- Abyssoninoe hibernica
- Acanthicolepis asperrima
- Adyte pellucida
- Aglaophamus malmgreni
- Aglaophamus rubella
- Alciopa reynaudii
- Alentia gelatinosa
- Alkmaria romijni
- Amaeana trilobata
- Amage auricula
- Amblyosyllis formosa
- Ampharete acutifrons
- Ampharete baltica
- Ampharete falcata
- Ampharete finmarchica
- Ampharete goesi
- Ampharete lindstroemi
- Amphicteis gunneri
- Amphicteis midas
- Amphicteis sundevalli
- Amphiglena mediterranea
- Amphitrite cirrata
- Amphitritides gracilis
- Amythasides macroglossus
- Anobothrus gracilis
- Antinoe finmarchica
- Aonides oxycephala
- Aonides paucibranchiata
- Aphelochaeta marioni
- Aphelochaeta multibranchis
- Aphelochaeta vivipara
- Aphrodita aculeata
- Aphropharynx heterochaeta
- Apistobranchus tenuis
- Apistobranchus tullbergi
- Apomatus similis
- Arenicola marina
- Arenicolides ecaudata
- Arenotrocha minuta
- Aricidea albatrossae
- Aricidea catherinae
- Aricidea cerrutii
- Aricidea minuta
- Aricidea simonae
- Aricidea suecica
- Artacama proboscidea
- Asclerocheilus intermedius
- Austrolaenilla mollis
- Autolytus alexandri
- Autolytus brachycephalus