Calanoid copepods are ubiquitous in marine, brackish, and fresh waters, comprising 48 families, approximately 330 genera and 2000 species of marine and freshwater copepods inhabiting pelagic, benthic and benthopelagic environments (Huys and Boxshall, 1991); (Walter and Boxshall 2019). However, compared to the diversity of cave-dwelling Cyclopoida and Harpacticoida, stygobiotic Calanoida is less diverse in terms of numbers of species" (Fosshegen & Iliffe, 1989).Calanoids can be distinguished from other planktonic copepods by having first antennae at least half the length of the body and biramous second antennae. However, their most distinctive anatomical trait is the presence of a joint between the fifth and sixth body segments. The largest specimens reach 18 millimetres (0.71 in) long, but most do not exceed 0.5–2.0 mm (0.02–0.08 in) long..

Calanoid copepods are the dominant animals in the plankton in many parts of the world's oceans, making up 55%–95% of plankton samples. They are therefore important in many food webs, taking in energy from phytoplankton and algae and 'repackaging' it for consumption by higher trophic level predators. Many commercial fish are dependent on calanoid copepods for diet in either their larval or adult forms. Baleen whales such as bowhead whales, sei whales, right whales and fin whales rely substantially on calanoid copepods as a food source.

"Nine species are recognized stygobionts and three more are possible. In the last few years the number of described stygobiotic taxa has increased and may continue to do so, as caves and gravel-beds are more intensively studied. Stygobionts are probably derived from a Miocene fauna that, under the constraint of a worsening climate, took advantage of the development of caves and found subterranean refugia. The Pleistocene glaciations accelerated this colonization process, which persists to the present". (from Brancelj & Dumont, 2007)

Recently Bradford-Grieve & Boxshall, 2019, using cladistic analysis of the non-‘Bradfordian’ genera improved the classification of a superfamily Clausocalanoidea to include Aetideidae Giesbrecht, 1893, Clausocalanidae Giesbrecht, 1893 and Pseudocyclopiidae G. O. Sars, 1902

Ten superfamilies are recognized: Pseudocyclopoidea Giesbrecht, 1893; Epacteriscoidea Fosshagen, 1973; Augaptiloidea Sars, 1905; Centropagoidea Giesbrecht, 1893; Megacalanoidea Sewell, 1947; Bathypontioidea Brodsky, 1950; Eucalanoidea Giesbrecht, 1893; Ryocalanoidea Andronov, 1974; Clausocalanoidea Giesbrecht, 1893; and Spinocalanoidea Vervoort, 1951.

THE BIOLOGYOF CALANOID COPEPODS

ACARTIIDAE Sars G.O., 1903 AETIDEIDAE Giesbrecht, 1892 ARCTOKONSTANTINIDAE Markhaseva & Kosobokova, 2001 ARIETELLIDAE Sars G.O., 1902 AUGAPTILIDAE Sars G.O., 1905 BATHYPONTIDAE Brodsky, 1950 CALANIDAE Dana, 1849 CANDACIIDAE Giesbrecht, 1893 CENTROPAGIDAE Giesbrecht, 1893 CLAUSOCALANIDAE Giesbrecht, 1893 DIAIXIDAE Sars G.O., 1902 DIAPTOMIDAE Baird, 1850 DISCOIDAE Gordejeva, 1975 EPACTERISCIDAE Fosshagen, 1973 EUCALANIDAE Giesbrecht, 1893 EUCHAETIDAE Giesbrecht, 1893 FOSSHAGENIIDAE Suárez-Morales & Iliffe, 1996 HETERORHABDIDAE Sars G.O., 1902 HYPERBIONYCIDAE Ohtsuka, Roe & Boxshall, 1993 METRIDINIDAE Sars G.O., 1902 NULLOSETIGERIDAE Soh, Ohtsuka, Imabayashi & Suh, 1999 ROSTROCALANIDAE Markhaseva, Schulz & Martínez Arbizu, 2009 RYOCALANIDAEAndronov, 1974 SCOLECITRICHIDAE Giesbrecht, 1893 SPINOCALANIDAE Vervoort, 1951

STEPHIDAE Sars G.O., 1902 SUBEUCALANIDAE Giesbrecht, 1893 KYPHOCALANIDAE Markhaseva & Schulz, 2009 LUCICUTIIDAE Sars G.O., 1902 MEGACALANIDAE Sewell, 1947 MESAIOKERATIDAE Matthews, 1961 PARAPONTELLIDAE Giesbrecht, 1893 PARKIIDAE Ferrari & Markaseva, 1996 PHAENNIDAE Sars G.O., 1902 PONTELLIDAE Dana, 1852 PSEUDOCYCLOPIDAE Giesbrecht, 1893 PSEUDOCYCLOPIIDAE Sars G.O., 1902 PSEUDODIAPTOMIDAE Sars G.O., 1902 RHINCALANIDAE Geletin, 1976 SULCANIDAE Nicholls, 1945 TEMORIDAE Giesbrecht, 1893 THARYBIDAE Sars G.O., 1902 TORTANIDAE Sars G.O., 1902 PARACALANIDAE Giesbrecht, 1893

Boholiniidae Fosshagen & Iliffe, 1989 syn. Pseudocyclopidae Giesbrecht, 1893; Calocalanidae Bernard, 1958 accepted as Paracalanidae Giesbrecht, 1893; Mecynoceridae Andronov, 1973 syn. Paracalanidae Giesbrecht, 1893; Phyllopodidae Brodsky, 1950 syn. Nullosetigeridae Soh, Ohtsuka, Imabayashi & Suh, 1999; Pseudocalanidae Giesbrecht, 1895 syn. Clausocalanidae Giesbrecht, 1893;Ridgewayiidae Wilson M.S., 1958 syn. Pseudocyclopidae Giesbrecht, 1893

PSEUDOCYCLOPOIDEA Bradford & Grieve, 2014

PSEUDOCYCLOPIIDAE Sars G.O., 1902

STYGOCYCLOPIA Jaume & Boxshall,1995 Stygocyclopia australis Jaume, Boxshall & Humphreys, 2001 [Australia; coastal karst waters] (left) Stygocyclopia balearica Jaume & Boxshall, 1995 [Balearic Islands;coastal karst waters] Stygocyclopia badinoi Belmonte, 2022 [anchialine; Zinzulusa cave, south Italy] Stygocyclopia philippensis Jaume, Fosshagen & Iliffe, 1999 [Panglao Island, Philippines; anchialine cave] Stygocyclopia badinoi Belmonte,2022 [Zinzulusa cave; southItaly; anchialine]

FRIGOCALANUS Schulz, 1996

Frigocalanus rauscherti Schulz, 1996

PSEUDCYCLOPIA T. Scott, 1892