Kinorhyncha is exclusively marine, holobenthic, free-living, meiofaunal species within a size range of 0.13 - 1.04 mm body length [1]. Their distributions are global, and are found in all marine habitats, often reported to have a high tolerance towards variations in sediment size, salinity and hypoxia levels [2]. The first Kinorhyncha species was described 165 years ago by Dujardin [3]. However, the knowledge of these animals is far from complete due to the fact that very small communities of researchers are interested in this phylum (currently less than few tens of researchers are working in this field, worldwide). Studies of Zelinka [4] [5] [6] [7] [8] focusing on Kinorhynch systematics and his monograph “Monographie der Echinodera” [9] completely revised the “Systematic Systems” of Reinhard [10] [11], and described 12 species from adult stages (in addition to four species described earlier), 34 species from juvenile stages, and for the first time provided information on all aspects of the morphology and biology of this taxon [9]. 20th century studies of Higgins have been considered to be a major step forward in the taxonomic studies of Kinorhyncha. Studies of Higgins and Co-workers after 1960 described 60 new species, re-described 14 species, and established six new genera [12] - [41]. During the last decade, several new species were described by various researchers [42] - [81]. Further, the existing literature says that Sørensen [82] listed 205 species of Kinorhyncha described from adult specimens, whereas Neuhaus [1] listed 191 species of Kinorhyncha described from adult specimens, and 50 species of juvenile. However, molecular phylogenetic studies by Yamasaki et al. [83] and Sørensen et al. [84], and description of new species from various geographical location warrant that the periodical revision of the Kinorhyncha classification is essential to make the taxonomical work in a single floor, for the clear-cut discussion. An attempt has been made on this line and their results are discussed herewith.

This review was carried out from the existing published results. The observed species are ordered alphabetically and grouped based on the taxonomic status under the following categories.

1) A Valid species (described from adult stage).

2) Nomina dubia (described only from Juvenile stage).

3) Taxon inquirendum (identity is uncertain or disputed by different experts).

A species known only from juvenile stages are generally regarded as nomina dubia, but names are available and valid according to the International Code of Zoological Nomenclature, which states that a new species may be described from any life history stage [85]. All taxon names were revised to employ currently accepted species names following WoRMS database [86].

At present the phylum Kinorhyncha comprises of 271 species belonging to 30 genera and 13 families. The families are distributed under three orders, Echinorhagata Sørensen et al. 2015, Kentrorhagata Sørensen et al. 2015, Xenosomata Zelinka, 1907. Classification and list of species of Kinorhycha are given below.

Class: Allomalorhagida Sørensen et al. 2015

Family: Dracoderidae Higgins & Shirayama, 1990

Genus: Dracoderes Higgins & Shirayama, 1990

Family: Franciscideridae Sørensen et al. 2015

Genus: Franciscideres Zotto et al. 2013

Family: Pycnophyidae Zelinka, 1896

Genus: Pycnophyes Zelinka, 1907

Genus: Leiocanthus Sánchez et al. 2016

Genus: Cristaphyes Sánchez et al. 2016

Genus: Higginsium Sánchez et al. 2016

Genus: Krakenella Sánchez et al. 2016

Genus: Setaphyes Sánchez et al. 2016

Genus: Fujuriphyes Sánchez et al. 2016

Family: Neocentrophyidae Higgins, 1983

Genus: Mixtophyes Sánchez et al. 2014

Genus: Neocentrophyes Higgins, 1969

Genus: Paracentrophyes Higgins, 1983

Class: Cyclorhagida Sørensen et al. 2015

Order: Echinorhagata Sørensen et al. 2015

Family: Echinoderidae Bütschli, 1876

Genus: Cephalorhyncha Adrianov, 1999

Genus: Echinoderes Claparède, 1863

Genus: Fissuroderes Neuhaus & Blasche, 2006

Genus: Meristoderes Herranz et al. 2012

Genus: Polacanthoderes Sørensen, 2008

Order: Kentrorhagata Sørensen et al. 2015

Family: Antygomonidae Adrianov & Malakhov, 1994

Genus: Antygomonas Nebelsick, 1990

Family: Cateriidae Gerlach, 1950

Genus: Cateria Gerlach, 1956

Family: Centroderidae Zelinka, 1896

Genus: Centroderes Zelinka, 1907

Genus: Condyloderes Higgins, 1969

Family: Semnoderidae Remane, 1929

Genus: Semnoderes Zelinka, 1907

Genus: Sphenoderes Higgins, 1969

Genus: Parasemnoderes Adrianov & Maiorova 2018

Family: Zelinkaderidae Higgins, 1990

Genus: Triodontoderes Sørensen & Rho, 2009

Genus: Zelinkaderes Higgins, 1990

Family: incertae sedis

Genus: Tubulideres Sørensen et al. 2007

Family: incertae sedis

Genus: Wollunquaderes Sørensen & Thormar, 2010

Order: Xenosomata Zelinka, 1907

Family: Campyloderidae Remane, 1929

Genus: Campyloderes Zelinka, 1907

Genus: Ryuguderes Yamasaki, 2016

Based on the literature collected, the present study reports 271 species valid species. Among these species, 82 species were added during the latter period of 2013 and was not included in the WoRMS data base (Table 1).

Before this study, Nehaus [1] was prepared the checklist for Kinorhyncha which exhibited only 189 species. The present study included another 82 species and designated as a newly described species. Besides, Nehaus [1] classified 4 species (Echinoderes druxi, E. masudai, E. steineri and Pycnophyes echinoderoides) as valid, although they are regarded as a nomen dubium. However, these 4 species are also currently classified as taxon inquirendum by WoRMS [86] due to its taxonomical uncertainty and described in the present work. Based on the above consideration, now the Kinorhyncha has 271speceis as valid species, 51 species under the category of nomendubium and 4 species as taxon inquirendum. Table 2, exhibited the nomendubium as discussed by the Nehaus [1] for 50 species and WoRMS [86] described 55 species.

According to WoRMS [86] data base and Nehaus [1], a total of 189 Species and 191 Species were reported, respectively (Table 3). E. druxi d’Hondt, 1973.E. masudai Abe, 1930, E. steineri (Chitwood, 1951), P. calmani Southern, 1914, P. echinoderoides Zelinka, 1928, and P. oshoroensis Yamasaki et al. 2012 were named to be valid. However, according to WoRMS [86], out of these 6 species, 4 species namely E. druxi, E. masudai, E. steineri and P. echinoderoides were termed as taxon inquirendum. Whereas, the status of P. calmani is unaccepted, and P. oshoroensis Yamasaki et al. [87], is not recorded.

Based on the data presented in Table 3 and Table 4, species such as Campyloderes vanhoeffeni var. kerguelensis Zelinka, 1913, Centroderes eisigii Zelinka, 1928, E. augustae Sørensen & Landers, 2014 and E. skipperae Sørensen & Landers, 2014 are valid according to WoRMS [86], however, not added in the works of Nehaus [1]. Of these, 2 species, namely E. augustae Sørensen & Landers, 2014, and E. skipperae Sørensen & Landers 2014, were identified after 2014, i.e., after the works of Nehaus [1].

Despite the fact that the distribution of Kinorhynca is widespread across the globe, research activities (more precisely taxonomy) are exclusively limited to few countries. Only few literatures are available from tropical shallow-water ecosystems. Though taxonomical studies have started long ago, it has received the attention of the researchers in the past two decades only. Here are a few of the notable taxonomical works given that are done across the globe.

Both Murakami et al. [89] and Adrianov et al. [92] have identified one species, each from Japan, which was followed by several other authors later, Yamasaki and Fujimoto [66] - (2Sp.); Yamasaki et al. [93] - (2 Sp.); Yamasaki [67] - (2Sp.), Yamasaki [73] - (1Sp.) and Sánchez and Yamasaki [94] - (2 Sp.). When compared to other countries, Korea adds to the species count with a total of 16 species in the recent years and proves to be active in the study of Kinorhyncha. Several authors have contributed to the identification of Kynorynchs like Thomsen et al. [61] - (1Sp.), Sørensen et al. [95] - (4Sp.), Sánchez et al. [60] - (4Sp.), Sørensen et al. [52] [96] - (4Sp.), Sørensen et al. [48] - (1Sp.), Sørensen et al. [47] - (1Sp.) and Altenburger et al. [68] - (1Sp.).

Spain has 8 species to its title Herranz et al. [55] - (1Sp.), Sørensen et al. [96] - (2Sp.) Sánchez et al. [53] - (2Sp.) and GaOrdóñez et al. [45] - (3Sp.). Australia, Lemburg [97] - (1Sp.), Sørensen and Thormar [51] - (1Sp.) and in Brazil, Zotto et al. [57] - (1Sp.), Sørensen and Landers [98] - (3Sp.) equally share 4 species each to his account.

Florida also has a near similar number of species identified such that of Spain, Herranz et al. [56] - (4Sp.), Sørensen et al. [43] - (2Sp.). Italy has the highest species identified across the globe, Zotto [69] - (1Sp.), Zotto and Todaro [99] - (36Sp.). Followed by Italy, the Iberian Peninsula has the highest Kinorynchs identified in the last two decades. Of this, the works of Sánchez et al. [53] - (1Sp.), Sánchez et al. [54] - (30Sp.), Sánchez et al. [62] [63] - (2Sp.) proves to be efficient with 33 identified species. Four species were identified in New Zealand by Neuhaus and Blasche [42]. Denmark has the least number of Kinorynch identified with a single species by Lundbye et al. [100].

The Panama sea exhibited 8 species [101] [102], followed by Mexico, Sørensen et al. [103] - (1Sp.); Sørensen and Landers [78] - (2Sp.) and Portugal [104] - (2Sp.).

Kinorhyncha is available from the intertidal zone to abyssal depths and from polar to tropical regions [1] [56] [95] [98] [105]. Still only a few dozen of literature is available from tropical shallow-water ecosystems. As far as India is concerned, works on Kinorhyncha remains scanty. The meiobenthic invertebrate group Cateria styx species has been reported from the east coast of India by Rao and Ganapati [106]. Ganapati and Rao [107] have identified species of Kinorhyncha present in Waltaire coast situated in Andhra Pradesh. Cateriagerlachi sp., were identified by Higgins [19] reported from the east coast of India. Pycnophyes sp., were collected from west coast of India by Dovgal et al. [108]. The specimen of Kinorhyncha family Pycnophyes sp., was collected from the west coast of India [108]. Although earlier work was carried out by Higgins [2] [21] in the Indian Ocean, the continuity of research was found to be negligible. According Vekatraman et al. [109] only nine species were reported in India till 1979. After 37 years, according to Jeeva and Mohan [110], works on Kinoryncha in India has taken a new shape with a first report of Echinoderes setiger (Greeff, 1869) from Andaman Islands (India). Thus, currently there are only 16 species (Table 4) that are identified in Indian waters.

Supplementary references where the species records are available given in [111] - [127].

The above list has a detailed status of all the species regarding the acceptance, non acceptance of species names and their acceptance as nomen dubium and taxon inquirendum in the last five years (i.e., after 2013), based on WoRMS [86] database and Nehaus [1]. This checklist record includes a total of 271 identified species out of which almost 82 species were published in articles. This research article could be useful for both present and future studies of Kinorhyncha, especially for controversial species status. Since, there is not much work on Kinorhyncha in India, this work could help initiate further research.

The authors thank Dr. Hiroshi Yamasaki (Japan) for his tireless help and suggestions. They also acknowledge the Head, Department of Ocean Studies and Marine Biology and authorities of Pondicherry University for providing the facilities to execute this project under the Ministry of Environment, Forest and Climatic Change (MoEFCC), New Delhi, through AICOPTAX funded programme.

The authors declare no conflicts of interest regarding the publication of this paper.

Jeeva, C., Mohan, P.M., Ragavan, P. and Muruganantham, V. (2020) A Review on Taxonomy of Phylum Kinorhyncha. Open Journal of Marine Science, 10, 260-294. https://doi.org/10.4236/ojms.2020.104020