Kinorhyncha

Diversity

Phylum Kinorhyncha is divided into two orders, Cyclorhagida and Homalorhagida , with 21 genera and over 200 species, some of which have only been described from juvenile stages. These animals are very small, generally less than 1 mm in length, segmented, and limbless. They are motile, marine (occasionally brackish), benthic invertebrates found worldwide, from intertidal to abyssal zones, on sediment layers, sandy beaches, in algae, or associated with other, larger invertebrates. Their bodies are covered with multiple spines, which are movable extensions of the body wall. The spines around the head are used to help pull these animals along the benthos, where they deposit feed on diatoms, bacteria, and other organic matter in the substrate. The head and pharynx can be retracted and covered with plates (called placids) on the second body segment, differences in the number and closing mechanisms of these plates are often used to distinguish species and higher taxonomic groups.

Geographic Range

Species belonging to this phylum have been described from marine and brackish waters around the world. They have been found at depths from 8 to 8,000 meters.

• Biogeographic Regions
• nearctic nearctic
  • native native
• palearctic palearctic
  • native native
• oriental oriental
  • native native
• ethiopian ethiopian
  • native native
• neotropical neotropical
  • native native
• australian australian
  • native native
• antarctica antarctica
  • native native
• oceanic islands oceanic islands
  • native native
• arctic ocean arctic ocean
  • native native
• indian ocean
  • native native
• atlantic ocean atlantic ocean
  • native native
• pacific ocean pacific ocean
  • native native
• mediterranean sea
  • native native

• Other Geographic Terms
• cosmopolitan cosmopolitan

Habitat

Kinorhynchs are generally found interstitially in upper, well oxygenated sediment layers. They may also be found on sandy beaches, in algae, or associated with other invertebrates such as sponges, hydroids and ectroprocts.

• Habitat Regions
• temperate temperate
• tropical tropical
• polar polar
• saltwater or marine saltwater or marine

• Aquatic Biomes
• benthic benthic
• reef reef
• coastal coastal
• abyssal abyssal
• brackish water brackish water

Systematic and Taxonomic History

The evolutionary relationships within phylum Kinorhyncha are still poorly resolved. Kinorhynchs are thought to be most closely related to loriciferans and priapulids , with whom they form superphylum Scalidophora . The inter-relationships of these phyla are still a matter of debate (although the majority of evidence points to a sister relationship between kinorhynchs and priapulids), as is the membership of this clade in the group Ecdysozoa (molting animals). The latter ambiguity is due to inadequate sampling of scalidophoran species in molecular examinations of possible ecdysozoans, as well as the lack of evidence that ecdysteroid hormones induce molting in this and other groups.

Some authors have additionally included nematomorphans as evolutionary relatives to scalidophoran phyla, in a clade called Cephaloryncha . Genetic evidence has provided support for a sister relationship between loriciferans and nematomorphans, which would render Scalidophora paraphyletic and support the future use of the name Cephaloryncha , as modern taxonomic systems of classification endeavor to give names only to monophyletic groups.

Physical Description

Kinorhynchs are yellowish brown and typically range from 0.13 to 1.04 mm, with bodies that can be divided into 13 zonites (segments). The first segment is the head, with a retractable oral cone, surrounded by anteriorly directed oral stylets (9 outer stylets and 3 to 4 rings of inner stylets, each ring with 5 stylets) and a mouth. Behind the oral cone are 5 to 7 rings of posteriorly directed scalids (sensory/locomotive spines). Each ring is made up of 10 to 20 circles, and each circle has 54 to 93 scalids. The neck is the second zonite, made of up to 16 placids, which can fold over the head when it is retracted. The remaining 11 zonites make up the animal’s trunk; each of these zonites is covered with a tergal (dorsal) and pair of sternal (ventral) plates, comprised of a chitinous basal layer and a membrane-like epicuticle. These form a rigid exoskeleton; articulation between the zonites as well as a flexible cuticle between the dorsal and ventral plates makes movement possible. There are no external cilia, but the entire body (including the organs) is lined in a thin cuticle with a chitinous basal layer. Some species are somewhat sexually dimorphic; males have outer spicules associated with their gonads.

Underneath the tergal and sternal plates, kinorhynchs have an epidermis which is non-ciliated and closely associated with the nervous system. Bands of dorsolateral and ventrolateral intersegmental, cross-striated muscles attach to the cuticle; strong fibers connect the sternal and tergal plates. Kinorhynchs have head retractor muscles as well as head cone retractor muscles, stretching between the base of the head scalids/mouth cone and posterior trunk segments. Additionally, there are circular muscles associated with the neck and mouth cone placids, but no circular body wall muscles. These animals are pseudocoelemates, and have a spacious anterior body cavity to allow for head retraction, their digestive and reproductive organs exist within blastocoelomic spaces.

The mouth leads into a buccal cavity that is followed by a sucking pharynx and esophagus, all of which are lined with cuticle. The esophagus is connected to a long, straight midgut, which is made of over 95% epithelial cells; these are interlocked and presumptively absorb nutrients. The midgut leads to a short hindgut, also lined with cuticle, terminating in an anus on the last zonite. Kinorhynchs excrete wastes via a pair of protonephridia located in the 8th and 9th trunk zonites, dorsolateral to the gut. These open through pores in the body cuticle and may have ciliate cells and/or microvilli. Gas diffusion takes place across the body wall and circulates by diffusion through the body cavity.

• Other Physical Features
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike
• sexes shaped differently

Development

Each ovary produces one large oocyte, fertilization is assumed to occur internally. Fertilized eggs are deposited in egg cases and embryos undergo direct development to juvenile stages, emerging with 11 of their 13 body segments fully formed. The final two segments, as well as a number of head scalids, are added during a series of six molts, as kinorhynchs progress to adulthood. Molting has only been observed in adults of two species.

Reproduction

Both males and females have paired gonads and short gonoducts. Males have two or three hollow, often rigid, spicules associated with their gonopores. When present, placement of these spicules varies from species to species. Females have both germ cells and nutritive cells in their gonads, and oviducts with seminal receptacles; their gonopores open between the 12th and 13th zonites. Females have been found with a brownish mucosal mass comprised of spermatozoa and spermatids, which is considered to be a spermatophore surrounding their posteriors. Mature spermatozoa may be as large as 1/4 of the total body length of the adult male. Mating has only been observed in one species, Pycnophyes kielensis . In this case, a male and female positioned themselves so their ventral posterior ends faced each other, with the mucosal mass surrounding their posterior ends.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Kinorhynchs are dioecious but males and females are generally indistinguishable visually. It is assumed that they can reproduce year-round and there is some evidence that 3 to 6 eggs are released at a time.

• Key Reproductive Features
• iteroparous iteroparous
• year-round breeding year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal
• oviparous oviparous

Little is known about egg production and early development in this group. Females provide yolk and an egg case for their eggs. Further parental investment is unknown.

• Parental Investment
• pre-hatching/birth
  • provisioning
    • female

Lifespan/Longevity

There is currently no information available regarding the typical lifespan of kinorhynchs .

Behavior

Kinorhynchs cannot swim. They move by extending the head and using anterior scalids to move the body forward to burrow in sediment or move through the water. Although they can sometimes be found in relatively high densities (up to 45 animals per 10 cm^2), these animals are considered solitary.

• Key Behaviors
• diurnal diurnal
• nocturnal nocturnal
• crepuscular crepuscular
• motile motile
• sedentary sedentary
• solitary solitary

Communication and Perception

These animals have simple nervous systems, with a series of ten ganglia connected and arranged in a ring around the pharynx. The epithelium is highly innervated. Scalids contain up to 10 monociliary sensory cells, which are connected to an exterior distal pore; these are likely chemo-and mechanoreceptive. All species in this phylum have species-specific patterns of sensory spots on their trunks, consisting of one or two pores surrounded by micropapillae. These sensory spots may have a cilium. Microvillar eyespots may be located on the pharyngeal nerve ring and some species have red-pigmented, light sensitive ocelli behind the mouth cone.

• Communication Channels
• tactile tactile
• chemical chemical

• Perception Channels
• visual visual
• tactile tactile
• chemical chemical

Food Habits

Kinorhynchs are likely direct deposit feeders, consuming benthic diatoms and organic material, including bacteria and other unicellular algae in the substrate. Some species have longitudinal cuticular rods, which may aid in filtering bacteria. Those that feed on diatoms may collect the algae on their head scalids to be ingested later, or directly suck them in through the pharynx; the pharynx may be protruded from the mouth for this purpose by pharynx protractor muscles. In these species, the oral stylets may break up the diatom's frustules (cell walls).

• Primary Diet
• planktivore planktivore
• detritivore detritivore

Predation

Kinorhynchs are likely included in the diets of most large marine invertebrates such as shrimp, snails, and other benthic feeders.

Ecosystem Roles

As meiobenthic animals, kinorhynchs are an important link in the food chain as prey for larger animals. Kinorhynchs may be parasitized by fungi, and some species may be commensal with bryozoans, hydrozoans, and sponges. They have been found in the gill chambers of intertidal bivalves, but the nature of this relationship is unclear.

Economic Importance for Humans: Positive

There are no known positive effects of kinorhynchs on humans beyond the place they occupy in the marine food chain and their use in scientific research.

• Positive Impacts
• research and education

Economic Importance for Humans: Negative

There are no known adverse effects of kinorhynchs on humans.

Conservation Status

As a broadly cosmopolitan phylum, kinorhynchs in general are not considered endangered or threatened, nor are there any conservation efforts directed at any individual species.

Additional Links

Encyclopedia of Life

Contributors

Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Texas State University.

References

Aleshin, V., I. Milyutina, O. Kedrova, N. Vladychenskaya, N. Petrov. 1998. Phylogeny of Nematoda and Cephalorhyncha derived from 18S rDNA. Journal of Molecular Evolution , 47: 597-605.

Brusca, R., G. Brusca. 1990. Invertebrates (2nd Edition) . Sunderland, MA: Sinauer Associates.

Halanych, K. 2004. The new view of animal phylogeny. Annual Review of Ecology, Evolution, and Systematics , 35: 229-256.

Higgins, R. 1982. "Phylum Kinorhycha or Echinodera " (On-line). Online Zoologists. Accessed March 15, 2013 at http://www.onlinezoologists.com/topics/taxa/kinorhyncha.html .

Higgins, R. 1978. Echinoderes gerardi n.sp. and E. reidli ( Kinorhyncha ) from the Gulf of Tunis. Transactions of the American Microscopical Society , 97/2: 171-180.

Higgins, R. 2007. Kinorhyncha , Loricifera , and Priapulida . Pp. 267-273 in The Light and Smith manual: Intertidal Invertebrates from Central California to Oregon, 4th Ed . Berkeley, CA: University of California Press.

Manylov, O. 1999. First finding of a microsporidian parasite in the gastrotrich, Turbanella lutheri ( Gastrotricha : Macrodasyida ). Protistology , 1: 17-19. Accessed March 15, 2013 at http://protistology.ifmo.ru/num1_1/manylov.pdf .

Margulis, L., M. Chapman. 2009. Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth . Boston, MA: Academic Press.

Nehaus, B. 2012. " Kinorhyncha " (On-line). World Register of Marine Species. Accessed February 23, 2013 at http://www.marinespecies.org/ .

Neuhas, B., R. Higgins. 2002. Ultrastructure, biology, and phylogenetic relationships of Kinorhyncha . Integrative and Comparative Biology , 42/3: 619-632.

Ruppert, E., R. Fox, R. Barnes. 2004. Invertebrate zoology: A functional evolutionary approach (7th Edition) . Belmont, CA: Thomson-Brooks/Cole.

Schmidt-Rhaesa, A. 2012. Handbook of Zoology: Nematomorpha , Pripulida , Kinorhyncha , Loricifera . Hamburg, Germany: de Gruyter.

Sørensen, M., M. Hebsgaard, I. Heiner, H. Glenner, E. Willerslev, R. Kristensen. 2008. New data from an enigmatic phylum: evidence from molecular sequence data supports a sister-group relationship between Loricifera and Nematomorpha . Journal of Zoological Systematics and Evolutionary Research , 46/3: 231-239.

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