Allometry of resource capture in colonial cnidarians and constraints on modular growth

1. Indeterminacy in growth of colonial organisms, such as corals, is commonly attributed to their modular construction which frees the colony from the allometric constraints that limit the size of single modules. However, as a colony grows, there may be a decrease in resource availability to interior modules because of active depletion and/or passive deflection by modules on the exterior. The effects of ‘self‐shading’ on resource capture in modular animals are modelled using a simple allometric growth function. 2. The model assumes that resource capture by a module scales as an exponent ( γ ) of colony size (i.e. number of modules). Data taken from the literature indicate that model values of γ for light and prey capture range from – 0·80 to – 1·16 for branching and encrusting corals. Module‐specific rates of resource use (i.e. metabolism) are less affected by colony size. Therefore, as a colony grows, net resource state eventually reaches zero, making further growth unsustainable or determinate. 3. The model also predicts an inverse relationship between module size and colony size such as that observed in Caribbean corals. This negative correlation results from the additive effects of module size and colony size on the degree of self‐shading. 4. Resource capture is affected by growth form and flow regime, and the interaction between them can account for some of the morphological variation in corals and other colonial suspension feeders.