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Related FAQs: Zoanthids, Zoanthids 2Zoanthids 3Zoanthid ID, Zoanthid Identification 2, Zoanthid ID 3, Zoanthid ID 4, Zoanthid ID 5, & Zoanthid Behavior,   Zoanthid Compatibility, Zoanthid Selection, Zoanthid System, Zoanthid Lighting, Zoanthid Feeding, Zoanthid Health, Zoanthid Reproduction

Related Articles: Sea Mat: An Ocean Of Color For The Aquarium by Blane Perun,  Water Flow, How Much is Enough

/The Conscientious Reef Aquarist

Mostly Colonial & Very Hardy;

The Sea Mats & Polyps That Are the Zoanthids 

To: part 2, part 3, part 4

By Bob Fenner

Zoanthid colony, aquarium

Amongst the stinging-celled organisms (corals, anemones, jellyfishes…) only the coral anemones (Corallimorpharia) rate near equal in hardiness and adaptability to captive care than the various polyps and sea mats that make up the Order Zoanthidea.

Though not as spectacular as other cnidarians, Zoanthids are common and widespread over the worlds' reefs, often occupying fringe physical environments (intertidal, back reef, other shallow areas, over dead corals). This ecoclinal tolerance points up their resistance in aquariums. Their toughness in the wild is of advantage to reef aquarists looking for sturdy coral-like livestock. Their only dark side, and it can be a huge one, is their capacity to chemically outcompete other Cnidarians placed too near, or in too small a volume.

The Place of the Zoanthids in Cnidarian 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 ("Nigh-dare-ee-yah"); in some older texts as the Phylum Coelenterata

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. 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 (the coelenteron).

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. 

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A Brief Classification of the Phylum Cnidaria :

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

Order Hydroida: Hydroids and hydromedusae, Hydra.  

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

Order Stylasterina: Hydrocorals; mostly deepwater. Hydrocorals; mostly deepwater.

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

Order Chondrophora. Planktonic Planktonic

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

Order Stauromedusae: Possess stalked medusa. Possess stalked medusa.

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

Order Semaeostomeae: Plate-shaped medusae. Plate-shaped medusae.

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

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

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

Order Stolonifera: Polyps arise from a creeping mat (stolon). Skeleton of calcareous tubes. Includes Tubipora (organ-pipe coral) 

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

Order Alcyonacea: Soft corals. Fleshy, rubbery. Mushroom or variously lobate growth forms. Skeleton of separate calcareous spicules. Soft corals. 

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. 

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

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

Order Scleractinia (Madreporaria): True or stony corals. Solitary or colonial polyps with calcareous skeletons. (

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

Order Zoanthidea: Colonial anemone-like polyps. One siphonoglyph, no skeleton.

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

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

Identification/Distinction from Related Forms:

For folks who have a hard time telling Zoanthids apart from the similar appearing sea anemones (Order Actinaria) and coral anemones (Order Corallimorpharia), you're not alone. Here is a Table comparing and contrasting external features of these groups.

Physical Trait  Differences between Zoanthids, true Anemones, Coral Anemones

Group Order Zoanthidea Order Actinaria (Anemones) Order Corallimorpharia
Siphonoglyphs 1 Usually 2 None
Solitary/Colonial Usually Colonial Usually Solitary Usually Solitary
Tentacles Usually 2 Rows, none around mouth Eight to Hundreds, have around mouth Radially Arranged, have around mouth
Tentacle Arrangement Poorly Developed Well Developed Poorly Developed
Body Column Button-Like Stout Usually Flat
Physical Appearance Usually Small, under 0.5" Well-Supported Poorly Developed

All three Orders members lack the heavy external calcareous skeletons of true or stony corals (Order Scleractinia); they're entirely soft-bodied polyps. In terms of mouth/body openings Zoanthids bear a single ciliated groove (called a siphonoglyph) at their edge, versus two in most anemones and none in the coral anemones.

Only the Zoanthids are truly colonial, sharing nutrient and nervous impulses directly through their membranous mats (hence some of their common names, sea mats) or stolonic basal connections. Each anemone and Corallimorpharian polyp is independent, though some may appear joined at their bases.

The true anemones have robust bodies and tentacles. Zoanthids are generally small (less than an inch in any dimension), button-like with two small rows of weak tentacles at their polyp margins. Corallimorphs have series of small tentacles radially arranged across their oral surface. (see images).

Species of Interest to Aquarists:

Zoanthids are sold as "polyps" of different sorts, mats and more creative names. There are several species offered year round from all tropical oceans.

Zoanthus sociatus, from the Caribbean, is relatively large (1.2" polyps) with 60 short tentacles; bright green when in good health. A few species of the genus Palythoa are also often collected out of the Western Atlantic for the trade.

Zoanthus danae from the Pacific is another excellent species that is commonly offered.

Several other Zoanthus, Parazoanthus and Palythoa spp. are offered out of the Pacific around Southeast Asia. The most popular by far is Parazoanthus gracilis imported from Indonesia. A similar, almost as pretty congener is P. swiftii, out of the Atlantic. (images)

Genus Palythoa:

Palythoa caesia Dana 1848. Rubbery appearing common mat, flattened polyps exposed to varying degrees to 3 cm. diameters. Tentacle ends look like knobs. Dark brown to tan in color. Western Pacific; Philippines, Indonesia. N. Sulawesi, and Hawai'i pix. 

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To: part 2, part 3, part 4

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