Carbonate Mineral Saturation State as the Recruitment Cue for Settling Bivalves in Marine Muds

After a pelagic larval phase, infaunal bivalves undergo metamorphosis and transition to the underlying sediments to begin the benthic stage of their life history, where they explore and then either accept or reject sediments. Although the settlement cues used by juvenile infaunal bivalves are poorly understood, here we provide evidence that carbonate saturation state is a significant chemical cue in both direct observation laboratory studies and field manipulations. In the laboratory, plantigrade-stage Mercenaria mercenaria (200 μm shell height) showed a significant positive relationship between percent burrowed and Ω_aragonite, with an increasing probability of settlement with increasing saturation state. In the field, we increased bivalve recruitment by a factor of three in a 30-day field study by raising the pH (∼0.3) and saturation state of surface sediments by buffering sediments with crushed shell (CaCO₃). The susceptibility of infaunal bivalves to dissolution mortality and the tight coupling of other sedimentary biogeochemical processes with carbonate dynamics suggest that mineral thermodynamics may be an overarching cue new settlers are responding to.