Harpacticoid copepods are primarily benthic organisms, which adapted to several kinds of fresh and marine environments, ground waters included. A few are planktonic or live in association with other organisms. In marine sediments, after the nematods, they represent the most aboundant meiofaunal group. Harpacticoids may be found also in semi-terrestrial habitats, such as damp moss and leaf-litter.

Presently the harpacticoid copepods are widespred across the world, some genera showing a cosmopolitan distribution, other ones are restricted to one continent or exibit more limited geonemy.

Most fresh water harpacticoids belong to the families Ameiridae, Parastenocarididae and Canthocamptidae with a great amount of subterranean species.

The order Harpacticoida contains over 3.600 species and subspecies belonging to 463 genera and 54 families; however, "the great majority of species is assumed to be still unknown, mainly because large regions of the earth (e.g. polar regions, Pacific, deep sea) have not yet been extensively sampled" (Willen, 2000).

"Thousands of species remain to be discovered and described. For example, an estimation of the number of species of Harpacticoida in the deep-sea regions quickly leads to a five-digit number" (Seifried, 2003)

A family key has been recently published by Huys et al. (1996), who pointed out that "the familial classification of the Harpacticoida is also in a state of flux and many changes are likely to occur in the near future".

Seifried & Schminke (2003) introduced remarkable changes and additions to harpacticoid systematics as follows:

Aegesthidae is senior synonym of Cervinidae, and Aegisthoidea is senior synonym of Cervinoidea

Aegisthidae enclose Aegisthinae Giesbrecht, 1892, Cervinidae Sars, 1903 and "Cervinopsinae" Brodskaya, 1963

Aegisthoidea enclose only Rometidae; Styracothoracidae and Rotundiclipeidae are enclosed in Syngnatharthra tax. nov.

Syngnatharthra tax. nov. enclose all Oligoarthra except Aegisthoidea, which is the sister group of Syngnatharthra

Maxillipedasphalea Lang, 1944 is polyphyletic and not maintained.

The following families: Chappuisiidae Chappuis, 1940; Darcythompsoniidae Lang,1936; Ectinosomatidae Sars, 1903; Neobradyidae Olofsson 1917 and Phyllognathopodidae Gurney 1932 are integrated in Syngnatharthra.

At the same time, Seifried (2003) introduced some changes and additions to harpacticoid systematics as follows:

Brotskayaia Huys, Møbjerg & Kristensen, 1997 was synonymized with Expansicervinia Montagna, 1981 (Aegisthidae Giesbrecht, 1892).

Neocervinia Huys, Møbjerg & Kristensen, 1997 and Pseudocervinia Brodskaya, 1963, were synonymized with Cervinia Norman, 1878 (Aegisthidae).

"Neobradyoidea" Olofsson, 1917 (Chappuisiidae Chappuis, 1940, Darcythompsoniidae Lang, 1936, Neobradyidae Olofsson, 1917, Phyllognathopodidae Gurney, 1932) is polyphyletic and therefore not maintained.

Chappuisiidae is part of Syngnatharthra Seifried & Schminke (2003). Darcythompsoniidae and Phyllognathopodidae were transferred to Podogennonta Lang, 1944.

Paramesochra australis Mielke, 1994 belongs to Ameiridae Monard, 1927 (Podogennonta) as Psammoleptomesochra australis Mielke, 1994.

Ectinosomatoidea Sars, 1903 was synonymized with Ectinosomatidae Sars, 1903, as both taxa enclose the same species.

The monophyly of Exanechentera Lang, 1944 was confirmed. Thompsonulidae Lang, 1944 was excluded from Exanechentera and was transferred to Podogennonta. Novocriniidae Huys & Iliffe, 1998, Paramesochridae Lang, 1944, Rotundiclipeidae Huys, 1988, and Superornatiremidae Huys, 1996 were integrated in Exanechentera.

Idyanthidimorpha Seifried, 2003 enclose Zosimidae Seifried, 2003 and Idyanthidae Lang, 1944.

Lang (1944) established Idyanthinae. Idyanthinae was excluded from Tisbidae Stebbing, 1910 sensu strictu and was raised to family rank.

Tachidiopsis Sars, 1911 was excluded from Idyanthidae and was transferred to Neobradyidae. Tachidiopsis bozici Bodin, 1968, T. ibericus Becker, 1974, T. laubieri Dinet, 1974, T. parasimilis Dinet, 1974, and T. sarsi Bodin, 1968 were moved to Marsteinia Drzycimski, 1968.

Styracothoracidae Huys, 1993 was synonymized with Idyanthidae in Moura & Martínez Arbizu, 2003).

Neoscutellidium Zwerner, 1967 was excluded from Idyanthidae and is integrated in Cholidyinae Boxshall, 1979 (Tisbidae sensu strictu).

Zosime Boeck, 1872, Peresime Dinet, 1974, and Pseudozosime Scott, 1912 were excluded from Idyanthidae and combined in Zosimidae Seifried, 2003.

Dactylopia Becker, 1974 together with Idyanthe Sars, 1909, Idyella Sars, 1906, Idyellopsis Lang, 1944, Styracothorax Huys, 1993, and Tachidiella Sars, 1909 represent the taxon Idyanthidae.

Idyanthopsis psammophila Bocquet & Bozic, 1955 belongs to Paramesochridae as Diarthrodella psammophila (Bocquet & Bozic, 1955).

"Tachidioidea" Sars, 1909 is polyphyletic and therefore not maintained, as Harpacticidae Sars, 1904 was transferred to Podogennonta by Willen (2000).

The monotypic Euterpinidae Brian, 1921 is synonymized with Tachidiidae Sars, 1909, as Euterpina acutifrons (Dana, 1848) belongs to Tachidiidae.

Novocriniidimorpha Seifried, 2003 (Novocriniidae -Superornatiremidae - Rotundiclipeidae) and Tisboidea Stebbing, 1910 (Peltidiidae Sars, 1904 - Tegastidae Sars, 1904 - Porcellidiidae Boeck, 1865 - Tisbidae sensu strictu) represent the taxon Palinarthra Seifried, 2003.

Clytemnestridae A. Scott, 1909 was synonymized with Peltidiidae. The species of Clytemnestra Dana, 1847 and Goniopsyllus Huys & Conroy-Dalton, 2000 belong to an advanced taxon within Peltidiidae (Seifried, 2003; Seifried, in litt.) [PDF-SUMMARY].

FAMILIES LIST (after Huys & Boxshall, 1991; Huys et al., 1996; Bodin, 1988,1997; Willen, 1999, 2000; Seifried & Schminke, 2003; Seifried, 2003: Boxshall & Halsey, 2004)

Adenopleurellidae Huys 1990, Aegisthidae Giesbrecht 1892, Ameiridae Monard 192, Ancorabolidae Sars 1909, Arenopontiidae Martinez Arbizu & Moura 1994, Argestidae Por 1986, Balaenophilidae Sars 1910 ****, Cancrincolidae Fiers 1990, Canthocamptidae Brady 1880, Canuellidae Lang 1944, Cerviniidae Sars 1903, Chappuisiidae Chappuis 1940, Cletodidae T. Scott 1904, Cletopsyllidae Huys & Lee, 1998/99 ***, Clytemnestridae A. Scott, 1909, Cristacoxidae Huys 1990, Cylindropsyllidae Sars 1909 *, Dactylopusiidae Lang, 1936, Danielsseniidae Huys & Gee 1996, Darcythompsoniidae Lang 1936, Diosaccidae Sars 1906, Ectinosomatidae Sars 1903, Euterpinidae Brian 1921, Hamondiidae Huys 1990, Harpacticidae Dana 1846, Heteropsyllidae Korney & Chertoprud, 2008, Huntemanniidae Por 1986, Idyanthidae Lang 1944, Laophontidae T. Scott 1904, Laophontopsidae Huys & Willems 1989, Latiremidae Boric 1969, Leptastacidae Lang 1948, Leptopontiidae Lang, 1948 sensu Martinez Arbizu & Moura, 1994, Longipediidae Brady 1880 , Louriniidae Monard 1927, Metidae Boeck 1872, Miraciidae Dana 1846, Neobradyidae Oloffson 1917, Normanellidae Lang, 1944 sensu Huys & Willems, 1989, Novocriniidae Huys & Iliffe, 1998, Orthopsyllidae Huys 1990 **, Paramesochridae Lang 1944, Parastenheliidae Lang 1936, Parastenocarididae Chappuis 1933, Peltidiidae Claus 1860, Phyllognathopodidae Gurney 1932, Porcellidiidae Brady 1880, Pseudotachiidae Lang, 1936, Rhynchothalestridae Lang, 1948, Rhizothricidae Por 1986, Rometidae Seifried & Schminke 2003, Rotundiclipeidae Huys 1988, Styracothoracidae Huys 1993 [recently moved to Idyantidae Lang 1944 (Moura & Martinez Arizu, 2003)], Superornatiremidae Huys, 1998, Tachidiidae Boeck 1864, Thalestridae Sars, 1905, Tegastidae Sars, 1904, Tetragonicipitidae Lang, 1944, Thompsonulidae Lang, 1944, Tisbidae Stebbing, 1910, Zosimidae Seifried 2003

* Martinez Arbizu & Moura (1994) considered the Cylindropsillidae as polyphyletic and desolved it, incorporating the Cylindropsyllinae in the family Canthocamptidae as well as raising to family level the subfamily Leptopontiinae.

More recently Recently Huys & Conroy-Dalton (2006) described 3 new species of the genus Evansula, including them in the re-established family Cylindropsillidae: Evansula cumbaensis Huys & Conroy-Dalton, 2006; Evansula polaris Huys & Conroy-Dalton, 2006; Evansula spinosa Huys & Conroy-Dalton, 2006.

** Huys (1990) created the family Orthopsyllidae, including the genera Orthopsyllus (removed from Canthocamptidae) and two new genera (Dionyx, Infrapedia). .

*** Huys & Lee (1998/99) created the family Cletopsyllidae to accomodate the genus Cletopsyllus Willey, 1935 (once included in the family Normanellidae Lang, 1944, subfamily Cletopsyllinae Huys & Willems, 1989) and three new genera, Retrocalcar, Bathycletopsyllus and Isocletopsyllus.

**** Most probably this family could belong to the Diosaccidae (Willen, 2000).

The family name Atergopediidae created by Martinez Arbizu & Moura (1997) is a junior synonym of the Novocriniidae, Atergopedia being the secon valid genus in the family.

The family name Paranannopidae Por, 1986 is as well invalid because it is based on the unavailable generic name Paranannopus Lang, 1936, and recently Willen (1999, 2000) integrated it into the family Pseudotachidiidae.

In the same occasion, the author pointed out a general review of the Thalestridimorpha Lang, 1948 raising the Pseudotachiidae Lang, 1936 to family rank, removing it from the family Thalestridae Sars, 1905 to which they had been previously assigned as a subfamily. In the same review the author reunited the families Ambunguipedidae Huys, 1990 and Hamondiidae Huys, 1990 with the family Rhynchothalestridae, and included the family Danielsseniidae Huys & Gee, 1996 in the Pseudotachidiidae, as well as subdivided the latter family into four subfamilies, viz. Paranannopinae, Pseudomesochrinae, Donsiellinae and Pseudotachidiinae.

FAMILIES AND GENERA AND SPECIES INCERTAE SEDIS : Ismardiidae Leigh-Sharpe, 1936 , Flavia Brady, 1899 ; Dactylopina Brady, 1910; Mawsonella Brady, 1918; Tisemus Monard, 1928; Viguierella coeca Ziegelmayer., 1923