (10%).
The dark grey anoxic sediments at the bottom of L'Atalante lake are covered with a loose black layer. Microbes found in the sediments are almost all (90%) various species of "Bacillus". In 2010, three metazoan species, all in the Loricifera phylum, were discovered living in the sediment, the first multicellular lifeforms known to live entirely without oxygen.
Tactical voting