Coral reefs today continue to mystify us. The wondrous, colorful masses of branches, inhabited by fish, crustaceans and other marine life truly are the Edens of the sea. We usually take the peculiar beauty of coral reefs for granted, but sometimes we realize that everything in nature must have (had) some purpose. An interesting example is the branched morphology of coral species. Recently theoretical biologists from the University of Amsterdam (UvA) found out why many species grow in this specific way. It seems that many corals branch during growth because it enables individual polyps to remove more nutrients from the water.

Corals are colonial animals, composed of thousands of polyps connected together by common tissue called coenenchyme. Coral polyps are clones of one another, which means that individuals in one particular colony have to collaborate as they all share the same genes - a common principle in evolution.

Figure 1, right: A branching Acropora sp., created by cloning hundreds of polyps. In the wild, these species grow into large tables which can be several meters across. They provide hiding places for many small fish. Photograph by Michael de Regt.

However, all polyps from a single colony also compete with each other for nutrients. Although many corals receive carbohydrates and glycerol from their symbiotic zooxanthellae, they also remove plankton from the water, which they ingest. This is where the competition starts, as each polyp has tentacles trying to remove a nice meal from the water column.

Modelling coral growth

Scientists from the UvA realized the importance of this inherent competitive behaviour, and constructed a mathematical model called PORAG (Polyp Oriented Radiate Accretive Growth, fig.2) which considers each polyp as an individual, fending for itself. This is not entirely true, as it is known that many coral species have polyps which are connected by a gastrovascular cavity (which runs through the coenosarc), allowing them to transfer nutrients between each other. The point is that direct food capture may still be beneficial, and therefore there still is some competition going on.

Figure 2: The PORAG model. Left, a: A coral tip is represented as a hemisphere or circle, the black dots are the initial polyps. b: The growth field is voxelized, meaning it is divided into discrete parts or voxels. c: The coral colony receives nutrients, which influences its growth direction. d: A new accretion (growth) step, after which a new layer of coral skeleton has been deposited onto the old one. From this stage, the model returns to (b), which starts a new growth cycle called an iteration (repetition).  Right: From this model, it becomes clear that the colony grows outwards. The black circles show the newly formed corallum (skeleton). The area for a single new polyp is also shown (modified from Merks et al, Journal of theoretical biology, 2004).

The model considered all polyps as individuals, which took up nutrients, deposited skeleton, were budding off new polyps and of course died occasionally. In their polyp-oriented model, spontaneous branching of colonies occurred without pre-programming. This told them that their model was getting close to the truth, as any good model shows what is really happening in nature.

Now why do corals often grow into branched colonies, exactly? This may have to do with capturing as much nutrients as possible. To understand this, we first need to have a look at the different shapes of biological structures.

Concave and convex

When regarding animal structures or tissue surfaces, different shapes exist. For example, we distinguish between convex and concave surfaces. Convex simply means that surfaces are round or dome-shaped. White blood cells (leukocytes) can be convex (fig.3). Concave means that surfaces are curved, or hollow. A nice example is a red blood cell which is biconcave, having two hollow surfaces (fig.3).

Figure 3, right: Different types of blood cells, having different shapes. A red blood cell, on the left, is biconcave. A white blood cell, on the right, is convex. In the middle, a blood platelet can be seen (copyright the Wikimedia Foundation).

Coral species such as Acropora, Stylophora and Seriatopora spp. have branches which have convex ends, because they are round (fig.1). Massive corals, such as Favia, Siderastrea and Montastrea spp. grow as spheres, and are also convex. Other corals, such as Echinopora and Turbinaria sp. (fig.4) build hollow or concave plate structures which resemble cups.

Figure 4: A large Echinopora lamellosa colony growing in an artificial laguna at NAUSICAA, Boulogne sur mer, France. These colonies are concave (photograph: Tim Wijgerde).

Having a concave surface doesn’t seem to be beneficial, as polyps will be residing more closely together. This makes the polyps highly competitive, as their tentacles will intermingle, which makes it more difficult to capture food. When coral polyps are placed on a convex surface, however, they radiate out into the water column. This provides plenty of room to capture plankton, without too much competition. The scientists found that this mechanism may drive many corals to grow into their common, branching shapes (or massive sphere shapes). They called their discovery the polyp fanning-effect, which causes polyps to grow into directions where most food is availabe. In this case, it means growing outward is the best thing to do.

Polyp spacing: creating bulky corals

The theoretical biologists also determined the effect of polyp spacing - the amount of space which exists between individual polyps. In their mathematical model, the size of inter-polyp space drastically altered a coral colony’s shape (fig.5). The larger the inter-polyp distance, the thicker and more compact the colony’s branches became (fig.5). They also experimented with diffusion of nutrients between polyps, which is known to occur in many species such a Stylophora pistillata. The higher the diffusion in the model, the more branching occurred as well (not shown). They found that this part of the model nicely described what biologists had seen at Caribbean reefs. Colonies of Montastrea annularis can be found there in three different shapes: bumpy, massive and columnar. Their calices, the depressions that house the coral polyps, are often more widely spaced in bumpy, round shapes which grow according to the PORAG model (fig.5, colonies g and h).

Figure 5: Effect of inter-polyp space on coral morphology. Left: Colonies were virtually grown in 84 repeating steps using computer software, in which the PORAG model was programmed. They repeated the experiment eight times, using increasing space values in between of the polyps, a to h. The colonies shown in a, b and c look quite natural, whereas colonies g and h have become very bulky. Right: Colony simulations using a small interpolyp space (a and c), and a larger interpolyp space (b and d). Modified from Merks et al, Journal of theoretical biology, 2004.

Nature and nurture

Coral growth is a complex matter, and it is now clear that this is controlled by both genes (also called 'nature') and the environment (also called 'nurture'). Genes seem to program how different coral species grow (branching, plate-like, massive, encrusting), and the environment apparently fine-tunes this process. For example, abiotic factors such as water flow and light availability will alter a coral's shape. More flow seems to stimulate thicker branches, as seen with Stylophora pistillata (personal observations by many), and more light seems to stimulate vertical instead of horizontal growth (low light may cause plate corals such as Montipora spp. to create a larger surface area by growing horizontally).

The PORAG model has helped scientists understand why factors such as polyp competition may have caused many species to be programmed to grow into branching colonies. This strategy may enable the individual polyps to take up more nutrients. It does seem puzzling that many concave, cup-shaped coral species exist, such as Echinopora and Turbinaria spp. Maybe future models will reveal this mystery...

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