• “Montereina” concinna (Alder & Hancock, 1864) represented as Discodoris concinna (Alder & Hancock, 1864)
• “Montereina” erubescens (Bergh, 1884) represented as Discodoris erubescens Bergh, 1884
• “Montereina” punctifera (Abraham, 1877) represented as Peltodoris punctifera (Abraham, 1877)
• Aaptos aaptos (Schmidt, 1864)
• Aaptos papillata (Keller, 1880)
• Aartsenia candida (Møller, 1842)
• Abaconia naufraga (Clench, 1938) accepted as Litiopa melanostoma Rang, 1829
• Abantennarius nummifer (Cuvier, 1817)
• Abarenicola claparedei (Levinsen, 1884) accepted as Abarenicola claparedi (Levinsen, 1884)
• Abarenicola claparedi (Levinsen, 1884)
• Abarenicola pusilla (Quatrefages, 1866)
• Abavopsis latosoleata (Salvini-Plawen, 1973)
• Abditoconus anselli (Harvey, 1996)
• Abditoconus brava (Romano, Pérez-Portela & D. Martin, 2014)
• Abditodentrix rhomboidalis (Millett, 1899) accepted as Tortoplectella rhomboidalis (Millett, 1899)
• Abedinium dasypus (Cachon & Cachon-Enjumet) Loeblich Jr & Loeblich III, 1966
• Abietinaria abietina (Linnaeus, 1758)
• Abietinaria filicula (Ellis & Solander, 1786)
• Abietinaria fusca (Johnston, 1847)
• Abietinaria interversa (Pictet & Bedot, 1900)
• Abietinaria pulchra (Nutting, 1904)
• Abludomelita aculeata (Chevreux, 1911)
• Abludomelita gladiosa (Spence Bate & Westwood, 1862)
• Abludomelita obtusata (Montagu, 1813)
• Abludomelita richardi (Chevreux, 1900)
• Abollifer prolabens Vørs, 1992
• Abothrium gadi Van Beneden, 1870
• Abra alba (W. Wood, 1802)
• Abra americana A. E. Verrill & K. J. Bush, 1898
• Abra fragilis Risso, 1826 accepted as Abra prismatica (Montagu, 1808)
• Abra longicallis (R. A. Philippi, 1844) accepted as Abra longicallus (Scacchi, 1835)
• Abra longicallis var. americana A. E. Verrill & K. J. Bush, 1898 accepted as Abra americana A. E. Verrill & K. J. Bush, 1898
• Abra longicallus (Scacchi, 1835)
• Abra longicallus americana A. E. Verrill & K. J. Bush, 1898 accepted as Abra americana A. E. Verrill & K. J. Bush, 1898
• Abra milashevichi Nevesskaja, 1963 accepted as Abra prismatica (Montagu, 1808)
• Abra nitida (O. F. Müller, 1776)
• Abra ovata (R. A. Philippi, 1836) accepted as Abra segmentum (Récluz, 1843)
• Abra prismatica (Montagu, 1808)
• Abra profundorum (E. A. Smith, 1885)
• Abra renieri (Bronn, 1831) accepted as Abra alba (W. Wood, 1802)
• Abra segmentina H. Adams & A. Adams, 1856 accepted as Abra segmentum (Récluz, 1843)
• Abra segmentum (Récluz, 1843)
• Abra tenuis (Montagu, 1803)
• Abra tenuis lorenzoi Cecalupo, Buzzurro & Mariani, 2008 accepted as Abra segmentum (Récluz, 1843)
• Abralia (Asteroteuthis) veranyi Rüppell, 1844 represented as Abralia veranyi (Rüppell, 1844)
• Abralia jattai Pfeffer, 1912 accepted as Abraliopsis (Abraliopsis) morisii (Verany, 1839) represented as Abraliopsis morisii (Vérany, 1839)
• Abralia lonnbergi Pfeffer, 1912 accepted as Abraliopsis morisii (Vérany, 1839)
• Abralia mediterranea Pfeffer, 1912 accepted as Abraliopsis morisii (Vérany, 1839)
• Abralia megalops A. E. Verrill, 1882 accepted as Ancistrocheirus lesueurii (A. d'Orbigny, 1835)
• Abralia veranyi (Rüppell, 1844)
• Abraliopsis (Abraliopsis) morisii (Verany, 1839) represented as Abraliopsis morisii (Vérany, 1839)
• Abraliopsis morisii (Vérany, 1839)
• Abraliopsis pfefferi Joubin, 1896 accepted as Abraliopsis morisii (Vérany, 1839)
• Abramis ballerus (Linnaeus, 1758) accepted as Ballerus ballerus (Linnaeus, 1758)
• Abramis bjoerkna (Linnaeus, 1758) accepted as Blicca bjoerkna (Linnaeus, 1758)
• Abramis brama (Linnaeus, 1758)
• Abramis sapa (Pallas, 1814) accepted as Ballerus sapa (Pallas, 1814)
• Absidia spinosa Lendn., 1905
• Abudefduf luridus (Cuvier, 1830) accepted as Similiparma lurida (Cuvier, 1830)
• Abudefduf vaigiensis (Quoy & Gaimard, 1825)
• Abylopsis eschscholtzii (Huxley, 1859)
• Abylopsis tetragona (Otto, 1823)
• Abyssascidia millari Monniot F., 1971
• Abyssidrilus profundus (Cook, 1970)
• Abyssocucumis abyssorum (Théel, 1886)
• Abyssocythere carpathica Pokorny, 1975 †
• Abyssoninoe abyssorum (McIntosh, 1885)
• Abyssoninoe hibernica (McIntosh, 1903)
• Abyssoninoe scopa (Fauchald, 1974)
• Abyssoninoe scopa aequilobata (Winsnes, 1981) accepted as Abyssoninoe hibernica (McIntosh, 1903)
• Abyssopathes lyra (Brook, 1889)
• Abyssorchomene abyssorum (Stebbing, 1888)
• Abyssorchomene chevreuxi (Stebbing, 1906)
• Abyssorchomene gerulicorbis (Shulenberger & Barnard, 1976)
• Abyssothyris atlantica Cooper, 1977 accepted as Abyssothyris wyvillei (Davidson, 1878)
• Abyssothyris wyvillei (Davidson, 1878)
• Acaenomolgus protulae (Stock, 1959)
• Acaenomolgus serpulae (Stock, 1960)
• Acanella arbuscula (Johnson, 1862)
• Acanthamoeba gigantea Schmoller, 1964
• Acanthancora clavatancora Topsent, 1927 accepted as Pseudohalichondria clavatancora (Topsent, 1927)
• Acanthancora schmidti (Topsent, 1898)
• Acantharctus posteli (Forest, 1963)
• Acanthaspidia decorata (Hansen, 1895) accepted as Iolanthe decorata Hansen, 1895
• Acanthaspidia laevis Chardy, 1975 accepted as Iolanthe laevis (Chardy, 1975)
• Acanthaspidia typhlops (G. O. Sars, 1879) accepted as Iolanthe typhlops (G. O. Sars, 1879)
• Acanthella acuta Schmidt, 1862
• Acanthella annulata Sarà, 1958
• Acanthella erecta (Carter, 1876)
• Acanthella mastophora (Schmidt, 1870)
• Acanthella multiformis Vosmaer, 1885
• Acanthella obtusa Schmidt, 1862 accepted as Dictyonella obtusa (Schmidt, 1862)
• Acanthephyra brevirostris Smith, 1885
• Acanthephyra curtirostris Wood-Mason in Wood-Mason & Alcock, 1891
• Acanthephyra eximea Smith, 1884 accepted as Acanthephyra eximia Smith, 1884
• Acanthephyra eximia Smith, 1884
• Acanthephyra pelagica (Risso, 1816)
• Acanthephyra purpurea A. Milne-Edwards, 1881
• Acanthephyra stylorostratis (Spence Bate, 1888)
• Acanthephyra tenuipes (Spence Bate, 1888)