Sediment burial caused by bioturbating shrimp negatively affects juvenile oyster survival and size

Epibenthic organisms on intertidal flats can be affected by underlying sediments and by the activities of bioturbating species. Bioturbating shrimp have 2 potential pathways to affect small clusters of juvenile oysters (seeded cultch): directly by moving sediment to the surface, or indirectly by affecting sediment properties (grain size, organic content, penetrability). We examined how Pacific oyster Magallana gigas survival and size responded to (1) ghost shrimp Neotrypaea californiensis density, (2) mud content and penetrability of sediment, and (3) shrimp density due to their effects on sediment properties (indirect pathway). Seeded cultch were deployed from spring through summer at 31 intertidal sites varying in both shrimp density and sediment properties within Willapa Bay, Washington (USA). Shrimp density was negatively associated with mud and organic content but positively with sediment penetrability, as expected from known ecosystem engineering effects of shrimp. However, neither mud content nor penetrability contributed statistically to the negative impact of shrimp density on oyster survival and size. No oysters survived the summer above 50-100 shrimp m -2 , and remaining oysters were smaller with increasing shrimp density. Overall, negative effects of shrimp on benthic oysters likely occur through the deposition of sediment (28.9 ml burrow -1 d -1 ) rather than alteration of sediment properties. Our study highlights how the antagonistic ecosystem engineering effect of shrimp on oysters occurs independently of sediment responses to bioturbation and deposit-feeding and quantifies the conditions ensuring the persistence of ecologically and commercially important foundation species.