Cautious clams? Energetic state modifies risk assessment in giant clams

Abstract

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.

Nutrient enrichment alters risk assessment in Giant clams

“…Therefore, the non‐light deprived control group of clams in Hayes et al. (2021) were more energetically comparable to the unenriched clams in our study. Thus, the energetically limited clams in Hayes et al.…”

“…Second, shaded clams in Hayes et al (2021) may have been so energetically limited that basic movements, such as mantle retractions, were not physiologically possible. This could explain why energetically limited clams in Hayes et al (2021) did not respond according to traditional risk assessment models. The methodological distinction between light deprivation and nutrient enrichment could have led to the seemingly contradictory results between our study and Hayes et al (2021).…”

“…The study conducted by Hayes et al. (2021) sought to deprive clams of energy through light manipulation, while the clams in our study were enriched with nutrient supplementation. Therefore, the non‐light deprived control group of clams in Hayes et al.…”

“…Before starting an experiment, we waited until the clam was relaxed which we defined as the shell being open and the mantle extended. Following previous studies (Doyle et al., 2020; Hayes et al., 2021; Johnson et al., 2017), we ran the eraser end of a No. 2 pencil along the full length of the clam's mantle and shell which prompted the clam to retract its mantle into its shell.…”

Welcome to the Pyrocene: animal survival in the age of megafire

“…Fourth, we expect that increased energetic demands will increase the relative cost of responding to fire cues, as is predicted for anti-predator responses (Trimmer et al, 2017). Hungry and subordinate animals take greater predatory risks (Hayes et al, 2020) and, because of widespread condition dependence seen in many behavioural decisions (Schmitz & Trussell, 2016), we expect that body condition will influence the response to fire cues.…”