Physiological and Morphological Plasticity in Larvae from Eilat, Israel, to Shallow and Mesophotic Light Conditions

Mesophotic reefs have been proposed as climate change refugia but are not synonymous ecosystems with shallow reefs and remain exposed to anthropogenic impacts. Planulae from the reef-building coral , Gulf of Aqaba, from 5- and 45-m depth were tested for capacity to settle, grow, and acclimate to reciprocal light conditions. Skeletons were scanned by phase contrast-enhanced micro-CT to study morphology. Deep planulae had reduced volume, smaller diameter on settlement, and greater algal symbiont density. Light conditions did not have significant impact on settlement or mortality rates. Photosynthetic acclimation of algal symbionts was evident within 21-35 days after settlement but growth rate and polyp development were slower for individuals translocated away from their parental origin compared to controls. Though our data reveal rapid symbiont acclimation, reduced growth rates and limited capacity for skeletal modification likely limit the potential for mesophotic larvae to settle on shallow reefs.