Atlantic Thyroscyphidae and Sertariidae (Hydrozoa, Cnidaria) collected during the CANCAP and Mauritania-II expeditions of the National Museum of Natural History, Leiden, The Netherlands CANCAP-project. Contributions, no. 120

Twenty-three species, subspecies or varieties of hydroids of the families Thyroscyphidae and Sertulariidae (Leptolida, Hydrozoa, Cnidaria) are described and figured, originating from north-eastern Atlantic localities visited during the CANCAP and Mauritania-II Expeditions of the Rijksmuseum van Natuurlijke Historie (now Nationaal Natuurhistorisch Museum) in the years 1976-1988 and covering waters around Azores, Canary Islands, Madeira, Cape Verde Islands and the Atlantic off Morocco and Mauritania. Besides notes on the variability of the various species in the large material available, vertical and horizontal distributions are discussed. Observations on the considerable material of Sertularella ellisii (Deshayes & Milne-Edwards, 1836) indicate that this species cannot be separated from Sertularella fusiformis (Hincks, 1861); both species are consequently merged under the oldest available name: Sertularella ellisii (Milne Edwards & Haime, 1836). Also included are the European records of Sertularella gaudichaudi Auct., not to be confused with Sertularella gaudichaudi (Lamouroux, 1824) from the Falkland Islands. Sertularella unituba Calder, 1991, is recorded from many localities in the Cape Verde Islands region; these records represent the first from the eastern Atlantic.