Evolution and systematics
Loricifera is traditionally considered a phylum in the group Introverta. The closest relatives of the loriciferans are the Ki-norhyncha (kinorhynchs) and Priapulida (priapulans). Loricifera includes 11 described species as of 2003. All species have been assigned to one order named Nanaloricida that includes two families, the Nanaloricidae and the Pliciloricidae. As of 2003, more than 100 species have been collected that have not yet been described.
Adult loriciferans range in size from 0.008 to 0.0157 in (200-400 pm). The largest specimen ever recorded was a giant larva from the deep sea that measured about 0.027 in (700 pm). The body is divided into five sections: a mouth cone; a head that can be introverted or drawn into the body; a neck; a thorax; and an abdomen. The mouth cone consists of 6-16 oral ridges and a cuticulanized, extrudable buccal tube. Furthermore, some nanaloricids may have six oral stylets.
The head, or introvert, consists of nine rows of sensory or locomotory structures called scalids. The scalids in the first row are called clavoscalids whereas the scalids in the remaining eight rows are called spinoscalids. There are usually eight clavoscalids; however, males of the family Nanaloricidae always have more clavoscalids than the females. The second and third rows of scalids vary greatly in the number of scalids in each row (from 7-15) and types. There are different kinds of leg-shaped scalids and a ventral double organ. The number of scalids in the fourth through the eighth rows is always 30. In the fourth row, the scalids may alternate between the typical spinoscalids and a noticeable claw-shaped variation on the typical form. The spinoscalids in the fifth through the sev enth rows always belong to the common, simple type. The spinoscalids in the eighth row resemble the trichoscalids of the neck region; sometimes they have alternating plates. The scalids in the ninth row in pliciloricids have beak-like structures. These scalids are not present in nanaloricids.
The neck of loriciferans consists of three rows of plates with 15 plates in each row and 15 appendages known as tri-choscalids. The trichoscalids are flattened with serrated margins; they either alternate between eight single and seven double scalids or have 15 single trichoscalids. The thorax has no appendages. The abdomen consists of a lorica with either 6-10 strong cuticularized plates or 22-40 plicae. A varying number of sense organs are present toward the rear of the abdomen. These specialized receptors are called flosculi and can be divided into two types; Nanaloricus-flosculi and Pliciloricus-flosculi.
Internally the body consists of a pharynx bulb, a digestive system with a short esophagus, and a straight midgut. The reproductive system consists of a pair of sack-shaped gonads with a pair of protonephridia, or primitive excretory organs, inside the gonads. The protonephridia are divided into anterior and posterior parts. Their presence inside the gonads is unique to Loricifera. In addition to these organ systems, the body of lori-ciferans contains a complex muscular system as well as a nervous system with a large brain and ventral nerve cord with ganglia (groups of nerve cells located outside the brain).
The postlarvae in the family Nanaloricidae look like adults although they lack one row of scalids and a reproductive system. The postlarvae in the family Pliciloricidae, however, differ significantly from the adults because their scalids are reduced in size and much simpler in appearance.
Scanning micrograph of Nanaloricus mysticus. Left: Adult male. Right: The Higgins larvae. Abbreviations: cs: clavoscalid; lo: lorica; lp: lorica plate; ls: locomotory seta; mc: mouth cone; ss: spinoscalid; to: toe; ts: trichoscalid. (Scanning micrograph by Reinhardt M. Kristensen. Reproduced by permission.)
The Higgins larva ranges in size from 0.003-0.0197 to 0.027 in (80-500 to 700 pm) and is easily recognized by the presence of two conspicuous toes. It is named for Professor Robert P. Higgins in honor of his longtime study of the In-troverta. The body of the Higgins larva is divided into the same sections as the adult. The mouth cone consists of either 6-12 double oral stylets (thin rods) and/or six oral ridges. The head, or introvert, consists of seven rows of scalids with eight clavoscalids in the first row. The second through the seventh row have spinoscalids as in the adult. The bilateral symmetry is pronounced in the arrangement of the larval scalids. The neck sometimes has a collar-like area that can close over the introvert when it is retracted into the body. The thorax resembles an accordion, having 5-6 ventral longitudinal folds. The abdomen also has longitudinal folds, but only on its sides and dorsal surface. Two or three pairs of sensory or locomotory setae (bristles) are located on the larva's ventral surface in the area between the thorax and the abdomen, with two or three pairs of setae and a pair of toes located toward the rear of the lorica. The toes are used for movement and have adhesive glands. There are several flos-culi of the Nanaloricus-type on the abdomen of the Higgins larva.
As of 2003, information is limited regarding the distribution of Loricifera. Representative organisms from this group have been recorded in relatively few locations. It is very likely, however, that these records reflect the scarcity of sampling rather than the actual distribution of loriciferans. The group is probably present in the marine meiofauna throughout the world, from shallow to deep coastal and abyssal waters. Meio-fauna refers to a category of microorganisms that are large enough to see with a basic microscope but too small to see with the naked eye. Loriciferans have been sampled in the coastal waters of France, Denmark, and North Carolina (USA); the Faroe Islands of the North Atlantic; the Mediterranean; the Coral Sea off Queensland, Australia; the Angola basin in the southeastern Atlantic; and from the Great Meteor Seamount in the North Atlantic. They have been sampled in abyssal waters in the Gulf of Mexico at a depth of 9,708 ft (2,959 m) and the Izu-Ogasawara trench at a depth of 27,100 ft (8,260 m) off Japan. Loriciferans are common in polar waters, especially in the deep sea.
Loriciferans are found exclusively in marine habitats, and live in the spaces between sand grains or in the mud at the bottom of the deep sea. The nanaloricids prefer sand with low levels of detritus (material derived from the decomposition of once-living organisms) or clean shell gravel, whereas the pli-ciloricids are often found in such deep-sea sediments as the white abyssal Globigerina ooze and the red deep-sea clay from the hadal zone (below 20,000 ft; 6,100 m). Both families of Loricifera have been recorded in Australian marine caves, and a new nanaloricid genus was discovered in 1998 in sediments influenced by hydrothermal vents in the deep waters of the Kilinailau Trench off Papua New Guinea.
Studies of loriciferan behavior are extremely scarce since most of the animals die before they can be examined. They have been studied live a few times, however, and researchers have observed their movement patterns and mating behavior.
The adults and the larvae both adhere strongly to the sediment with a kind of glue made by the adhesive glands located toward the rear of the adults and on the toes of the larvae. The adults usually crawl by using their many scalids and their mouth cones. Observers have noticed that the mouth cone telescopes out to its full length, fastens itself to a sand grain, and then draws in again so that the loriciferan's body is pulled forward.
The larvae may use their scalids and setae to crawl between grains of sand. They can also swim by using their toes. Larvae of the family Nanaloricidae, whose members have large and flipper-shaped toes, have sometimes have been found swimming free among the plankton. The larvae of the family Pli-ciloricidae have thin spine-shaped toes and are probably unable to swim; their movement is apparently restricted to crawling.
Little is known about the mating behavior of loriciferans. It is assumed, however, that the branched clavoscalids in the males of the family Nanaloricidae are used as chemorecep-tors to locate female sex pheromones. The males in this family also have one ventral modified hook-shaped pair of trichoscalids that hold the female during copulation.
Loriciferans probably feed on suspended organic particles, microalgae, and bacteria. They eat bacteria or algae by piercing them with their oral stylets and then sucking out the contents. The spinoscalids of nanaloricids are often completely covered with bacteria. It is still uncertain why the bacteria are present in such numbers. Some researchers suggest that the animals collect the bacteria from the interstitial water, whereas others maintain that the animals cultivate the bacteria on the mucus that they secrete. What is certain is that the loriciferans feed on these bacteria; this is confirmed by the fact that their digestive tracts are filled in some cases with partially digested bacteria.
Loricifera have very complicated life cycles with both sexual and asexual forms of reproduction. Members of the family Nanaloricidae reproduce only sexually and have distinct sexual dimorphism; that is, different body forms related to gender. The dimorphism is illustrated in the males by some of the clavo- and trichoscalids that are modified pheromone receptors as well as hooks to hold the female during copulation. Fertilization may be either internal or external. The primary larva, the Higgins larva, hatches from the fertilized egg and grows by molting. After at least 2-5 larval instars (stages between molts), the larva metamorphoses into a postlarva. The postlarva is a dormant stage and never feeds. A male or female with fully developed gonads emerges from the postlarva and the life cycle repeats itself.
The members of the family Pliciloricidae follow two different life cycles, named the Rugiloricus cauliculus and Rugiloricus carolinensis cycles respectively. The Rugiloricus cauliculus cycle includes a cycle of sexual reproduction like the one in the family Nanaloricidae. The only difference is that the postlarva looks different from the adult. The scalids of the postlarva are reduced in size and much simpler in form. The Rugiloricus cauliculus cycle, however, also comprises an asexual cycle in which the Higgins larva develops a mature ovary. The maturation of larval ovary is usually called neoteny, hence the matured larval stage is named the neotenous larva. Several (4-8) Higgins larvae form inside the neotenous larva. The larvae are formed by parthenogenesis, which means that they develop from female gametes without fertilization and are therefore genetically identical to the neotenous larva. The parthenogenetic larvae are completely identical to the Hig-gins larva and may develop either into new neotenous larvae or into a postlarva that will molt later into an adult male or female.
In the Rugiloricus carolinensis cycle, the postlarva is reduced so that only the cuticle of the postlarva is left inside the larva. The adult then emerges directly from the larva. Several reductions occur in the life cycle of an undescribed order from the Faroe Islands in the North Atlantic. The cuticles of the postlarva and the adult are found inside a cyst-like larva, and a mature ovary with several eggs develops inside these cuticles. The eggs mature into normal Higgins larvae that eat the maternal individual. The new larvae emerge from an opening in the rear of the empty exuvium (cast-off covering) of the cyst-like larva. This mode of reproduction is called pe-dogenesis and is usually seen only in nematodes and insects.
No species of loriciferans are listed by the IUCN.
Loriciferans may have some significance to humans. They are found in large numbers in areas where methane seeps from the sediment. It may be possible in the future to use lori-ciferans or other meiofauna as indicators for the presence of methane or other gases.
1. Japanese deep-sea girdle wearer (Pliciloricus hadalis); 2. Bucket-tailed loriciferan (Rugiloricus cauliculus); 3. French girdle wearer (Nanaloricus mysticus); 4. American diamond girdle wearer (Rugiloricus carolinensis). (Illustration by Barbara Duperron)
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