Modelling aspects of growth and form in hard corals.

Dave Barnes
Australian Institute of Marine Science

Colonial hard corals comprise a small mass of tissue intimately associated with a large mass of skeleton. Tissue and skeletal growth are not necessarily controlled by the same environmental factors, although skeletal growth requires tissue and tissue growth depends upon skeletal growth. There then exists a need to accommodate the two forms of growth, one to the other.

Early in my career, I examined how tissue and skeletal growth might be harmonized in hard corals. Colonies start growing from a single point and tend to form spheroidal shapes. Thus, in branching colonies, new branches are added between diverging, existing branches and, in "solid" colonies, new calices (polyps) are added between diverging, existing calices. However, there is a decrease in the rate of creation of space between branches and calices as colonies increase in size and colonies must change their tissue or skeletal growth rates, or change their growth form. Genetically controlled variations in colony form can be interpreted as strategies to adapt to this geometric restraint on colony growth, while environmentally linked variations in growth form can be interpreted in terms of differing rates and tissue and skeletal growth.

In recent years, I have become interested in the annual density banding pattern in massive (rounded) coral colonies. Since some coral colonies grow continuously for up to 1000 years, their annual density banding patterns offer proxy environmental records analogous to those obtained from tree rings. This fabulous potential has not been realised until very recently, largely because coral researchers could not agree about fundamental aspects of density band formation. The principal disagreement was about the dating of density bands. This was compounded by unexplained differences in the appearance of the bands and by the presence of fine (monthly) density bands only in some species corals.

Some years ago, my group recognised that not all skeletal growth occurred at the outer surface of a colony. Tissue occupies the outer several millimetres of skeleton and skeletal growth occurs as formation of scaffolding that is later bricked in. Additional to these skeletal extension and thickening processes is periodic (monthly) uplift of the lower margin of the tissue layer. We could see that these processes would affect density bands but found it impossible to build conceptual pictures. We turned to numerical modelling. Results of such modelling explained apparently conflicting results regarding density bands and, later, apparently conflicting results associated with inclusion of trace materials in the skeleton. Interestingly, this work on coral growth and density bands is throwing up new ideas about how tissue and skeletal growth are harmonized.