Interacting effects of temperature, habitat and phenotype on predator avoidance behaviour in Diadema antillarum: implications for restorative conservation

Abstract

Caribbean long-spined sea urchin Diadema antillarum populations crashed following a mass mortality event in 1983-1984 with cascading effects on reef health. Population restoration efforts may be hampered by unknown effects of short- and long-term elevated sea surface temperature (SST). We investigated how a key behavioural trait, predator avoidance behaviour (PAB; percentage of long defensive spines that moved in response to shadow stimuli), was affected by elevated SST in 180 individuals from 2 contrasting Honduran reefs: Utila (flattened reef structure, dearth of predation refugia) and Banco Capiro (complex reef structure, abundant refugia). Initiation of PAB is mediated by melanin, which breaks down at elevated water temperatures; thus, as SST rises, D. antillarum may become vulnerable to predation. We compared local current SST (CSST; 29.7°C) with 2 IPCC predicted long-term climate change scenarios under laboratory conditions. PAB decreased by 13.98-15.37% at CSST +1.4°C and 31.67-42.44% at CSST +3.1°C. Trial temperatures were similar to maxima recorded in the Caribbean during the 2016 El Nino, so our results also represent likely responses to worst-case short-term acute temperature anomalies. Juveniles maintained higher PAB than adults, indicating increased reliance on anti-predation behaviours. White-spined phenotypes from Utila’s flattened reef maintained higher PAB than black-spined counterparts, likely due to increased conspicuousness to visual predators. Habitat complexity may mitigate temperature-driven losses in natural behavioural defences. D. antillarum may be resilient to near-term (<2039) SST increases and periodic temperature stresses but may struggle under long-term, worst-case scenario conditions. Restoration of D. antillarum populations must be coupled to augmented reef complexity to improve future resilience.