Size structure of populations (shell height in mm) and oocyte size (in µm) distribution of Alviniconcha hessleri from hydrothermal vents along the Mariana back-arc. Individuals were collected at five vent sites in the northern and central regions of the Mariana back-arc (~18.2 to 15.4° N) during the FK161129 Schmidt Ocean Institute cruise in 2016. These data were collected through histological analyses on several individuals from each site.