D. João de Castro seamount has an elevation of 1000 m and is located between the islands of Terceira and São Miguel, Azores, on the hyper-slow spreading Terceira Rift. The summit of the seamount shoals to 13 m below sea level and has hydrothermal vents at a depth of 20 m. This is the first vent system to be described on a hyperslow spreading centre. Gas discharging from the vents is dominated by CO2 (90%) with lesser H2S, H2 and CH4. Two distinct vent areas were identified that had different physical and chemical characteristics. The white area had vent orifices with white bacterial mats, while the yellow area had yellowish coloured vent openings. Vents in the white area had lower temperatures (≤ 43 °C), but higher concentrations of H2S, CH4 and H2. Beggiatoa sp. was found in this area. The yellow area had vent temperatures up to 63 °C. No typical hydrothermal vent fauna was found at these vents. The macro species present were similar to those found in coastal and seamount areas of the Azorean Archipelago. There was evidence for the presence of deeper vents on the flanks of the D. João de Castro seamount, at depths between 150–400 m.

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