seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of coral reef taken from a video "A delightful feature of healthy coral reefs is that so many organisms are crammed together in one small space. Although it looks benign, there are competitive interactions taking place all over the reef. Let's put together how these interactions work to regulate the distribution and numbers of reef organisms." - Turneffe Island, 2002 Video courtesy Andy Stockbridge, Belize

Sessile organisms

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of coral/sponge competition taken from a video "How often do we swim past something like this without noticing...just a sponge and a coral...but they are actually competing for space, and quite intensely." - Turks & Caicos 2003

photograph of  crowdedreef A rich crowded Caribbean reef may have space for colonising organisms, but not much. The reality is that on a healthy reef, most colonisable spots may be already occupied and competition for the remaining spots by resident invertebrates may be intense.

Scientists have recorded the number of competitive interactions per square meter among sessile reef organisms on various Caribbean reefs. Think about your last swim on a reef and how many interactions you observed, then compare with what the experts saw in these areas:

Florida Keys: 1 (Hill 1998 Oecologia 117: 143, data given for sponge-coral interactions only)

St. Croix: 6 (Suchanek et al. 1982 NOAA)

Puerto Rico: 10 (Vicente 1990 In, New Perspectives in Sponge Biology, Smithsonian Instit Press)

How does your recollection compare with that of the experts? And keep in mind that they were counting per square meter! If your answer was less than their average, or even zero, perhaps you just need some guidance in what to look for.

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.

Competition in motile reef organisms is usually for space/territory, food, and mates, and is presented in its own MOTILE ORGANISMS section.

NOTE incapable of movement, as a coral, barnacle, or gorgonian; opposed to sedentary, where movement is possible but not usual, as in sea anemones, clams, or mussels, and motile, where movement is freely possible, as in crabs, sea cucumbers, or fishes

hot buttons for competition part of Biology of Caribbean Coral Reefs website hot button for preemptive competition part of Biology of Caribbean Coral Reefs hot button for overgrowth competition part of Biology of Caribbean Coral Reefs hot button for chemical competition part of Biology of Caribbean Coral Reefs

Preemptive competition

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of reef taken from a video "Each of these sessile organisms, sponges, corals, and algae is occupying space that could be occupied by another. Ecologists term this preemptive competition." - Cayman Brac, 2001

photograph of crowded Caribbean reef

In a healthy reef, space is at a premium. Only on the death of the corals, sponges, anemones, hydroids, and algae featured here can other organisms find space to settle and grow. Thus, the presence of one organism preempts another from occupying the space.




A rich bottom cover will exclude algal spores
and invertebrate larvae from settling. The
reproductive propagules may be chemically
repelled, physically brushed away, or eaten
outright by the residents 0.5X

photo collage of corals competing for light

Just as trees compete for light in a tropical rainforest, so plate corals Agaricia spp. grow in such a way as to intercept as much light as possile, and they often shade one another in preemptive competition for light.

NOTE corals need light to meet the energy needs of its photosynthesising symbionts. See REEF ORGANISMS THAT LIVE TOGETHER: MUTUALISM

photo collage of worms competing woth corals
Worms that reside in corals could be considered parasites but, by preempting space that could be occupied by the coral, they are also acting as competitors.

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of spinyhead blenny taken from a video By occupying abandoned worm-holes in corals, spinyhead and other hole-dwelling blennies preempt other worms and other organisms from occupying the same space - Turks & Caicos, 2003

photograph of a spinyhead blenny in its hole
Studies in the Virgin Islands show that spinyhead blennies Acanthemblemaria spinosa compete for worm-holes in coral heads. If blennies are caught and moved to other parts of the reef, larger individuals will competitively dominate smaller ones for access to pre-existing holes. If experimental holes are drilled in the coral they are quickly colonised. As they grow the blennies will explore up to several meters from their home hole looking for new, larger holes. Buchheim & Hixon 1992 J Exp Mar Biol Ecol 164: 45.



If a spinyhead blenny goes out looking for a new and bigger hole in which to live, it should know how to find its way back should it fail in its quest. The following images describe an experiment done in the Virgin Islands to test the blenny's homing fidelity.

NOTE the ability of an animal to return to a home site

photograph of spinyhead blenny's home site in a coral head
The blenny's homesite coral
photograph of spinyhead blenny's homesite hole in the coral
The blenny's hole in the coral

data for experiment on homing fidelity in spinyhead blennies in the Virgin Islands
When the blenny is removed from its hole and released at varying distances from it, its ability to return falls off quickly with increasing distance from the hole. The % values indicate the probability of its safe return from the indicated distance. Buchheim & Hixon J Exp Mar Biol Ecol 164: 45.

Note that the blenny's navigational skills seem poorly developed - much less so than in fishes that are used to moving freely around the reef.

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of gorgonians taken from a video "Gorgonians feed on small planktonic organisms which they catch in their tentacles. I'd guess that the polyps on these different gorgonians are competing for available food." - Cayman Brac 2001

photo collage of fliter-feeding reef organisms
Preemptive competition also occurs among sessile filter-feeding invertebrates for food, as downstream availability of food must depend upon how much feeding activity is going on upstream
photograph of gorgonian
What follows is an experiment on food competition among gorgonians. Design of experiment and results from Kim & Lasker 1997 J Exp Mar Biol Ecol 215: 49.
photos of 5 gorgonian pieces to be used in an experiment 5 gorgonian pieces attached to a supporting platform tray with gorgonian pieces set out into reef area
The researchers first cut off 5 equal-sized branches from 5 different gorgonian colonies The pieces are then positioned in an orderly way on a supporting tray The assembly is placed in an area of the reef characterised by oscillating water flow
results for experiment on competition among gorgonians
Here are the results after 1yr. Note that growth is greatest at either end of the array, diminishing towards the middle.

So, how do we interpret these results? Review the options below and select the most convincing one. Then CLICK HERE for explanations.

Different gorgonians have different growth rates.

The end portions intercept food items borne in the water currents and deprive the innermost portions of them.

The end portions out-compete the innermost portions for oxygen.

The end portions are most advantageously sites with respect to light.

Predators selectively feed on the innermost portions owing to their protected location.

The innermost portions are negatively affected by metabolic and digestive wastes coming from the outmost portions.