Energy allocation in a reef coral under varying resource availability

An organism’s pattern of resource allocation to reproduction and growth over time critically impacts on its lifetime reproductive success. During times of low resource availability, there are two fundamental, mutually exclusive strategies of energy investment: maintenance of somatic tissues to support survival and later reproduction or investment into an immediate reproductive event at the risk of subsequent death. Here, we examine energy allocation patterns in the coral Montipora digitata to determine whether energy investment during periods of resource shortage favours growth or reproduction in a sessile, modular marine species. We manipulated light regimes (two levels of shading) on plots within a shallow reef flat habitat (Orpheus Island, Great Barrier Reef, Australia) and quantified energy uptake (rates of net photosynthesis), energy investment into reproduction (E _R ), tissue growth per unit surface area (E _T ) and energy channelled into calcification (E _C ). With declining resource availability (i.e. reduced photosynthesis), relative energy investment shifted from high (~80%) allocation to tissue growth (E _R :E _T :E _C  = 11:81:8%) to an increasing proportion channelled into reproduction and skeletal growth (20:31:49%). At the lowest light regime, calcification was maintained but reproduction was halted and thus energy content per unit surface area of tissue declined, although no mortality was observed. The changing hierarchy in energy allocation among life functions with increasing resource limitation found here for an autotrophic coral, culminating in cessation of reproduction when limitations are severe, stands in contrast to observations from annual plants. However, the strategy may be optimal for maximising fitness components (growth, reproduction and survival) through time in marine modular animals.