Sessile organisms

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Competition in sessile reef organisms takes 3 forms: 1) preemptive, where through its presence one organism prevents another from occupying the same space, 2) overgrowth, where one organism, like a seaweed, grows over or otherwise crowds out and kills another organism like a coral, and 3) chemical, where through release of a toxic material one organism prevents another from settling and surviving. Access each type via the icons.

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of corals competing taken from a video "Here's a typical example of chemical competition between corals. You can see why this is something that might be overlooked on a dive. The boundary between them doesn't look too severe, but it's actually a zone of intense chemical activity as the two species compete for space." - Little Cayman Island 2001

Sessile organisms: chemical competition

photograph of coral and sponge competing for space
At first glance, sessile reef invertebrates seem to share space amicably, but the situation may be less benign than it appears. Note in the photograph on the Left the "stand-off" distance maintained by the coral knobs from the barrel sponge. Similarly, in the photograph on the Right nothing much seems to be going on, photograph of a sponge and coral competing but the red sponge and the sea-fan gorgonian are actually actively competing.


Madracis-type coral grows
within the main cavity of a
barrel sponge Xestospongia


A red encrusting sponge is engaged
with the base of a sea-fan gorgonian
Gorgonia sp. in chemical competition 0.5X

photo collage of chemically mediated stand-off competitions
More examples of chemical stand-offs:

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of coral and sponge chemically competing, taken from a video "Here's some chemical competition for you. Looks like the rope sponge is dissolving where it contacts the coral...or, maybe it's the other way round." - Turks & Caicos 2003

Competition for space between sessile organisms is often indicated by lines of dissolution or necrosis as one species' defenses battle another's. The defenses may involve humoral responses (cells and agglutinins), release of toxins or, in the case of corals and other cnidarians, employment of stinging-cell weaponry. Some examples are shown in the photographs:
photo collage of chemical competitions

photograph of sponges Ircinia felix and Ageles conifera growing in close associationSponges growing adjacent to one another don’t always show evidence of chemical competition. In such cases we can ask whether the sponges may be benefitting from the close association, perhaps by exchange of nutrients or other chemicals by diffusion. The chemicals could be from the sponges themselves or from algae or other microorganisms growing on them. Tests on 8 or so pairings of different sponge species in Curacao, however, disclose no such exchange. Each sponge in a paired association shows its own unique pattern of alkaloids...oh, well, another good idea into the waste basket! Schaft & Mebs 2002 Coral Reefs 21: 130.

NOTE the researchers monitor different types of alkaloids. These are nitrogen-rich, ring-like molecules often associated with toxicity (e.g., poison arrows). Other common alkaloids are morphine, quinine, nicotine, cocaine, caffeine, and many others

Stinker sponge Ircinia felix wrapped around
a brown tube sponge Ageles conifera 0.6X


In the case specifically of corals, the zone of contact involves a battle of polyps, and the weapons are nematocysts. A sequence of fighting polyps in the zone of interaction of 2 brain corals Diploria labyrinthiformis is shown below.

NOTE lit. "thread cysts", also known as "stinging cells". They are not cells; rather, they are special capsules housed within the skin or epithelial cells of cnnidarians. Within each capsule is a coiled thread wound in amongst a gel-like protein toxin. On appropriate stimulus, like the touch of a competitor's tentacle, the gel swells with water, the capsule bursts, and the thread is rapidly propelled outwards by the pressure until its tip penetrates the tentacle of the other coral. The thread is hollow and, in a stinging kind of nematocyst (some types of nematocysts are sticky and tangling, but not toxic), the toxin is conducted down the thread and injected into the potential aggressor

sequence of drawings showing chemical competition between brain corals

photograph of sea anemone with acontia protruding
Nematocysts are found not only on tentacles of corals and other cnidarians, but also on mesenterial filaments. These filaments extend from the bases of mesenteries that hang curtain-like from the pharynx into the digestive cavity. The filaments possess highly toxic nematocysts and function to entangle and sting live prey swallowed as food. In some species, they can also be extruded from the mouth or through pores in the body wall and used in defense.

NOTE also known as digestive filaments or, in sea anemones, as acontia.

In some anthozoans, such as this temperate-latitude
sea anemone Metridium sp., the filaments can be
squeezed out of pores in the body wall 3X

illustration showing the mesenterial filaments in cnidarian polyps
In a cnidarian species with transparent skin, as the one featured here, the mesenterial filaments or acontia can be seen hanging into the digestive cavity.

Corals are colonial, and so the polyps communicate with one another. This allows nutrients and energy materials to be shunted from one part of the colony, where food may be plentiful, to other areas where food is in low supply.

photograph of coral Mussa angulosa
Studies in Jamaica show that the fleshy coral Mussa angulosa tops the pecking order amongst corals. Its aggressive dominance is explained by a superior ability to extrude mesenterial filaments over its competitors and digest them in situ. Lang 1973 Bull Mar Sci 23: 260.

photo/diagram showing coral aggressione via acontia
Other studies in Jamaica on interactions between lettuce corals and other corals, such as boulder corals, reveal that while initial aggression may be via protrusion of mesenterial filaments from one polyp to another, a second type of tentacle may be brought into play. These are known as a fighting or sweeper tentacles, and originate from the polyp margin from among the regular feeding tentacles.

The sequence is shown below. Chornesky 1989 Ecology 70: 843.

drawing of fight sequence between corals 1 drawing of fight sequence between corals 2 drawing of fight sequence between corals 3 drawing of fight sequence between corals 4 drawing of fight sequence between corals 5
Tentacle contact elicits protusion of filaments from one polyp Days or weeks later the attackee may develop fighting tentacles The fighting or sweeper tentacles develop at the polyp margin Later, the first attacker may develop its own fighting tentacles And so the battle seewaws back and forth

Injuries to the polyps result in an increase in separation distance between them. Aggression abates until the polyps heal and begin to grow again, bringing the corals back into proximity. Continued aggression usually leads to a standoff, with growth of the colonies proceeding in other, safer, directions.

Not all cnidarian polyps have fighting tentacles, and probably even fewer have mesenterial filaments. The advantage of the former is that their reach is longer, and where better to site one's weaponry than on the peripheral parts of the polyps? The advantage of the latter is that their nematocyst complements are more potent, and they do double-duty in terms of function. Of course, if you have both, then all the better.

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of anemone-coral chemical competition taken from a video "Well, it's easy to see who is the superior competitor here. The anemone has made quite a mess of the coral. Note, though, that the fire coral doesn't seem to be bothered by the anemone." - Bonaire 2003

photographs of different types of chemical competition on the reef
In these photographs there is little doubt as to which cnidarian is the superior competitor. However, size and nematocyst potency are the main determinants of the dominance heirarchy.

cartoon showing nematocyst discharge into different food items by a coral polyp
Nematocysts, the most important mediators of chemical competition in corals and their relatives, actually have strict limitations on discharge. One aspect of this is simply (and crudely) shown here, testing nematocyst discharge when a coral-polyp's tentacles are touched with carbohydrate (a potato chip), sugar, and protein (the last in the form of a small crustacean). Note that discharge is only into the test item with protein components. This is helpful to know, but of course nematocyst discharge is much more complex than this, and for more information on the subject see LEARNABOUT NUTRITION: CORALS: PREY CAPTURE.

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