The bigger they are the better they taste: size predicts predation risk and anti‐predator behavior in giant clams

The decision to hide from predators and the time allocated to hiding are economic decisions that integrate the benefits of escaping a predator and the costs of reduced resource acquisition. Body size is a factor that influences many antipredator decisions. Giant clams, Tridacna maxima, depend on photosynthesis as their main source of energy; thus, retracting their mantle into their shell inhibits energy acquisition and ultimately growth. We experimentally encouraged 95 individual clams to hide by touching them and found that after accounting for variation explained by observer, larger clams remained closed longer. When we looked at the response of these 95 clams to a total of four consecutive experimentally induced closings over about 10min, we found that larger clams on average hid longer and that clams had individually consistent hiding times and generally habituated to repeated experiments. We then focused on a subset of 30 clams and continued this experiment every other day over 6days for a total of four sessions. Over this longer duration, clams consistently habituated to repeated disturbance, but the effect of size disappeared. We also found that larger clams were bitten by predators more often than smaller clams. Large clams pay an immediate cost to hiding from benign stimuli and apparently learn to modify their behavior to repeated benign experiences.