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Coral Compatibility: On Reducing Captive Negative Interactions Cnidarians 

Bob Fenner

Stichodactyla gigantea

Too many people envision the reefs of the world to be some aquatic equivalent of "Bambi crossing a meadow" with some sort of miraculous "balance" amongst the life that occurs there. This is a dangerous precept for aquarists, as these areas are veritable battle zones with the life competing for space, food, mates... at a "tooth and claw" level. Most all benthic life we keep continues with this behavior in captivity. Overgrowing, shading, poisoning, stinging, eating its neighbors... Sometimes at great distances. Herein will be my statements re approaches to alleviate the major thrusts of this issue with stinging-celled life... All out competition is an omnipresent situation in captive reefs. Proper set-up... Starting with smaller specimens, rinsing them, quarantine, placing the less noxious to more in order, careful maintenance... and observation are all useful tools here.


            Looking at most television and other media presentations re the worlds reefs, one gets the notion that these are places of pastoral perfection With static stands of dominant inplaced/attached life and shoals of colorful fishes passing by. This is a far cry from the truth, as most reef aquarists can attest. In fact, just thinking about what one sees on tropical shallows should cause a thinking person to wonder Just why is there so much polypoid life here? Does it make sense that predators wouldnt consume such ready fare? Looking a bit closer, one can clearly make out demilitarized zones between various specimens and with a bit more cursory examination, that there are definite patterns in the abundance, distribution and proximity of one organism type and its relation to others.

            These facts are of use, importance to home hobbyists, as obviously all Cnidarian life has mechanisms for extending itself in space and time to the detriment of others near it. Various tools are used as we will see to outcompete neighbours for space, light and food and to prevent them in turn from outright being killed.

Various Competitive Means:

            There are a few ways to classify the mechanisms used by Cnidarians to attain and retain their piece of paradise on reefs. Here is mine.

n     Growth Means



Physically blocking


n     Physical Means

            Digestive Dominance

            Stinging Mechanisms

                                    Proximal: Nematocysts/Cnidocysts

                                    More Distal & Specialized: Acrorhagi

                                    Most Distal: Sweeper Tentacles


n Chemical Allelopathy

            Toxins produced that are deleterious to cnidarians, other invertebrates, often fishes and even algae.

Competitive Means Details:

Growth Means: Overshadowing/Shading:

            This is a principal strategy of the more fast-growing, reef-building families of stony corals, the Acroporids, Pocilloporids and some Poritids. Simply growing faster, taller, overhanging and crowding out other life forms below and aside their colonies to deprive them of light and settling foods.


An Acropora sp. So-called Table Top coral

A stand of Turbinaria. Most scleractinian families have large, arborose or blade-like species.

Growth Means: Overgrowing:

Faster growing, more dominant by way of being more noxious or better stinging species can overcome and even possibly derive nutrition from slower growing, less noxious, less stinging ones that lose to them. This phenomenon is especially common among stoloniferans (polyps to hobbyists) and gorgonians (sea fans).

A Briareum (encrusting Gorgonian) overtaking a stony coral  in the Caribbean

An encrusting  stony coral seems to be getting the upper hand on a sponge


            Though the contestants may not be chemically challenging to each other, simple proximal exclusion can deprive other life of light, current, foods, waste-dilution

A large Fan in the TWA.

Especially useful in the mobile family of Plate corals, the Fungiidae.

Physical Means: Digestive Dominance:

Some Cnidarian groups can use their tentacles and extend other feeding structures through their body wall (mesenterial filaments) whose cnidocytes contain digestive elements. Some stinging-celled organisms can egest (throw up) buccal pellets, digesting near competitors.

The giant green anemone, Anthopleura gigantea and

The closely related clone anemone A. elegantissima, with an obvious DMZone twixt clonal communities where anything between gets eaten/dissolved when the tide is up.

Stinging Means: Proximal:

            Nematocysts/Cnidocysts are cellularly derived structures that are everted (turned inside out) given chemical (smell) and physical (pressure) clues with either barbed ends of various sorts or agglutinant (sticky) processes. These can be shot out with extreme speed and pressure penetrating even a thickened skin Some are VERY sticky, others are EXTREMELY toxic. And theyre the gift that keeps on giving; can continue to sting, agglutinate off-animal, at much later times as part of released mucus that can sting, poison other life. Theyre especially prevalent in hard and soft corals and anemones. 

More Distal Stinging Mechanisms: Acrorhagi:  

            This structural defense is widespread amongst coldwater Anemones, and found in some hard and soft coral groups. The actual stinging structures are called holotrichs, mastigophores.  

 Even More Distal Stinging Mechanisms: Sweeper Tentacles: 

            This is the most common occurrence and source of trouble with stony coral keepers but sweeper tentacles also occur in soft corals, other Classes.  Their mode of action appears as burning of adjoining species, different genotypes of the same species. Occur in response and in the direction of strangers. And sweepers can break off keep stinging, digesting. 

The size of species, polyps is no indication of the length of sweepers, nor most importantly, the number of cnidocysts For example, even one inch Oculinds (Galaxeas) have sweepers of up to 12 length. Caryophylliids/Euphylliids very common as well. hierarchy of stinginess can be arranged with species of the genera Fungia, Galaxea, Goniopora being high, Lobophyllia being intermediate and Montiporas being low.  


And the winnah is: Galaxea

Plerogyra extending sweeper tentacles

Chemical Allelopathy: Poisoning Others: 

Mostly gorgonians and soft corals, but about half of cnidarians produce allelopathic compounds in some quantity; some of their effects are profound, especially in aquariums vs. wild settings, where concentrations can be orders of magnitude higher due to a lack of dilution. Can be lethal to other livestock, including fishes and algae; however, the effects, degrees of toxicity are species to species specific and variable.

Allelopathogens are used for more than offense/defense. They also serve to shed/avoid space and metabolic parasites. Chemically, theyre mainly 'terpenoid' and  'sarcophine' <a class of Terpenes> compounds, but there are many others.  Higher reduction-oxidation potential (e.g. the use of Ozone) is useful in destruction of these materials.  

Some of the greatest volume producers:



Leather corals, e.g. Sarcophyton

Gorgonians, sea fans

Sidebar: What are Terpenoids:

Terpenoids are:

Monomers of Isoprenes five Carbon chains.

Largest group of organic molecules for instance, Lipids are terpenoids.

Some common examples: Plant Aromatics (Eucalyptus, Ginger, Cloves): Cannabinoids... Camphor, Menthol and precursors (make up) Steroids, Sterols

Important biological molecules/hydrocarbons, involved in identification of proteins



Practical Matters: What Aquarists Can and Should Do (or at least be aware of) 

         Signs/Symptoms of Overt Aggression/Poisoning


  1.  Space: For dilution and spacing specimens, providing varied habitats
  2.  Careful stocking plan, selection, placement, order
  3.  Securing, sequestering of individuals, colonies.
  4.  Adequate feeding, lighting/regimens, circulation
  5.  Biological, Physical and Chemical filtration, augmentation, efficient skimming
  6.  Good maintenance Water changes et al.

         Treatment for Effects

  1. Moving in emergencies
  2. Water changes
  3. Use of chemical filtrants, cleaning of skimmer/s


Recognize the Signs/Symptoms of Overt Aggression/Poisoning: 

      For stinging-celled life:

  1. Polyps not opening, wilting, burned appearing, decolorizing.
  2. Skeletal septa and skeleton showing through.
  3. Slowing, ceasing, redirecting of growth.


For fishes:

Rapid or slowed breathing and movement death. 

For algae:

Bleached appearance, especially in direction of cnidarians. Slimy dissolution.  


Means of Prevention: Selection, Acclimation & Quarantine: 

            The best methods of treating these issues of negative interaction is obviously to avoid them altogether. Though not entirely possible, nor even practical, the following are valuable to prevent extreme reactions.  

Selection: Look for the best in apparently healthy specimens; these are less likely to over-react in the presence of established Cnidarian life in your syste
If at all possible, less noxious, stinging species should be placed first. 

Acclimation of New Specimens: Is best done in a plastic tray best, rinsing specimen/s individually, with clean, system water, and discarding mixed water. 



Two freshly collected stands of Sarcophytons are rinsed in a plastic tote in advance of further rinsing, processing, before being wrapped in sheets of polyethylene bagging (to prevent their touching each other) and hauled back to shore to Walt Smiths facility in Fiji.


            Quarantining: For a week or two allows you time to examine for new specimens for health, possible undesirable hitchhikers, and allows them to rest, shed slime, stinging cells, wastes...            

Space: For dilution and spacing specimens, providing varied habitats: The bigger the system/volume the better. The larger the system, the more stable and the more hodge-podge and even crowded an assortment can be. Wastes are diluted and specimens can be more widely spaced initially, giving them more time/space to grow, time to undergo classical habituation (to ignore each other).

Bob James 300 plus gal. system in Toronto with Carpet Anemone, Soft, Hard Corals Giant Clams Most animals, including fishes are 17 plus years old

Typical Nano death trap of mostly shrooms.



Careful stocking plan: Species selection, placement, order: STUDY! Only through careful studying, reading, conferring with other reefers assemble a working assortment. Buy/Procure captive-produced stocks they do get along better. Provide adequate initial, stinging, expansion and growth space between specimens. Go slowly often cnidarians get used to each other over time. Keep your system in dynamic equilibrium through regular monitoring and maintenance.  

Proper spacing and placement of a mixed hard and soft coral system

A fully-grown together reef in the Red Sea.


Securing, sequestering of individuals, colonies: Falling of specimens into each other or not must be avoided by strapping, gluing, wedging, isolating in place ON/IN a stable setting. Place rock, other structure on bottom of tank; larger, flatter pieces first. Drill, tie, anchor structure together if necessary.  

Look to the wild, photos, films for examples of

Natural arrangements.


Adequate feeding, lighting/regimens, circulation 

All cnidarians need to be fed even principally photosynthetic species/groups. Nutrition can be provided purposely or indirectly via refugium culture, ancillary feeding of other stocks, or biochemically. 

For most set-ups, mixes of livestock, lighting should be consistent. Light systems should be on timers, cycled on/off regularly.  

Circulation should be vigorous and non-laminar. On the plus side of poor water movement is the lack of chemical communication and distribution of chemical and physical materials. On the plus side of high water movement are export of same, distribution of foods, oxygen, elimination of wastes, carbon dioxide, provision of steady pH, RedOx, Biominerals  

Practically speaking there can not be too much

Dynamic circulation


Biological, Physical and Chemical filtration, augmentation, efficient skimming  

The more complex the make-up of the system, incorporating live elements, ozone/high/er ReDox, mud, reasonable levels of biominerals and alkalinity on a steady basis the better. This is accomplished with the use of refugium/s, macroalgae culture, DSBs, a regular routine of established maintenance.  


Not Cnidarians Alone: Other Phyla: A Bit More Fuel on the Fire 

Cnidarians arent (by far) the only organisms with competitive mechanisms on/in reefs All animal, algal, plant and micro-organism groups have varied means of avoiding predation, securing space, other resources Some are much more potent than stinging-celled animals e.g. Sponges, BGA About the best one can do is again set up a good-sized system properly, stock it appropriately, maintain it adequately and remember the oxymoron dynamic equilibrium in keeping it so.

And the beat goes on Our old pal, BGA twixt a Gorgonian being overtaken by the Hydrozoan Millepora.

Phyllodesmium Ever wonder why Nudibranchs can get away with being so flamboyantly colored and not be consumed?


Cloze: Overall Considerations 

At times Extreme Competition in the wild is mirrored in various fashions and degrees in captive systems.

Much overt aggression and poisoning amongst cnidarians and their tankmates can be prevented, ameliorated by:

  1. Having a large/r, better filtered, maintained system.
  2. Picking out healthy specimens, cleansing/diluting their shipping water, quarantining them.
  3. Starting SLOWLY with small/er specimens/colonies of less stinging, noxious species first, and
  4. Placing them apart to reduce awareness and interaction.

Evidence of Negative Interaction, Suggested Reactions  

  1. Can be discerned by careful observation, and needs to be readily addressed by:
  2. Moving the offending and/or offended parties, possibly out of the system.
  3. Making a large water change.
  4. Cleaning up skimmer/s and adding ozone, chemical filtrants.


Selected Biblio.  

New eBook on Amazon: Available here
Livestocking Pico, Nano, Mini-Reefs; Small Marine Aquariums
Successfully discovering, determining, picking out the best species, specimens for under 40 gallon saltwater systems.
Book 1: Principles, Algae, Invertebrates

by Robert (Bob) Fenner

Ates, R. 1989. "Aggressive behavior in corals". Freshwater and Marine Aquarium 12(8):104-105, 107, 110, 112.  

Calfo, Anthony. 2007. Book of Coral Propagation, 2d ed. Reading Trees, Pittsburg PA.

Delbeek, Charles. 2005. Reef Aquariums: Coral Compatibility. AquariumFish.com.

Dubinsky, Z. [Ed.] 1990. Coral reefs. Ecosystems of the world. V. 25. Elsevier Sci. Publ., Amsterdam. 550 p.

Hauter, Stan & Debbie, undated. Coral Competition;Turf Wars in Coral Reef Tanks. About.com. Saltwater Aquariums http://saltaquarium.about.com/library/blank/bl_CoralCompetition.htm

Hidaka, Michio. 1985. Nematocyst Discharge, Histoincompatibility, and the Formation of Sweeper Tentacles in the Coral Galaxea fascicularis. Biological Bulletin, Vol. 168, No. 3 (Jun., 1985), pp. 350-358

J. B. C. Jackson and Leo Buss. 1975. Allelopathy and Spatial Competition among Coral Reef Invertebrates. PNAS | December 1, 1975 | vol. 72 | no. 12 | 5160-5163

O. Langmeada and C. Sheppard. 2004. Coral reef community dynamics and disturbance: a simulation model. Ecological Modelling Volume 175, Issue 3, 15 July 2004, Pages 271-290  

Paletta, Michael, undated. Coral Aggression.  Pet Education.com. Dr.s Foster & Smith. http://www.peteducation.com/article.cfm?cls=16&cat=1988&articleid=2955

Sammarco, P.W., J.C. Coll, S. La Barre and B. Willis. 1983. "Competitive strategies of soft corals (Coelenterata: Octocorallia): allelopathic effects on selected scleractinian corals." Coral Reefs 2:173-178.

Sebens, Kenneth P. & Julia S. Miles. 1988. Sweeper Tentacles in a Gorgonian Octocoral: Morphological Modifications for Interference Competition. Biological Bulletin, Vol. 175, No. 3 (Dec., 1988), pp. 378-387

Shimek, Ron. 2007. Coral crowding and combat. AFI 12/07.

WetWebMedia.com: Cumulative: Cnidarian Compatibility & FAQs on: Cnidarian Compatibility,



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