Environmental context Animals that eat marine algae strongly affect the rate at which a compound that algae synthesise, dimethylsulfoniopropionate (DMSP), is converted to the important atmospheric climate gas dimethylsulfide (DMS). In studying the processing of DMSP by algae-eating molluscs, we have discovered that some molluscs accumulate and retain DMSP exceptionally well, but this can be very variable. With this knowledge, investigators will be able to design improved experiments to understand the effects of molluscs on DMS production in local ecosystems. Abstract Molluscs often play major roles in processing phytoplankton-synthesised dimethylsulfoniopropionate (DMSP) in local ecosystems. We find that some mollusc species retain tissue DMSP exceptionally tightly and exhibit unusually great and statistically non-normal interindividual variation in DMSP accumulation and retention. Individual mussels (Mytilus, Geukensia) living within a single clump, for example, range 6- to 11-fold in tissue [DMSP] and are often non-normal in statistical distribution. These properties cannot be explained by the elevation of the substrate on which the mussels are living or by mussel position in a clump. When mussels (Mytilus edulis) are deprived of DMSP for up to 5 weeks in depuration experiments, some individuals retain high tissue [DMSP], whereas others exhibit reduced [DMSP]. Such interindividual divergence helps explain non-normal distributions of tissue [DMSP] after depuration. We re-analyse published data from which the half-time for tissue DMSP loss during depuration can be calculated. In the only mollusc studied in this way (Haliotis), the half-time is 13–25 times longer than in similar size fish. Besides posing a challenge for DMSP mass balance studies, retention and interindividual variation may point to as yet unknown properties of molluscs: tight retention suggests functional roles for DMSP, and non-normal statistical distributions suggest discontinuities among individuals in DMSP metabolism.