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/Diversity Series

Cnidaria: Stinging Celled Animals

By Bob Fenner

Stichodactyla gigantea

With the explosion of interest in "reef" and other specialized types of marine systems there has been a tremendous (re)surgence of interest in anemones, corals and other Cnidaria for captivity. Many species have proved hardy/practical. Like other invertebrates and fishes, success rests in proper collection and distribution and careful estimation on the consumer's part in selection, habitat, placement and feeding.

This overview presents notes on what these animals are, how to go about picking out healthy specimens, keep them alive, and some items on their interesting biology.

What: Classification:

Bothersome hydras, jellyfish, sea anemones, tube anemones, reef and non-reef-building corals, soft and hard, black, horny and stony, sea pens, sea pansies , sea wasps, Portuguese men of war and sea fans; these are the stinging-celled animals, the Cnidaria.

These organisms are radially symmetrical (hence inclusion in a sub-Animal Kingdom, the Radiata), are made up of two basic tissue layers and possess salient "stinging" cells. These are grouped as thread-like (volvent) barbed/spined with and without toxins (penetrant), and "sticky" for anchoring (glutinant).

Their body shapes come in two basic formats; a sessile polyp-like and free-swimming medusa-like (illustration). The basic features of these body-types are evident; most species are either one or the other, medusoid or polypoid, others pass through both in their life cycle. The body cavity formed by the tissue layers bears series of tentacles around a single body/mouth/"anus" opening/mouth.

There are few freshwater members (e.g. Hydras), most are marine in shallow tropical waters, though they are found worldwide at all depths. There are some ten thousand described species with a rich fossil history dating back before the Cambrian period.

Phylum Cnidaria (Coelenterata). Three Classes

Class Hydrozoa: simple, tube-like stomach. Small medusa stage and a small to large polypoid stage.

Order Hydroida: Hydroids and hydromedusae, Hydra.

Lytocarpus nuttingi at right, Puerto Vallarta 2015. A Sea-fern Hydroid

Order Milleporina: stinging corals. Have very heavy calcareous skeletons. Millepora

A typically fawn-colored Millepora colony. Here in the Red Sea, though worldwide in tropical oceans.

Order Stylasterina: Hydrocorals; mostly deepwater.

Distichopora violacea, Red Sea

Order Siphonophora: Typically planktonic, colonial. Physalia, the "sailor by the wind".

                    Order Chondrophora. Planktonic

        Class Scyphozoa: Jellyfishes. Have four-chambered stomachs. Most have large medusa and small polyp stages. @ 200 species

                    Order Stauromedusae: stalked medusa

                    Order Cubomedusae: Sea wasps. Medusae with four groups of tentacles.

Order Semaeostomeae: Plate-shaped medusae.

                    Order Rhizostomeae: Lack tentacles on the margin of the bell. Cassiopea

        Class Anthozoa: Polyp stage only, stomach divided into numerous compartments.

            Subclass Octocorallia/Alcyonaria. Octocorals. Anthozoa with eight-multiple tentacles. Almost all colonial.

Bigger PIX: The images in this table are linked to large (desktop size) copies. Click on "framed" images to go to the larger size.
An unknown... Octocoral sp.

                    Order Stolonifera: Polyps arise from a creeping mat (stolon). Skeleton of calcareous tubes. Includes 

                        Tubipora (Organ-Pipe Coral)

                        Star Polyps, Clavulariids  

                    Order Telestacea: Lateral polyps on simple or branched stems; skeletons of calcareous spicules.                

Order Alcyonacea: the Soft Corals. Fleshy, rubbery. Mushroom or variously lobate growth forms. Skeleton of  separate calcareous spicules. At right, a colony of the Xeniid Anthelia glauca in the Red Sea.

Order Coenothecalia: Comprised of only the Indo-Pacific Blue Coral, Heliopora. Massive calcareous skeleton.                                

Order Gorgonacea: Sea fans, Sea Whips, precious red jewelry coral (Corallium). Long stiff internal skeletons.    Horny, upright plantlike growth. Colonial. The Whip Coral Juncella juncea, family Ellisellidae in the Red Sea at right.                   

                    Order Pennatulacea: Sea Pens. Colonies as fleshy, flattened or elongate. Anchored in mud or sand bottom. 

            Subclass Zoantharia (Hexacorallia): Solitary or colonial. Eight or more tentacles, multiples of six. Reef and non-reef building corals, anemones.                  

Order Actinaria: Sea Anemones. Solitary or clone polyps without a skeleton. Two siphonoglyphs.         

Free Anemones! Whee! Aiptasia.

Order Scleractinia (Madreporaria: True or Stony Corals. Solitary or colonial polyps with calcareous skeletons.     

Agaricia lamarcki

                    Order Rugosa, or Tetracoralla. An extinct coral order.

                    Order Tabulata: Extinct, colonial. Heavy calcareous skeletons with horizontal platforms (tabulae).

                    Order Ceriantharia: Tube Anemones, elongate tapered bodies. Live in secreted mucus tubes.

                    Order Zoanthidea: Colonial Anemones. One siphonoglyph, no skeleton.

                    Order Antipatharia: Black Corals. Colonial. Polyps arranged around an axial skeleton.

                    Order Corallimorpharia: False Corals or Coral-Anemones, Mushrooms. Solitary or colonial, flattened coral-like           anemones. Tentacles radially arranged. Look like true corals, but lack skeletons.


General Notes re Selection: I concur with John Tullock; the cleanliness, knowledge and helpfulness of a given dealer is a good indication of the potential chance of procuring decent stock from them.

More specific information per group, species is offered through succeeding articles and in the references below. As a rule:

1) Purchase specimens that have been "hardened" for a week or two at your dealers. A small deposit should ensure timely holding.

2) Observe the specimen feeding at the dealers. Are all "parts" fully operational?

3) Has the specimen been "colored" artificially? Do you care?

4) Smell the water and the critter itself if possible. Good one's smell okay and vice versa.

5) Check for necrotic, rotting "whitish" areas, evidence of regrowth/colonization. Torn, dissolving sections are indications of almost certain doom/failure.


Another highly variable area. Some groups tolerate "poor" water quality. As an example, tube anemones (Order Ceriantharia) are found thriving in some of the muckiest muck. Others seem very sensitive to metabolite build-up, withdrawing physically, not feeding in the presence of low concentrations of nitrates, low re-dox potential...


You'll have to (if you want to keep them alive) and want to (unless you're super-wealthy) read up on your new charges requirements and meet them more than half way.

Some organisms are attached (e.g. sea fans, leather corals) and must/should come with that part of their natural substrate. This is in turn wedged into an appropriate area. Tube anemones and other anthozoans demonstrate preferences for depths, sizes and textures of gravels.


Cnidarians of all living orders have been maintained in synthetic water in captive systems. The general rules apply. Shallow water varieties are more tolerant of environmental range and rapid change, metallic contamination and treatment is a no-no. Frequent partial water changes, with higher/normal (@1.025) specific gravity, temperature (@75-82 degrees for tropical systems), higher pH's (8.0-8.2 plus) will support initially healthy individuals. For folks who can afford other windows for checking-in (monitoring) or adjusting water quality, it all helps. Trickle filters, protein skimming with or without ozone, chemical filtration, some to more "live-rock" are my notable favorites.

Lighting can be important in terms of strength, duration and quality. Some stinging-celled animals are not-at-all desirous of being in the lime-light. Deeper water, shielded, colorless specimens are included here. Alternately, endo-symbiotic algae-containing species (like reef-building corals) require lots of strong, long day-cycle, light of proper wave-length mix.

Water movement; some species need it rough, some-don't. Sea fans utilize currents extensively for growth, feeding and reproduction. Other groups do fine in still waters, others require it.

Buffering capacity/alkalinity I will make a special mention of. Calcareous species can absorb vast quantities of carbonates, sulfides, fluorides et al of alkaline earth metals. Water changes may not be enough to keep up with healthy growing systems. A test-kit and augmentation may well be warranted.


Under-crowding and under-feeding are the best adjuncts to filtration for all types of captive systems. Cnidarian specialty systems may be adequately filtered through all existing popular modes. The more carrying capacity and redundancy you can afford, the wider the margin of safety/disaster-proofedness you will enjoy. I personally prefer an outside canister filter in concert with a functioning protein skimmer/fractionator and an absence of undergravel filter use.


For tissue-grade life that seems to forever live in the slow lane, coelenterates display an amazing diverse/rich repertoire of activity. They may move slow, but they are decidedly aware of and interacting with their living and non-living environment.


Can be extremely important. Many corals and mobile cnidarians; anemones, tube anemones, sea fans (by waving), can get themselves, their tankmates and you harm by stinging/waging digestive dominance/territorial "war" on each other. Buffer zones between unknown quantities/qualities and adequate studying-up is warranted before purchase or introduction.


Once again, in very general terms:

1) Handle as little as possible. Orient the specimens where you want, if they move, leave them alone.

2) Do not lift them into the air. Some are normally exposed in the wild, most are not. When lifted they tear, take in gas, basically get over-stressed. Where possible, slip them into the transport bag/container underwater.

3) On that note, move them with care. They shouldn't rub themselves raw, nor roll around due to too large/small a device.

4) Do not allow the shipping water into your system. Deleterious changes may have occurred during transport, even if there were no pests, parasites or pollution from the source. "Mixing" of water before release is of no practical use and is often harmful as a function of ammonia toxicity coupled with high pH.

5) Don't over-react if a specimen seems flaccid, seems un-eager to feed... Check system parameters, do a partial water change, change foods, temperature, specific gravity..., but mainly just be patient. Unless other tank-mates seem ill-effected, leave the animal alone. They can and do go for long periods without doing much.

6) I do not regularly endorse quarantine or prophylactic "dipping" of stinging-celled organisms. Both these practices, on average, do more harm than good with this group.

Predator/Prey Relations

Ates (1991) presents a recent survey of coral and anemone eaters. Cnidarians are known to be consumed to some degree by most all animal groups. Many worms, crabs, starfishes, snails, nudibranchs should be left out of captive systems or at least closely watched. Of fishes, triggers, most large angels, some butterflies, Parrotfishes and Puffers are to be avoided

Corals and anemones are known to eat/digest other species/genotypes not to their liking.

Cnidarians are opportunistic omnivores and will at least attempt to ingest/digest most anything that comes their way, including unfamiliar geographic types, nearly planktonic (e.g. Seahorses) species.  


See above paragraph. Where in doubt, which is where I'm at perennially, underfeed or less. Most all members of the phylum are filter-feeders of zoo- and phytoplankton. A lot of this stuff is really small, hence the designation nano-plankton. Some get a good deal of their nutrition through photosynthetic co-op, some through slime-farming sediment, offal. Some species/specimens will eat whole organisms, shrimp, fish, beef, poultry, don't do it!

Get in the habit of preparing finely divided fresh, frozen or prepared mashes with or without expensive adjuncts and administer these while temporarily suspending particulate filtration.


A real "grey" area at this time. See above for selection criteria, suitable environment and acclimation pointers.


My how time flies. Didn't even mention reproduction, neurobiology, locomotion, symbioses...Read on.

Bibliography/Further Reading

Marine Hitchhiker/Critter ID (Maughmer, Toonen, Tompkins)

Ates, Ron. 1991. Fishes Eating Corals. FAMA 7/91.

Baensch, Hans & Helmut Debelius. 1994. Marine Atlas, v.1. MERGUS, Germany. 1215pp.

Barnes, Robert D. 1974. Invertebrate Zoology. W.B. Saunders Co.

Friese, U. Erich. 1972, Sea Anemones. T.F.H. Publications

Tullock, John. 1989. Selecting Healthy Marine Invertebrates, Part 1- Coelenterates other than corals. Marine Fish Monthly 4/89

Wells, J.G.. 1982. Encyclopedia of Marine Invertebrates. T.F.H. Publ. Neptune City, New Jersey.

Wilkens, Peter. 1977. Flower Animals, More Flower Animals. Marine Aquarist, 7:9,10 1977.

Wilkens, Peter. 1990. The Mysterious World of Anthozoans, Parts 1 & 2. 7,8/1990.

Wood, Elizabeth M. 1983. Corals of the World. T.F.H. Publ.

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