also serve as parasites. Algal parasites such as Coccomyxa ophiurae cause spinal malformation. Unlike in sea stars and sea urchins, annelids are not typical parasites. Diversity and taxonomy Between and species of brittle stars are currently known, but the total number of modern species may be over . This makes brittle stars the most abundant group of current echinoderms (before sea stars). Around 270 genera are known, these are distributed in 16 families, which makes them at the same time a relatively poorly diversified group structurally, compared with the other echinoderms. For example, 467 species belong to the sole family of Amphiuridae (frail brittle stars which live buried in the sediment leaving only their arms in the stream to capture the plankton). There are also 344 species in the family of Ophiuridae. List of families according to the World Register of Marine Species, following O'Hara 2017: subclass Myophiuroidea Matsumoto, 1915 infra-class Metophiurida Matsumoto, 1913 super-order Euryophiurida O'Hara, Hugall, Thuy, Stöhr & Martynov, 2017 order Euryalida Lamarck, 1816 family Asteronychidae Ljungman, 1867 family Euryalidae Gray, 1840 family Gorgonocephalidae Ljungman, 1867 order Ophiurida Müller & Troschel, 1840 sensu O'Hara et al., 2017 suborder Ophiomusina O'Hara et al., 2017 family Ophiomusaidae (O'Hara, Stöhr, Hugall, Thuy, Martynov, 2018) family Ophiosphalmidae (O'Hara, Stöhr, Hugall, Thuy & Martynov, 2018) Ophiomusina incertae sedis suborder Ophiurina Müller & Troschel, 1840 sensu O'Hara et al., 2017 family Astrophiuridae Sladen, 1879 family Ophiopyrgidae Perrier, 1893 family Ophiuridae Müller & Troschel, 1840 Ophiurina incertae sedis Ophiurida incertae sedis super-order Ophintegrida O'Hara, Hugall, Thuy, Stöhr & Martynov, 2017 order Amphilepidida O'Hara, Hugall, Thuy, Stöhr & Martynov, 2017 suborder Gnathophiurina Matsumoto, 1915 super-family Amphiuroidea Ljungman, 1867 family Amphiuridae Ljungman, 1867 family Amphilepididae Matsumoto, 1915 super-family Ophiactoidea Ljungman, 1867 family Ophiactidae Matsumoto, 1915 family Ophiopholidae O'Hara, Stöhr, Hugall, Thuy & Martynov, 2018 family Ophiothamnidae O'Hara, Stöhr, Hugall, Thuy & Martynov, 2018 family Ophiotrichidae Ljungman, 1867 suborder Ophionereidina O'Hara, Hugall, Thuy, Stöhr & Martynov, 2017 super-family Ophiolepidoidea Ljungman, 1867 family Hemieuryalidae Verrill, 1899 family Ophiolepididae Ljungman, 1867 (restricted) super-family Ophionereidoidea Ljungman, 1867 family Amphilimnidae O'Hara, Stöhr, Hugall, Thuy & Martynov, 2018 family Ophionereididae Ljungman, 1867 suborder Ophiopsilina Matsumoto, 1915 super-family Ophiopsiloidea Matsumoto, 1915 family Ophiopsilidae Matsumoto, 1915 order Ophiacanthida O'Hara, Hugall, Thuy, Stöhr & Martynov, 2017 suborder Ophiacanthina O'Hara, Hugall, Thuy, Stöhr & Martynov, 2017 family Clarkcomidae O'Hara, Stöhr, Hugall, Thuy & Martynov, 2018 family Ophiacanthidae Ljungman, 1867 family Ophiobyrsidae Matsumoto, 1915 family Ophiocamacidae (O'Hara,