The growth of regular branching corals in flowing water: pilot studies.

Roeland Merks, Jaap A. Kaandorp and Peter M. A. Sloot

Section of Computational Science
Department of Computer Science
University of Amsterdam

Stony corals are colonies of polyps, as a living sheet surrounding stony material that was deposited by earlier generations. The growth process of a coral colony can be described as the iterative deposition of new layers on top of the previous layers.

The shape of a stony coral seems to depend strongly on the amount of water flow it experiences. Corals growing at an exposed growth site in general show more compact growth forms than corals that are found at more shallow sites. A possible explanation for this observation has been given by a previous simulation model~\cite{Kaandorp.prl96}. In this model the thickness of a newly deposited layer depends locally on the concentration of food particles. Under ``shallow'' growth conditions the availability of food is limited by the diffusion rate of the food. As a result, the growth rate will be highest at the ``tips'' of the coral, sticking into nutritious water that has not yet been depleted by competing branches. This effect is diminished if the water flow ``mixes'' the water, leveling off the differences in food availability.

In these simulations, however, quite irregular growth forms develop. These shapes contrast the growth forms found in nature, that are often quite regular branching structures. In the present studies, we aim to test possible explanations for these regular shapes. As an explanation for the development of regular shapes we are testing a hypothesis that has been put forward by Rinkevich~\cite{rinkevich.bms85} to explain the prevention of branch fusion in some coral species. Rinkevich explains the absence of branch fusion in these coral species by hypothesizing a buffer zone surrounding the branches, presumably created by a diffusing chemical. Within this buffer zone, the growth of neighboring branches is either inhibited or redirected.

Although in our present model the fusion of branches is technically impossible, we expect that the inclusion of a buffer zone around the coral branches will result in more regular growth forms. At the same time, we expect that such buffer zone might be ``flushed away'' in a water flow. We will study the behavior of the new model in different flow conditions. Preliminary results will be presented and discussed.

References

J.~A. Kaandorp, C.~P. Lowe, D.~Frenkel, and P.~M.~A. Sloot. Effect of nutrient diffusion and flow on coral morphology. Phys. Rev. Lett., 77:2328--2331, 1996.

Baruch Rinkevich and Yossi Loya. Coral isomone: a proposed chemical signal controlling intraclonal growth patterns in a branching coral. Bulletin of Marine Science, 36(2):319--324, 1985.