Cautious Clams? Energetic State Modifies Risk Assessment In Giant Clams

A fundamental trade-off exists between the essential activities of acquiring energy and avoiding predators, thus animals are expected to make decisions that optimize foraging and avoid predation. These assessments are often state-dependent with hungrier animals taking greater risks when foraging. Previous studies have explored state-dependent risk assessment in a variety of taxa, yet no studies have focused on giant clams, genus Tridacna. These organisms provide a unique system to test foraging-risk trade-offs because they have two main energy sources: photosynthesizing symbiotic algae and siphoning nutrients from the water column. These activities can only occur when the clams' shells are open and the mantle is vulnerable to predation. Here, we tested whether risk assessment in giant clams was state-dependent. We designed three experiments of different shading durations (within-day and multiday) to block photosynthesis while allowing limited water flow for siphoning. We measured the latency of the clams to reopen after a simulated predator touch to determine whether different duration of shading modified their antipredator response. Our single-day experiment did not show a change in the hiding times across the three treatments. However, clams increased their hiding time as the treatments increased food deprivation (no restrictions on flow or photosynthesis, restriction on flow, restriction on flow and photosynthesis) when exposed to treatments for multiple days. Overall, we found that clams that were more energy-deprived had a longer hiding time. This contrasts with findings from previous state-dependent risk assessment literature and suggests that clams are more cautious when energy deprived, a result that may be generalizable to other sessile invertebrates.