The basin-wide, second International Indian Ocean Expedition (IIOE-2; 2015-2030) is yielding vast amounts of biodiversity information through >50 endorsed research projects spread from Africa to Australia. The repeat of Australia’s 110° E line in the SE Indian Ocean yielded vast amounts of high-seas biodiversity observations (viruses to whales) with most beyond Australia’s national jurisdiction. As an example, mesopelagic fish diversity along 110°E was studied across 30° of latitude through day and night, depth-stratified, multi-net sampling from the RV Investigator. Along the transect, >150 species of fishes were recorded, which were mostly larval and transforming stages although juveniles and adults were also sampled. Species richness, abundance and diversity increased towards lower latitudes. Horizontal and vertical fish distributions analysed relative to concurrent hydrographic profiles indicated clear bio-geographic patterns, repeated by most lower taxa. South of the Subtropical Front <10 fish species were observed whilst at tropical stations, influenced by the Indonesian Throughflow, >60 species were recorded. The latter were mainly mesopelagic Myctophidae, Phosichthyidae and Gonostomatidae, epipelagic Gempylidae and various tropical reef-associated fishes. International research programmes such as the IIOE-2, with a collaborative approach across the Indian Ocean basin, provide a fundamental contribution to modelling whole systems and achieving future ocean sustainability.