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Loricifera (from Latin, lorica, corselet + ferre, to bear) is a phylum of very small to microscopic marine sediment-dwelling animals with twenty-two described species, in eight genera. Aside from these described species, there are approximately 100 more that have been collected and not yet described. Their size ranges from 100 µm to ca. 1 mm.  They are characterised by a protective outer case called a lorica and their habitat, which is in the spaces between marine gravel to which they attach themselves. The phylum was discovered in 1983 by Reinhardt Kristensen, in Roscoff, France. They are among the most recently discovered groups of Metazoans. They attach themselves quite firmly to the substratum, and hence remained undiscovered for so long. The first specimen was collected in the 1970s, and later described in 1983. They are found at all depths, in different sediment types, and in all latitudes.
The animals have a head, mouth and digestive system as well as a lorica. The armor-like lorica consists of a protective external shell or case of encircling plicae. There is no circulatory system and no endocrine system. Many of the larvae are acoelomate, with some adults being pseudocoelomate, and some remaining acoelomate. Development is generally direct, though there are so called Higgins-larvae, which differ from adults in several respects. The animals have two sexes as adults. Very complex and plastic life cycles of pliciloricids include also paedogenetic stages with different forms of parthenogenetic reproduction.  They are not known to be present in the fossil record.
Their closest relatives are thought to be the Kinorhyncha and Priapulida with which they constitute the taxon Scalidophora. The three phyla share four characters in common — chitinous cuticle, rings of scalids on the introvert, flosculi, and two rings of introvert retracts.
In anoxic environment
Three species of Loricifera have been found in the sediments at the bottom of the L'Atalante basin in Mediterranean Sea, more than 3000 meters down, the first multicellular organisms known to spend their entire lives in an oxygen-free environment. They are able to do this because they rely on hydrogenosomes (or similar organelles) instead of on mitochondria for energy.
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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 a deep basin at the bottom of the Mediterranean Sea, where they inhabit a nearly salt-saturated brine that, because of its density (> 1.2 g/cm3), does not mix with the waters above. As a consequence, this environment is completely anoxic and, due to the activity of sulphate reducers, contains sulphide at a concentration of 2.9 mM. Despite such harsh conditions, this anoxic and sulphidic environment is teeming with microbial life, both chemosynthetic prokaryotes that are primary producers, and a broad diversity of eukaryotic heterotrophs at the next trophic level.
- Gad, G. 2005. Successive reduction of the last instar larva of Loricifera, as evidenced by two new species of Pliciloricus from the Great Meteor Seamount (Atlantic Ocean). Zoologischer Anzeiger. 243: 239–271.
- Ruppert, Edward E., Richard S. Fox, and Robert D. Barnes. Invertebrate Zoology. 7th ed. Toronto: Brooks/Cole — Thomson Learning, 2004. 776.
- Heiner, I. 2005. Preliminary account of the loriciferan fauna of the Faroe Bank (NE Atlantic). Biofar Proceedings 2005: 213–219.
- Heiner, I., Kristensen, R.H. 2005. Two new species of the genus Pliciloricus (Loricifera, Pliciloricidae) from the Faroe Bank, North Atlantic. Zoologischer Anzeiger. 243: 121–138.
- Kristensen, R.M. 2002. An Introduction to Loricifera, Cycliophora, and Micrognathozoa. Integrative and Comparative Biology. 42: 641–651.
- Fang, Janet (4-6-2010). "Animals thrive without oxygen at sea bottom". Nature. Nature Publishing Group. doi:10.1038/464825b. Retrieved 4-6-2010. Check date values in:
- Mentel M. & Martin W. (2010) "Anaerobic animals from an ancient, anoxic ecological niche". BMC Biology 2010, 8:32. doi:10.1186/1741-7007-8-32.
- Danovaro R., Dell'Anno A., Pusceddu A., Gambi C., Heiner I. & Kristensen R. M. (2010). "The first metazoa living in permanently anoxic conditions". BMC Biology 8: 30. doi:10.1186/1741-7007-8-30
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