Map of the study area (Gulf of Cadiz) and location of sampling sites. squares with numbers, mud volcanoes with chemosymbiotic bivalves: full black circles, mud volcanoes visited during the study but bivalves not found: grey circles: mud volcanoes and other structures not visited during the study. Bon, Bonjardim MV; CA, Captain Arutyunov MV; CR, Carlos Ribeiro MV; Dar, Darwin MV; Gem, Gemini MV; Gin, Ginsburg; JB, Jesus Baraza MV; Kid, Kidd MV; Mek, Mèknes MV; Mer, Mercator MV; PDE, Pen Duick Escarpment; Por, Porto MV; Sag, Sagres MV; Yum, Yuma MV. The numbers inside the squares indicate the presence of the following species. 1 Acharax gadirae 2 Petrasma elarraichensis 3 Lucinoma asapheus 4 Thyasira vulcolutre 5 Spinaxinus sentosus 6 Isorropodon megadesmus 7 Isorropodon sp. indet. 8 Christineconcha cf. regab 9 Bathymodiolus mauritanicus 10 Idas sp. 11 Laubiericoncha chuni (empty shells only) 12 Callogonia cyrili (empty shells only) 13 Pliocardia sp. (empty shells only).