Competition
Competition

Sessile organisms

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
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 coral with algal overgrowth "These Porites-type corals look healthy, but look closely at the reddish-coloured overgrowth that is killing some of the coral. I've heard about a coral-killing red alga introduced from the Mediterranean. I wonder if this could be it?." - Turneffe Island, Belize 2000 courtesy Andy Stockbridge, Belize

Sessile organisms: 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 competition involves one organism growing over another, thereby denying it space to live. The aggressor may smother the other, thus denying it of oxygen, light, food, or other essential resource, or it may physically displace it. Of the many examples of overgrowth competition on coral reefs, 5 stand out in importance. Overgrowth by algae/cyanophytes is considered here, while information on other taxa can be accessed via the icons.

Sessile organisms: overgrowth competition: algae/cyanophytes


seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of algaeovergrowth taken from a video "Lots of algae here. Mostly the brown alga Dictyotera. Why is it so abundant? Must have some sort of defense to protect it." - Cayman Brac 2001

photograph of rich mass of green alga Chaetomorpha
Algae are ubiquitous on reefs and provide sustenance for many fishes and invertebrates, but unusually heavy overgrowths may be symptomatic of nutrient enrichment from sewage, agricultural runoff, or other sources. Fertilisers may be carried to the reef in runoff waters from farmlands, golf courses, lawns, and gardens.

 

 

 

Green alga Chaetomorpha linum growing in
nutrient-rich Discovery Bay, Jamaica 0.03X

photograph of algal overgrowth in the Cayman Islands
Overgrowths of algae damage the reef by suffocating sessile animals underneath, by blocking lilght and thereby reducing photosynthesis in corals and gorgonians, and by various toxic effects.

 

 

 

 

 

Excessive growth of filamentous algae, perhaps
Chaetomorpha
sp., on a reef in Cayman Brac

  photograph of Caulerpa overgrowth

Rich overgrowth of the green alga
Caulerpa racemosa
even spilling into a
barrel sponge Xestospongia muta 0.5X

 
photograph of a spiny lobsterin its algal-overgrown crevice home
This spiny-lobster's Panulirus argus home in a crevice is overgrowth with at least 4 types of green algae.
If sufficient nutrients are available for their growth, algae may aggressively invade wound sites, scars, or other areas of weakened defenses on corals, sponges, gorgonians, and other sessile reef animals. If the "algae" is finely divided and fuzzy to the touch, it may be a bacterial cyanophyte, also known as "blue-green algae".
photo collage of examples of algal overgrowths

photograph of green alga Caulerpa racemosa
Caulerpa racemosa can grow from a single vegetative
fragment and increase its surface coverage 10-fold
in a single growing season 0.5X


Species of the green alga Caulerpa are chemically defended against herbivorews and are aggressive overgrowers. So dominant is Caulerpa taxifolia that its introduction from the Caribbean into the Mediterranean near Monaco in the mid-1980s led to serious overgrowth of subtidal habitats and displacement of resident species.
photograph of green alga Caulerpa racemosa close view
Detail of a variety of C. racemosa showing vegetative growth 1X
 

photograph of invasive red alga Ramicrusta textilis overgrowing and presumably killing a mound coral Montastraea faveolataAfter its first invasive appearance in the Caribbean area in 2009 the encrusting red alga Ramicrusta textilis is now a dominating overgrowth competitor of both hard and soft corals in areas of Puero Rico. The species is fast growing but not long-lasting, as its attachment to the substratum soon degrades and it washes away easily from wave and storm effects. Presumably, though, the overgrown corals are fatally damaged by its presence. Ballantine & Ruiz 2013 Coral Reefs 32: 411; photograph courtesy the authors.

 

 

 

Invasive red alga Ramicrusta textilis overgrowing and
presumably killing a mound coral Montastraea faveolata


   
seahorse dive leader for Biology of Caribbean Coral Reefs photograph of parrotfish feeding taken from a video "When we hear parrotfishes crunching away we think of the devastation being wreaked. But it may not be all bad, at least not for the corals, if removal of competing algae gives more room for their larvae to settle and grow." - Turks & Caicos 2003

photograph of redtail parrotfish feeding
Grazing by herbivorous parrotfishs is thought to benefit corals because the fishes consume potentially competing seaweeds...

 

 

 

 

 

Redtail parrotfish Sparisoma
chrysopterum
feeds on algae 0.25X

photo collage of parrotfishes scraping coral
..even though parts of the corals may be scraped and eaten during the process.

photograph of blue-green alga
photogaph of blue-green alga colonising a dead fan gorgonianCYANOPHYTES are another type of aggressive overgrowing reef organism - actually a type of bacterium. Growths of cyanophytes may be brown, purple, or other hues depending upon the species. They are toxic and are eaten by only a few specialised types of animals.

 

A degraded fan gorgonian Gorgonia sp.
is overgrown by a dark purplish-coloured
cyanophyte (middle), as well as algae (upper
L) and other unidentified organisms 0.5X

 

Cyanophytes often grow in large
gooey mats on the sea bottom. In
so doing, they smother and kill
other sessile organisms 1X


photograph of blue-green algae overgrowing mound coral Montastrea cavernosa Although cyanobacterial growths are seen commonly throughout the Caribbean region, scientists are not always certain whether the growths are on dead organisms or whether the bacteria are actively killing live organisms that they overgrow. Ritson-Williams et al. 2005 Coral Reefs 24: 629. photograph of blue-green algae overgrowing
 
Cyanobacteria Lyngbya confervoides overgrows mound coral Montastrea cavernosa in the Florida Keys   Purple Lyngbya sp. overgrows brown algae Dictyota sp. on a reef in Panama

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