Herbivorous and deposit-feeding gastropods are a major component of archaeological shell middens worldwide. They provide a wealth of information about subsistence, economy, environment, and climate, but are generally considered to be less than ideal for radiocarbon dating because they can ingest sediment while they graze, inadvertently consuming terrestrial carbon in the process. However, few studies of C-14 activity in herbivores or deposit-feeding gastropods have been conducted into this diverse range of animals that inhabit many environmental niches. Here, we present results investigating ¹⁴C variability in shells belonging to the families Strombidae and Potamididae from the Bogi 1 archaeological site, Caution Bay, southern coastal Papua New Guinea (PNG). These shells make up 39.3% of the shell MNI8 in the excavation units studied and some of these species are the most common taxa of neighboring sites. It would therefore be advantageous to establish if there are any ¹⁴C offsets associated with such animals, and identify those that can give reliable calendar ages. Our methodology combines a high-resolution excavation protocol, selection of short-lived samples identified to species level, and a tri-isotope approach using ¹⁴C, δ¹³C, and delta ¹⁸O to evaluate the source of variability in shells. Our results indicate that considerable variation exists between different species of Strombidae with some inhabiting muddier environments that act as sinks for limestone-derived sediments with depleted ¹⁴C content. The magnitude of variation is, however, overshadowed by that measured in the mudwhelk, Cerithidea largillierti, which has the largest spread in ¹⁴C of any shellfish studied so far at Caution Bay. This animal ingests sediment within the estuary that contains ¹⁴C derived from both enriched and depleted sources.