The Mariana Arc of submarine volcanoes has recently been the site of an international, interdisciplinary study into the structure and function of the associated hydrothermal systems. A broad size range of juvenile alvinocaridid shrimp, Opaepele loihi and Alvinocaris sp. M (diagnosis in preparation by R. Webber), were collected from active sites on NW Rota-1 and NW Eifuku volcanoes. Fatty acid biomarkers and stable isotopes of carbon and nitrogen revealed a photosynthetic signal in small juveniles of both species, probably acquired during a pelagic larval phase. Size-related changes in the fatty acid composition of both species indicated a dietary switch from pelagic feeding on photosynthetic material to ingestion of bacteria at vent sites after settlement. This is especially true for O. loihi, where carbon isotopic signatures implied ingestion of bacteria with form II RuBisCo. Juvenile Alvinocaris sp. M also appear to have eaten bacteria, although probably those with form I RuBisCo; detritus may also feature in their diet at an early stage. With increasing size, the fatty acid and isotopic composition of Alvinocaris sp. M implied a lesser dependence on bacterivory and a possible switch to carnivory. Generally, Alvinocaris sp. M and O. loihi are more similar in their biochemical composition to opportunistic alvinocaridids than to strict bacterivores. We suggest that as juveniles both species rely to varying degrees on bacteria and that opportunism and scavenging are likely sources of nutrition in older individuals.