• Abra alba (W. Wood, 1802)
• Abra nitida (O. F. Müller, 1776)
• Abra ovata (R. A. Philippi, 1836) accepted as Abra segmentum (Récluz, 1843)
• Abra renieri (Bronn, 1831) accepted as Abra alba (W. Wood, 1802)
• Abra segmentum (Récluz, 1843)
• Abramis ballerus (Linnaeus, 1758) accepted as Ballerus ballerus (Linnaeus, 1758)
• Abramis brama (Linnaeus, 1758)
• Abramis sapa (Pallas, 1814) accepted as Ballerus sapa (Pallas, 1814)
• Acanthobothrium coronatum (Rudolphi, 1819) Blanchard, 1848
• Acanthocardia paucicostata (G. B. Sowerby II, 1834)
• Acanthocephaloides irregularis Amin, Oğuz, Heckman, Tepe & Kvach, 2011
• Acanthocephaloides propinquus (Dujardin, 1845) Meyer, 1933 accepted as Solearhynchus rhytidotes (Monticelli, 1905) Belofastova, 2006
• Acanthochitona fascicularis (Linnaeus, 1767)
• Acanthogyrus (Acanthosentis) tylosuri (Yamaguti, 1939)
• Acanthomysis strauchi (Czerniavsky, 1882) accepted as Paramysis (Metamysis) ullskyi Czerniavsky, 1882
• Acarochelopodia delamarei Angelier, 1954
• Acartia (Acanthacartia) tonsa Dana, 1849-1852
• Acartia (Acartiura) clausii Giesbrecht, 1892
• Acartia clausi Giesbrecht, 1889 represented as Acartia clausii Giesbrecht, 1892 represented as Acartia (Acartiura) clausii Giesbrecht, 1892
• Acartia clausii Giesbrecht, 1892 represented as Acartia (Acartiura) clausii Giesbrecht, 1892
• Acartia tonsa Dana, 1849-1852 represented as Acartia (Acanthacartia) tonsa Dana, 1849-1852
• Acipenser gueldenstaedtii Brandt & Ratzeburg, 1833
• Acipenser huso Linnaeus, 1758
• Acipenser persicus Borodin, 1897
• Acipenser ruthenus Linnaeus, 1758
• Acipenser stellatus Pallas, 1771
• Acmostomum dioicum Metschnikoff, 1865
• Acontiolaimus zostericola Filipjev, 1918 (uncertain > taxon inquirendum)
• Acrorhynchides bivittatus (Uljanin, 1870) Strand, 1928 (uncertain > taxon inquirendum)
• Acrorhynchides graciosus (Uljanin, 1870) Strand, 1928 (uncertain > taxon inquirendum)
• Acrorhynchides reprobatus (Pereyaslawsewa, 1892) Strand, 1928 (uncertain > taxon inquirendum)
• Acrorhynchides spiralis (Pereyaslawzewa, 1892) Strand, 1928 (uncertain > taxon inquirendum)
• Acrorhynchus bivittatus (Uljanin, 1870) Graff, 1882 accepted as Macrorhynchus bivittatus (Uljanin, 1870) Pereyaslawzewa, 1892 accepted as Progyrator mamertinus (Graff, 1874) Reisinger, 1926
• Acrorhynchus dolichocephalus (Pereyaslawzewa, 1892) Graff, 1905 accepted as Paulodora dolichocephala (Pereyaslawzewa, 1892) Artois & Schockaert, 1998
• Acrorhynchus graciosus (Uljanin, 1870) Graff, 1882 accepted as Acrorhynchides graciosus (Uljanin, 1870) Strand, 1928
• Acrorhynchus reprobatus Graff, 1905 accepted as Progyrator mamertinus (Graff, 1874) Reisinger, 1926
• Acrorhynchus spiralis (Pereyaslawzewa, 1892) Graff, 1913 accepted as Acrorhynchides spiralis (Pereyaslawzewa, 1892) Strand, 1928
• Actacarus bacescui Konnerth-Ionescu, 1970
• Actacarus bacescui bacescui Konnerth-Ionescu, 1970
• Actacarus ponticus Bartsch, 1999
• Actinia equina (Linnaeus, 1758)
• Actinothoe clavata (Ilmoni, 1830) accepted as Sagartiogeton undatus (Müller, 1778) accepted as Cylista undata (Müller, 1778) accepted as Sagartia undata (Müller, 1778)
• Agaue chevreuxi (Trouessart, 1889)
• Agauopsis brevipalpus (Trouessart, 1889)
• Agauopsis ibssi Bartsch, 1996
• Agauopsis marinovi Petrova, 1976
• Agetus typicus Krøyer, 1849
• Aglaophenia picardi Svoboda, 1979
• Alitta succinea (Leuckart, 1847)
• Allostoma austriacum (Graff, 1882) accepted as Allostoma durum (Fuhrmann, 1898)
• Allostoma capitatum Uljanin, 1870 (uncertain > nomen dubium)
• Allostoma catinosa (Beklemischev, 1927) accepted as Allostoma catinosum (Beklemischev, 1927) Mack Fira, 1974
• Allostoma catinosum (Beklemischev, 1927) Mack Fira, 1974
• Allostoma durum (Fuhrmann, 1898)
• Allostoma pallidum Beneden, 1861
• Alona quadrangularis (O.F. Müller, 1776)
• Alona rectangula G.O. Sars, 1862 accepted as Coronatella rectangula (G.O. Sars, 1862)
• Alosa immaculata Bennett, 1835
• Alosa pontica (Eichwald, 1838) accepted as Alosa immaculata Bennett, 1835
• Alpheus dentipes Guérin, 1832
• Amage adspersa (Grube, 1863)
• Amalthaea sarsii (Steenstup, 1854) accepted as Corymorpha sarsii Steenstrup, 1855
• Ameira parvula (Claus, 1866)
• Ameira parvula parvula (Claus, 1866) accepted as Ameira parvula (Claus, 1866)
• Amonardia similis (Claus, 1866)
• Ampelisca diadema (A. Costa, 1853)
• Amphiascopsis similis (Claus, 1866) accepted as Amonardia similis (Claus, 1866)
• Amphiascus parvulus (Claus, 1866)
• Amphiascus similis (Claus, 1866) accepted as Amphiascopsis similis (Claus, 1866) accepted as Amonardia similis (Claus, 1866)
• Amphibalanus amphitrite (Darwin, 1854)
• Amphibalanus eburneus (Gould, 1841)
• Amphibalanus improvisus (Darwin, 1854)
• Amphiura stepanovi Djakonov, 1954
• Ancyrocephalus cobitis Ergens, 1963 accepted as Haliotrema cobitis (Ergens, 1963) Merella, Dmitrieva, Piras, Huyse, Gerasev & Garippa, 2010 accepted as Xenoligophoroides cobitis (Ergens, 1963) Dmitrieva, Sanna, Piras, Garippa & Merella, 2018
• Angulus donacinus (Linnaeus, 1758) accepted as Moerella donacina (Linnaeus, 1758)
• Angulus fabula (Gmelin, 1791) accepted as Fabulina fabula (Gmelin, 1791)
• Angulus tenuis (da Costa, 1778) accepted as Macomangulus tenuis (da Costa, 1778)
• Anisakis simplex (Rudolphi, 1809) Dujardin, 1845
• Anisocladium fallax (Rudolphi, 1819) Looss, 1902
• Anisocladium gracile (Looss, 1901) Vlasenko, 1931
• Anisocoelium capitellatum (Rudolphi, 1819) Lühe, 1900
• Anisocycla pointeli (de Folin, 1868) accepted as Ebala pointeli (de Folin, 1868)
• Anomalocera patersoni Templeton, 1837 accepted as Anomalocera patersonii Templeton, 1837
• Anomalocera patersonii Templeton, 1837
• Anoplodactylus petiolatus (Krøyer, 1844)
• Anoplodactylus stocki Bacescu, 1958
• Anoplostoma viviparum (Bastian, 1865) Bütschli, 1874
• Antho (Antho) involvens (Schmidt, 1864)
• Antho involvens (Schmidt, 1864) represented as Antho (Antho) involvens (Schmidt, 1864)
• Anticoma acuminata (Eberth, 1863) Bastian, 1865
• Aonides oxycephala (Sars, 1862)
• Aonides paucibranchiata Southern, 1914
• Aphalloides coelomicola Dollfus, Chabaud & Golvan, 1957
• Aphallus tubarium (Rudolphi, 1819) Poche, 1926
• Aphanurus stossichii (Monticelli, 1891) Looss, 1907
• Apherusa bispinosa (Spence Bate, 1857)
• Aphia minuta (Risso, 1810)
• Aponurus galeatus (Looss, 1907)
• Apseudes acutifrons Sars, 1882 accepted as Apseudopsis acutifrons (Sars, 1882)
• Apseudes ostroumovi (Bacescu & Carausu, 1947) accepted as Apseudopsis ostroumovi Bacescu & Carausu, 1947