This study focuses on quantitative analysis of the species composition and distribution of living and dead mollusks to assess fidelity of death assemblages in a protected tidal flat and a shallow subtidal lagoon behind a fringing reef from the Indo-Pacific province. We evaluate how well the death assemblage (DA) reflects the original living community and whether there was a recent ecological shift in species composition. Quantitative analysis of 18 samples from unvegetated sandy substrata shows that dead individuals dominate in the shallow subtidal lagoon, but living and dead individuals were roughly equally abundant on the tidal flat. This difference points to rapid degradation or export of dead shells from tidal flats, leading to smaller potential for time averaging. Rarefied species richness and diversity of DAs is higher than that of the living assemblages (LAs) at the scale of habitats and at the scale of the study area, and this difference in richness is stronger and live-dead (LD) agreement in composition is smaller in the subtidal than in the intertidal habitats. Distinct assemblages characterized intertidal and subtidal habitats both in LAs and DAs, and the rank-abundance distributions of DAs generally corresponded to that of LAs. We suggest that anthropogenic impact in the area did not result in a major environmental change in the subtidal environment during the past few decades because existing small differences between LAs and DAs can be explained by some degree of time averaging and small-scale redistribution of shells. On the tidal flat, however, the lower time-averaging of the DA and the generalistic life habits of prominent members in the LA do not exclude a major shift.