Oithona Baird, 1843

Oithonids  are abundant cyclopoid copepods that  are common  in marine  planktonic communities  worldwide.  They  occur  over  a  wide latitudinal range in coastal  and offshore waters, both in epipelagic and mesopelagic zones of the open ocean, as well as  in brackish areas.  Most  species  are  less  than  1  mm  in  body  length  and  are most efficiently captured by small-mesh nets (< 200 μm). This leaflet presents the taxonomic and ecological characteristics of 14 Oithona species that are widely distributed in  the North  Atlantic  and  the  Mediterranean  Sea: O.  atlantica, O.  brevicornis, O.davisae, O. colcarva, O.  decipiens, O.  longispina, O.  nana, O.  parvula, O.  plumifera, O.  robusta, O. setigera, O.similis-group, O. tenuis,andO. vivida.The family Oithonidae is also briefly presented, together with ataxonomic key to distinguish among the three genera that belong  to  it. This  work  is largely  based  on  the  paper  by  Nishida  (1985),the  book  by Boxshall and Halsey (2004), and the extensive information provided by the website on marine planktonic copepods by Razouls et al. (2005–2018). This is a new leaflet.