Giant clams (genus Tridacna) are the largest living bivalves and, like reef-building corals, secrete massive calcitic skeletons and host symbiotic algae (zooxanthellae) from which they obtain a significant portion of their respiratory carbon supply. However, both processes (biomineralization and algal photosynthesis) require mechanisms for carbon concentration. In corals, vacuolar-type H+-ATPases (VHAs) have been show to acidify the algal symbiosome, increasing symbiont photosynthesis. Because giant clams occupy similar habitats and perform similar biochemical functions to corals, we hypothesized that VHAs may play analogous roles in tridacnid tissues. We assayed for the presence, and investigated the potential functional roles of VHAs within various tissues of the small giant clam (Tridacna maxima). VHAs were present in all tissues assayed (gill, azooxanthellate byssal mantle, and algae-bearing siphonal mantle), but were ~ 3x more abundant in siphonal mantle where they were located in close proximity to algae-bearing tubules and sites of calcification. Further, VHA contribution to algal photosynthesis was substantial, with inhibition of VHAs reducing symbiont photosynthetic production by nearly 50% in vitro. Algal productivity in light-exposed mantle increased overall energy turn-over by ~ 100%, and therefore VHA activity likely confers strong ecological and energetic benefits to the host clam. These results confirm the presence of VHAs in tridacnid clams and suggest that, like in corals, they play an important functional role in algal energy generation, potentially sustaining massive growth and permitting the evolution of gigantism in the Tridacna lineage.