Sessile organisms
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. hot button for preemptive 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

Overgrowth competition

hot buttons for overgrowth competition hot button for fire-coral overgrowth competition hot button for sponges-overgrowth competition hot button for coral/zoanthid overgrowth competition hot button for tunicate-overgrowth competition hot button for algae/cyanophyte-overgrowth competition
Overgrowth of other organisms by corals/zoanthids is considered here, while information on other taxa can be accessed via the icons.


seahorse dive leader for Biology of Caribbean Coral Reefs Website photograph of zoanthid overgrowth taken from a video "Zoanthids are relatives of corals and gorgonians, and this type is often found on corals." - Turks & Caicos 2003

As noted elsewhere in BCCR, competition amongst corals is often for light - required for the photosynthesising plant symbionts found in their tissues. This type of competition is seen when corals grow in 3-dimensional array with one coral shading another. Two-dimensional competition for space in corals invariably involves one protagonists' humoral system chemically fighting another's. Some images of this type of competition are found below, and also in the section CHEMICAL COMPETITION.

NOTE humoral systems are internal defensive systems. In vertebrates, these involve antibodies, B- and T-cells, and other white cells involved in mopping up. In invertebrates, they involve protein hemagglutinins and mopping-up phagocytic cells (cells that engulf foreign proteins, bacteria, and parasites in the same way that the various white cells do in vertebrates). Antibodies and hemagglutinins do similar jobs in identifying foreign or "non-self" protein for the various phagocytic cells to act on and remove

photo collage showing various types of competition among corals
photographs showing interactions of corals and tubeworms
As we have seen, corals mount active resistance to encroachment by other corals, sponges, gorgonians, and to some extent zoanthids, but not evidently to tubeworms. Whether the tubeworms are chemically camouflaged in some way is not known.


seahorse dive leader for Biology of Caribbean Coral Reef website photograph of mat zoanthids taken from a video Mat-type zoanthids, such as this Palythoa sp., are in some areas so competitively dominant that they may blanket and smother all sessile bottom-inhabiting invertebrates - St. Maarten 2005

photograph of mat zoanthid overgrowing a coral
photograph of zoanthid overgrowing a hydroid ZOANTHIDS are strong overgrowth competitors and may form large, dominant colonies overtop of other cnidarians, such as gorgonians and hydroids, on the sea bottom.

NOTE Cnidaria: a phylum of invertebrates that includes jellyfishes, anemones, corals, sea pens, hydroids, and zoanthids



Mat zoanthid Zoanthus pulchellus
overgrowing a coral 5X



Hydroid zoanthid Parazoanthus
growing on, and killing,
its favoured hydroid host
Dentitheca dendritica 0.6X

photograph of white encrusting zoanthid
Growth rates of white encrusting zoanthids Palythoa caribaeorum in the U.S. Virgin Islands have been measured at 4mm per day, making it arguably the fastest growing of any anthozoan species. Green 1981 Proc 4th Int Coral Reef Sympos 2: 679.

NOTE Class Anthozoa in Phylum Cnidaria includes corals, sea anemones, gorgonians, sea pens, and zoanthids

list of features of zoanthids that make them superior overgrowth competitors
There are several features of zoanthids, for example, species of Palythoa, that contribute to their success as overgrowth competitors, as shown in the accompanying illustration. The role of one of these features, namely, presence of photosynthesising symbionts, is considered in detail elsewhere: LEARNABOUT NUTRITION: CORALS: A CASE STUDY: PHOTOSYNTHESIS.

NOTE an organism that lives on, with, or in another organism. The most common examples are commensalism, mutualism, and parasitism