a, ArcGIS Online map (left; Map data, 2019 Esri) and topographic base map (right; plotted using Generic Mapping Tools software (see Methods) of the collection locations within the Mariana Trench. b, RV Tan Suo Yi Hao. c, The deep-sea lander Tianya. d,e, In situ observations of the MHS (P. swirei). f, Fresh MHS specimen (holotype; Liparidae sp. 2 MT-2016; female). g, A Tanaka’s snailfish caught off the coast of Matsue city, Shimane Prefecture, Japan. Credit: photo of Tanaka’s snailfish taken by Takuya Morihisa in 2003.