Co-variation between autotrophy and heterotrophy in the Mediterranean coral Cladocora caespitosa

Abstract

This study quantifies the relative contributions of autotrophy and heterotrophy to the energy budget of the temperate scleractinian species Cladocora caespitosa Linnaeus 1767. Colonies were incubated under different light and feeding regimes, and changes in carbon acquisition through photosynthesis and feeding were measured during a 2-month time period. This approach allowed us to quantify the rate at which adjustments to physiology occurred, as well as the magnitude of up- or downregulation of both feeding modes. In addition, we explored how shifts in carbon acquisition mode influenced tissue biomass (protein content), energy stores (lipid content) and colony growth (calcification). Increases in feeding capacity during prolonged exposure to darkness were correlated with decreases in photosynthetic capacity. However, feeding effort did not decrease when photosynthesis was high. In fact, feeding was maximal under high light conditions when food was not available. During starvation, colonies used their lipid stores to partially meet their metabolic requirements. Colonies kept in low light used nutrients from feeding to supplement calcification, whereas those kept at high light converted carbon from feeding into tissue biomass. This work provides the first estimates of rates of adjustment of heterotrophic feeding capacity in a Mediterranean scleractinian coral. For the study species, flexibility in carbon acquisition through heterotrophy was approximately equal to flexibility in photosynthesis both in magnitude and in the rate at which physiological adjustments occurred. The ability to alter feeding effort rapidly and strongly may explain the wide depth distribution of C. caespitosa, and its ability to survive in turbid coastal waters where light is often limited.