Nutrient enrichment alters risk assessment in Giant clams

Many species exhibit state‐dependent risk assessment. Photosynthetic giant clams (e.g. Tridacna maxima) retract their mantles and close their shells as an anti‐predator response. Although being open allows clams to photosynthesize or siphon‐feed, staying open increases vulnerability to predation. Prior studies indicate that giant clam risk assessment is state‐dependent. Hiding time differs based on body condition, shown by the light deprivation of clams for varying amounts of time. Nutrient enrichment has been shown to impact giant clam growth and photophysiology through their symbiotic zooxanthellae (Symbiodinium sp.). Although previous work by Hayes et al. (2021) examined risk assessment in light‐deprived giant clams, nutrient enrichment and its potential effects on giant clam risk assessment are unstudied. Here, we tested whether nutrient enrichment would alter risk assessment of giant clams by conducting two experiments that quantified hiding time for nutrient‐enhanced clams and two control treatments following an experimentally induced mantle closure: one with a singular simulated predator probe per trial and then with repeated simulated predator probes in the same trial. In our single predation probe experiment, nutrient‐enriched clams significantly increased their hiding time over multiple days. Nutrient‐enrichment did not, however, modify their response to repeated probes. Overall, our results support that nutrient enrichment modulates clam risk assessment. This indicates support for prior state‐dependent risk assessment literature, suggesting nutrient enriched clams will take less risks to forage. This is in direct contrast to results found by Hayes et al. (2021) that suggested that energetically depleted clams were taking less risks to forage, warranting additional research into the mechanisms behind clam energetics and behaviour. Giant clams (Tridacna maxima) acquire energy by siphon feeding and through the photosynthesis of their symbiotic zooxanthellae (Symbiodinium sp.). When they perceive a predatory threat they retract their mantel and close their shell but this comes at the cost of not being able to feed. We conducted a nutrient enrichment experiment to see whether modifying physiological state influenced clam risk assessment that we quantified by the time they remained closed following an experimental probe. Over time, nutrient enriched clams stayed closed for a longer time compared to control treatments. Clam antipredator behaviour, like