Zoanthids (order Zoantharia also called Zoanthidea or Zoanthiniaria) are an order of cnidarians commonly found in coral reefs, the deep sea and many other marine environments around the world. These animals come in a variety of different colonizing formations and in numerous colors. They can be found as individual polyps, attached by a fleshy stolon or a mat that can be created from small pieces of sediment, sand and rock. The term "zoanthid" refers to all animals within this order Zoantharia, and should not be confused with "Zoanthus", which is one genus within Zoantharia.
The name of the order is controversial. Non-specialists often use the term Zoanthidea whereas most taxonomists use Zoantharia. The term Zoantharia in turn is used temporarily instead of Hexacorallia. However, major taxonomic papers published since 1899 by specialists (O. Carlgren and F. Pax have described more species than all other authors combined) use Zoantharia, and most recent specialists on the order  continue to use the term Zoantharia.
Zoanthids can be distinguished from other colonial anthozoans and soft coral by their characteristic of incorporating sand and other small pieces of material into their tissue to help make their structure (except for the family Zoanthidae). The main characteristic of the order is that their tentacles are organised in two distinct rows.
While the most well-known zoanthids are the zooxanthellate genera found in tropical and sub-tropical waters (primarily Zoanthus and Palythoa), many other species and genera exist, some still relatively unknown to science. Many zoanthids (in particular the genera Epizoanthus and Parazoanthus) are often found growing on other marine invertebrates.
Often in zooxanthellate genera such as Zoanthus and Palythoa there are a large number of different morphs of the same or similar species. Such zooxanthellate genera derive a large portion of their energy requirements from symbiotic dinoflagellates of the genus Symbiodinium (zooxanthellae), similar to many corals, anemones, and some other marine invertebrates.
Families and genera
The families and genera within the order Zoantharia (also known as Zoanthidea) are:
* Family Abyssoanthidae
Additionally, there are other zoanthid genera such as Neozoanthus or Paleaozoanthus for which there are currently only few data available, those zoanthids having never been found again since their original description.
Some zoanthids contain the highly toxic substance palytoxin. Palytoxin is one of the most toxic organic substances in the world, but there is an ongoing debate over the concentration of this toxin in these animals. However, even in small quantities, the toxin can be fatal should it be ingested or enter the blood stream. If delivered immediately, vasodilators injected into the ventricle of the heart can act as an antidote.
In order for this toxin to be dangerous to humans, the average aquarist would need to ingest the zoanthid in sufficient quantities, or brush a recent cut over it. Average handling, propagation and aquarium maintenance is unlikely to pose any danger beyond a localized skin reaction.
Palytoxin is a tumor promoter, and is being studied in relation to signaling pathways in skin cancer genesis.  Contrary to common belief, palytoxin can be absorbed through intact skin.  The danger of acute poisoning from venomous zoanthids is quite real. An aquarist was poisoned through skin injuries on fingers by a Parazoanthus species, but recovered after 3 days. His zoanthid was found to contain 2-3 milligram of palytoxin per gram.  For comparison, the intravenous LD50 dose of palytoxin for a grown man is less than 8 microgram. Thus each gram of the offending zoanthid contained enough venom to kill at least 125 grown men.
Zoanthids feed both by photosynthesis, aided by the zooxanthellae they contain, and by capturing plankton and particulate matter. Although photosynthesis aids in their nutrition, even species that do not actively capture plankton cannot live through photosynthesis alone. Zoanthids can eat meaty foods, such as lancefish, brine shrimp, krill and bloodworms.
1. ^ Sinniger F., Montoya-Burgos J.I., Chevaldonne P., Pawlowski J. (2005) Phylogeny of the order Zoantharia (Anthozoa, Hexacorallia) based on the mitochondrial ribosomal genes. Mar. Biol. 147 (5): 1121-1128.
Source: Wikispecies, Wikipedia: All text is available under the terms of the GNU Free Documentation License