Documentation of Stony Coral Growth, and Photos of Coral Aggression

(not clickable photos)

 

Photo sizes adjusted to give approximately the same scale for each photo

 

Photos document growth for:

• Purple-Tipped Acropora (in center of all three photos)
• Rose Montipora digitata (lower center, last two photos)

(Also note growth of the Red Sea Xenia soft coral behind and to the left of the purple-tipped Acropora. All that we had of this species in March 1999 can be seen in the first photo. By the time of the second photo, these colonies in the reef tank were not only much larger and had spread, but cuttings had been moved to the seagrass tanks where they covered about 1/3 of one of the right side wall of the tank, and many cuttings had been given away to other reefkeepers)

 

March 29, 1999

 

The purple-tipped Acropora in the center of this photo was purchased in early February 1999. At that time, it was a small Y-shaped branch tip broken off a larger colony and attached to a rock using superglue. It had two small branch tips and only a small amount of tissue grown out onto the rock.

 

Note: The brown Pocillopora damicornis coral to the right of the purple-tipped Acropora coral was moved out of view after this photo was taken, so it is absent in photos below.

 

November 12, 1999

 

Note: Several new corals were added since the above photos, including a long-polyped green Acropora and a "Rose" Montipora digitata (the orange coral bottom center), and others.

 

The red arrow points to place where a branch of a long-polyped green Acropora stony coral had grown in contact with the purple-tipped Acropora.

 

The white material at the arrow tip is a clump of mesenterial filaments (digestive filaments) released by the terminal polyp of the encroaching green Acropora branch in an attempt to kill a portion of the the purple-tipped Acropora and clear space for its own growth. This attempt was not successful (see August 23, 2000 photo below).

 

August 23, 2000

 

Note: The long-tentacled green Acropora and several other corals were moved out of view shortly before this photo was taken.

 

The red arrow points to a shelf of tissue and skeleton that grew out from the purple-tipped Acropora coral in response to the attack by the long-polyped green Acropora shown above. This effectively stopped the intrusion of the green Acropora into the purple-tipped Acropora's space by putting a cap over the growing branch.

 

Due to this cap of tissue as well as the continual growth of the purple-tipped Acropora's branches (which cast shade below), the long-polyped green Acropora was in danger of being killed. It had stopped growing and was starting to die off, so it was moved to a new location shortly before this photo was taken.

 

In the last two photos above, also note the substantial growth by the "Rose" Montipora digitata stony coral (orange in color). At least two large branches had been removed from this coral (to start new colonies) between the dates of these two photos , so the actual growth was actually greater than seen here. The green variety of Montipora digitata (not visible in these photos) grows significantly faster than this orange variety.

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Closeup of aggressive mesenterial filaments produced by the long-polyped green Acropora coral in response to contact with the purple-tipped Acropora

(November 12, 1999)

Coral Aggression

 

The white tufts (near the center of the photo) are aggressive mesenterial filaments (also called acontia) released from the ends of two branches of long-polyped green Acropora in response to contact with the purple-tipped Acropora. Normally part of the digestive sturctures within the polyps, these filaments can be released in aggressive encounters with other corals.

 

Space and access to light are at a premium on coral reefs, and corals engage in a variety of different forms of aggression against neighbors competing for space:

• Mesenterial filaments (acontia) digest the living tissue of neighbors
• Some stony corals produce extremely long tentacles called sweeper tentacles that are many times the length of ordinary tentacles and are packed with nematocysts (stinging cells). These sweeper tentacles reach out and kill susceptible neighbors. In many cases, these sweeper tentacles are only produced when neighboring corals are present within a certain range, and they are directed toward the neighbor.
• The ordinary tentacles of corals and other cnidarians often sting and kill neighbors on contact as well.
• A variety of toxic compounds are also produced by corals and other cnidarians. Soft corals (such as Xenia, the pale-colored colonies very conspicuous behind the purple-tipped Acropora in the Nov 12 photo above) tend to have especially impressive chemical arsenals. These toxins help protect the corals from predators, but they are also often harmful to neighboring corals.

As can be seen in the above photos, not all corals are susceptible (or equally susceptible) to the attacks of other corals, and there is a hierarchy of aggressiveness/susceptibility among different species. The two Acropora battling for space in these photos were equally matched in some respects, neither able to harm the other with acontia or nematocysts, but the purple-tipped Acropora won this conflict over space by overgrowing and shading its competitor. In contrast, contact between this same long-polyped green Acropora that lost this battle and Montipora digitata (the orange coral in the bottom right) results in death of all contacted Montipora tissue wihin a day or two of contact. Release of mesenterial filaments is not even necessary in this case.