Molluscs associated with a cold seep on the Mtwara Mud Volcano off Tanzania (Mollusca: Gastropoda and Bivalvia)

Molluscs living in cold seep habitats often exhibit adaptations to extreme environmental conditions like high concentrations of oil, methane, hydrogen sulphide, heavy metals, and high acidity. This paper reports on the mollusc fauna encountered on the Mtwara Mud Volcano (MV) with a crestal cold seep, which was discovered off Tanzania during the cruise SO306 by the German RV Sonne in 2024. Living specimens of the cold seep fauna include the bivalve Gigantidas tanzaniensis Hoffman, Westbeek & Kihara sp. nov., and the gastropods Bathyacmaea nipponica Okutani, Tsuchida & Fujikura, 1992, Eatoniella kahawa Hoffman sp. nov., Provanna castanea Hoffman & Kihara sp. nov., Provanna mzunguko Hoffman sp. nov., and Enigmaticolus nipponensis (Okutani & Fujiwara, 2000). Empty shells of species belonging to cold seep fauna include the bivalves Gigantidas komekahawia Hoffman, Pfennings & Freiwald sp. nov., Lucinoma mviringo Hoffman sp. nov., Calyptogena pemba Hoffman, Pfennings & Freiwald sp. nov., Isorropodon mtwaraense Hoffman sp. nov., and Isorropodon cowioae Hoffman sp. nov. A number of gastropods with, most likely, a large distribution along the east African margin were found as well: Puncturella mrembo Hoffman sp. nov., P. alternata Hoffman sp. nov., Fissurisepta kilimanjaro Hoffman sp. nov., Cornisepta infundibulus Hoffman sp. nov., Anatoma agulhasensis (Thiele, 1925), A. japonica (Adams, 1862), A. chaki Hoffman sp. nov., Satondella goudi Geiger, 2012, Sinezona macleani Geiger, 2006, Toroidia marisindica Hoffman sp. nov., Skenea kitovu Hoffman sp. nov., Seamountiella laini Hoffman sp. nov., Cerithiella kupamba Hoffman sp. nov., and Pedicypraedia stylasteriophila Hoffman, Pfennings & Freiwald sp. nov. At species level, most of the molluscan taxa at the Mtwara MV are different from those reported from other cold seep locations. Two exceptions are the gastropods Bathyacmaea nipponica and Enigmaticolus nipponensis, both were originally described from cold seeps off Japan and their distributions are herein extended to the continental slope off Tanzania.