Kinorhyncha

(invertebrate zoology) A class of the phylum Aschelminthes consisting of superficially segmented microscopic marine animals lacking external ciliation.

(Kinorhynchs)

Phylum: Kinorhyncha

Number of families: 10

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Meiobenthic, superficially segmented, and spined marine free-living cephalorhynch worms

Evolution and systematics

The name "Kinorhyncha" comes from the Greek words kinema (motion) and rhynchos (proboscis or snout). Kinorhyncha is considered either a class within the phylum Aschelminthes or a separate phylum with close relationships to aschelminth worms. However, the Kinorhyncha was later included as a class in the phylum Cephalorhyncha, established for four classes of Aschelminthes: Priapulida, Kinorhyncha, Loricifera, and Nematomorpha.

Treated here as a phylum, Kinorhyncha encompasses two orders, five suborders, 10 families, 15 genera, and about 150 species. The two orders are: Cyclorhagida (with families Zelinkaderidae, Antigomonidae, Cateridae, Semnoderidae, Centroderidae, Echinoderidae, Dracoderidae, and Cephalorhynchidae), and Homalorhagida (with families Pycnophyidae and Neocentrophyidae). There is no fossil record for the Kinorhyncha.

Physical characteristics

Adult kinorhynchs range in length from 0.008 in (0.2 mm) in certain Echinoderes, to 0.05 in (1.2 mm) in the arctic Pycnophyes. Most are transparent; but there are yellowish and reddish species that live on macroalgae. A few have red or brown cup-shaped eyes located in the head near the brain.

The body is subdivided into two main regions: an eversible head, or introvert, and trunk, metamerically segmented into 11 cuticularized trunk segments. The spherical head is joined to the trunk by a short eversible neck with closing plates, called placids. The head and neck are not serially gomologous to trunk segments but are traditionally considered as the first and second segments; thus, there is a total of 13 body segments.

The head terminates with a protrusible mouth cone surrounded by nine oral styles. Internally, the terminal mouth is followed by 20 pentamerously arranged buccal styles, which are partly eversible when feeding. The spherical head bears 5–7 rings of as many as 89–91 posteriorly directed spines called scalids. Scalids are sensory and are also used for forward locomotion. The neck is composed of a series of closing plates, which retract over the head when it is withdrawn into the trunk. Cyclorhagids typically have 14–16 radially arranged placids. Homalorhagids have 2–4 dorsal and 2–4 ventral trapezium-like placids.

Kinorhynchs are characterized by metamerical trunk armor (exoskeleton). The trunk segments are variously subdivided longitudinally into a series of cuticular plates (dorsal tergites and ventral sternal plates). In most Cyclorhagida, segment 3 (the first trunk segment), is entire and composed of a complete ring of cuticle. In the Semnoderidae, segment 3 is divided into bivalved plates forming a clamshell-like closing apparatus, which acts with placids to close off the inverted head. In the Pycnophyidae, segment 3 is composed of one arched dorsal plate, or tergite, and three mobile ventral, or sternal, plates. Mobile ventral plates close the anterior region of the trunk when the head is introverted. Segment 4 is entire in the Echinoderidae or subdivided into one tergal and two sternal plates by midventral and lateral articulations extending posteriorly from segment 4 through segment 13 in most other kinorhynchs. Articulations, the flexible junctionsof the arthrocorial cuticle allow movement between plates and segments, as well as inversion and eversion of the head.

Trunk spines are usually located middorsally (DS), laterally (LS), midterminally (MTS), or lateroterminally (LTS). In some species, there are accessory lateral (LAS) and accessory lateroterminal (LTAS) spines, middorsal processes (MP), and modified spinelike appendages. Some spines are adhesive tubes (AT). The spines are sensorial and related to locomotion.

The internal anatomy of kinorhynchs is related to the outer segmentation, nervous system, muscles, and glandular system, which are all distinctively segmented. Simultaneous contraction of segmental dorsoventral muscles increases the pressure of the body-cavity fluid in the trunk, displaces it forward, and everts the head. Scalids move forward, plow backward through the interstices around and propel the kynorhynch forward. Special head retractor muscles retract the introvert back into the trunk in synchrony with relaxation of the dorsoventral muscles.

Distribution

Kinorhynchs occur worldwide, from polar to tropical seas.

Habitat

Kinorhynchs are eurybathic (from zero to 17,390 ft [0 to 5,300 m]), euryhaline (from 7 ppt in estuaries to 60 ppt in tide pools), and eurythermic (from 29.3 to 104°F [-1.5 to 40°C]).

Mesobenthic or interstitial species of kinorhynchs live in the interstices between large sediment particles; endobenthic species burrow by displacing small sediment particles in muddy sediments; and epibenthic species live at the water-sediment interface or in the suspended flocculent material on the surface of marine algae and invertebrates. They usually are found within the few uppermost inches of muddy sediments, depending on the oxygen gradient. In sand or shell-gravel of high-energy beaches, kinorhynchs are found at depths of 3.3 ft (1 m) or more. Most prefer mud, or mud mixed with sand, with a high organic content.

Behavior

Kinorhynchs are relatively abundant representatives of the permanent meiofauna, occasionally ranking third or fourth in number within a meiobenthic sample. The mean annual population density is about 10,000 –15,000, but may reach 50,000 specimens per square meter. Sex ratio in the populations is about 1:1.

Being placed into sedimented particles, kinorhynchs can agglutinate the detritus and amass the particles into concretions glued together by mucus. As also noted for nematodes, these activities result in soft bottoms acquiring a particular framework.

Parasitic Suctoria and sessile Peritricha infusorians (Ciliophora) are often found on the trunk surface of kinorhynchs.

Feeding ecology and diet

Kinorhynchs are herbivorous and detrivorous. Most cyclorhagids are diatom-feeders. They collect pennate diatoms using their rigid articulated oral styles as multiple pincers to locate one end of the shell and manipulate the diatom to their terminal mouth. Kinorhynchs bring diatoms into their buccal cavity with an action of their mobile buccal styles and with pumping movements of their sucking pharynx. The sharp buccal styles then move against the diatom, damaging the girdle and causing the two valves to separate. The diatom cell is emptied by the sucking of the pharynx, and the emptied frustule is rejected in the substrate. Echinoderes also can strip off terminal spines and collect diatoms between their head scalids. The diatoms are then collected by the oral styles using a repetitive withdrawing of the scalids and a protrusion of the mouth cone.

Most homalorhagids are selective deposit-feeders. They have flexible nonarticulated oral styles, which they use to collect detritus and bacterial clots. The sharp buccal teeth of the eversible anterior part of the buccal cavity also act as scrapers to graze on bacterial film and fungi.

Numerous glands secrete mucus, which is released onto the trunk surface. This secretion is also used to entrap detritus, bacteria, diatoms, and fungi, which are subsequently browsed on together with the mucus. Flexible and spiny cyclorhagids also can strip off terminal spines and collect attached diatoms between their head scalids. The diatoms are then collected by the oral styles using a repetitive withdrawing of the scalids and a protrusion of the mouth cone.

The parasitic protozoan Kinorhynchospora japonica (phylum Microspora) infects all midgut cells of Kinorhynchus yushini. Sulfur gram-negative bacteria are found in a few special midgut cells (bacteriocytes) of Pycnophyes kielensis from the intertidal mud with sulfuretted hydrogen and are thought to be symbiotic.

Reproductive biology

Kinorhynchs are dioecious, with external sexual dimorphic characters (AT and penile spines in males, and LTAS in females). Sexes copulate with spermatophore transfer; fertilization is internal. Mature ovaries commonly develop a single large oocyte. Fertilized eggs (60–80 microns) are attached to sand grains or detritus. Cleavage has not been observed. The early embryo in the egg shows no segmentation. Just before hatching, the juvenile kynorhynch has 11 segments: the head with scalids, the neck with placids, and nine trunk segments, which are not divided into sternal and tergal plates. The time required for development from oviposition to hatching is about 10 days. At the time of hatching, the juvenile straightens and simultaneously everts its head, thus tearing open the egg envelope. Kinorhynchs have a direct development through five to six juvenile stages, each derived from a molt.

Just after hatching, the first juvenile can feed on diatoms and detritus. The first molt establishes a 12-segmented juvenile. All trunk segments are discernible in the third stage, but the complete separation of the most posterior segments becomes evident in the fourth or fifth stages.

To distinguish various juvenile stages of kinorhynchs, terms previously proposed for some invalid genera are used. In Cyclorhagida, two morphological states of juveniles are designated: "Centropsis," with MTS and "Habroderes," having LTS and without MTS. In Homalorhagida, there are three morphological states of juveniles: "Centrophyes," with MTS; "Hyalophyes," with LTS and without MTS; and "Leptodemus," without any terminal spines. Cyclorhagids with MTS usually have series of only "Centropsis" stages. In Echinoderidae, first three "Centropsis" stages are usually followed by three "Habroderes" stages. In Pycnophyidae, three "Centrophyes" or "Leptodemus" stages are usually followed by three "Hyalophyes" stages or there are only "Leptodemus" stages.

Conservation status

No species of Kinorhyncha is listed by the IUCN.

Significance to humans

None known.

Species accounts

Resources

Adrianov A. V., and V. V. Malakhov. Kinorhyncha: Structure, Development, Phylogeny and Classification. Moscow: Nauka Publishing, 1994. ——. Cephalorhyncha of the World Oceans. Moscow: KMK Scientific Press Ltd., 1999. ——. "Kinorhyncha." In Reproductive Biology of Invertebrates. Vol. 9, edited by K. Adiyodi, R. Adiyodi, and B. Jamieson. [n.p.], 1999.

Higgins, R. P. "Kinorhyncha." In Reproduction of Marine Invertebrates. Vol. 1, edited by A. Giese and J. Pearse. [n.p.], 1974.

Kristensen, R. M., and R. P. Higgins. "Kinorhyncha." In Microscopic Anatomy of Invertebrates, Vol. 4, Aschelminthes, edited by F. W. Harrison and E. E. Ruppert. New York: Wiley-Liss Inc., 1991.

Zelinka, K. Monographie der Echinoders. Leipzig: Engelman, 1928.

Adrianov A. V., and V. V. Malakhov. "Phylogeny and Classification of the Class Kinorhyncha." Zoosystematica Rossica 4 (1995): 23–44.

Higgins, R. P. "A Historical Overview of Kinorhynch Research." Smithsonian Contributions to Zoology 76 (1971): 25–31. ——. "The Atlantic Barrier Reef Ecosystem at Carrie Bow Cay, Belize, II Kinorhyncha." Smithsonian Contributions to the Marine Sciences 18 (1983): 1–131.

Kozloff, E. N. "Some Aspects of Development in Echinoderes (Kinorhyncha") Transactions of the American Microscopical Society 91 (1972): 119–130.

Neuhaus, B., and R. P. Higgins. "Ultrastructure, Biology, and Phylogenetic Relationships of Kinorhyncha." Integrative and Comparative Biology 42 (2002): 619–632.

[Article by: Andrey Adrianov, PhD]

A phylum of free-living marine invertebrates less than 0.04 in. (1 mm) long. They are segmented and lack external ciliation (see illustration). Kinorhynchs are benthonic, so-called because they generally dwell in mud or sand from intertidal to deep-sea habitats. Two orders are generally recognized, Cyclorhagida and Homalorhagida.

Echinoderes sp., a cyclorhagid kinorhynch (ventrolateral view).

The body is covered by a transparent cuticle secreted by an underlying epidermis. The cuticle is molted only in the process of juvenile growth. Three body regions are recognized: a head segment, a neck segment and an 11-segment trunk. The head is completely retractable. When everted, it extends its five to seven circles of recurved spines called scalids. The neck consists of plates called placids which function to close the anterior opening of the trunk when the head is retracted. Most kinorhynchs have a pair of adhesive tubes on the ventral surface of the third or fourth segment.

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Kinorhynchs
Scientific classification
Kingdom: Animalia Phylum: Kinorhyncha Reinhard, 1887
Subdivisions
Order Cyclorhagida Suborder Cyclorhagae Family Echinoderidae Family Zelinkaderidae Family Centroderidae Family Dracoderidae Suborder Conchorhagae Family Semnoderidae Suborder Cryptorhagae Family Cateriidae Order Homalorhagida Suborder Homalorhagae Family Pycnophyidae Family Neocentrophyidae

Kinorhyncha (Gr. κίνηω, kīneō 'move' + ρυνχος, rhynchos 'snout') is a phylum of small (1 mm or less) marine pseudocoelomate invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos. They are also called mud dragons.

They are segmented, with a body consisting of a head, neck, and a trunk of eleven segments. They do not have external cilia, but instead have a number of spines along the body, plus up to seven circles of spines around the head,^[1] which they use for locomotion, withdrawing the head and pushing forward, then holding with the spines while drawing up the body. The spines are part of a cuticle secreted by the epidermis; this is molted several times while growing to adulthood. The head is completely retractable, and is covered by a set of neck plates called placids when retracted.

Kinorhynchs eat diatoms and other things found in the mud.

There are two sexes that look alike, and the larvae are free-living, but little else is known of their reproductive process.

Their closest relatives are thought to be the phyla Loricifera and Priapulida. Together they constitute the Scalidophora.

The two groups of Kinorhynchs are still generally characterized as orders rather than classes, about 150 species are known.

References

1. ^ Brusca and Brusca, Invertebrates, pp. 347

External links

• Introduction to the Cephalorhyncha
• Drawings of Kinorhyncha

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