A QUICK WORD ON IDENTIFICATION
Zoanthus are a genus of corals within the order Zoantharia, an order it shares withΒ PalythoaΒ andΒ Parazoanthus. You may have also heard zoanthus referred to as zoanthids which is correct, but if you want to be a stickler for details, the term zoanthids refers to all the corals in the order while zoanthus is specific to the genus.
There is quite a lot of active research on Zoanthid taxonomy and the cutting edge is DNA classification. It is not nearly as straight forward as it may seem. There is a LOT of genetic consistency so it is a chore to find small segments of DNA that are actually different enough to base a scientific classification on. Over 90% of the coralβs genome is identical so a lot of current research is delving into that. Genetic research is further complicated by the effect the environment has on the expression of the genetic code. The genes themselves donβt change but the how the organism reads the genetic code in response to the environment does so you could see two very different traits in corals originate from an identical genetic sequence. I expect quite a lot of reclassification to occur in this space.
What was once 300 identified species has been whittled down to around 50-60 in the past several years depending on the criteria used to differentiate the different morphs. There are new classification insights as more genetic testing is being done, but for the purposes of this hobby-focused website, we've chosen to arbitrarily lump larger polyp individuals into Palythoa and smaller polyp specimens into Zoanthus.
As a practical matter, there is some degree of fogginess you will encounter in this industry whether something is called a Zoanthus or a Palythoa because there is a wide range of physical variation within the genus. On the far ends of the spectrum the two are easy to differentiate but once you get into large polyp Zoanthus variants and smaller polyp Palythoa variants, the naming convention breaks down at the hobbyist level.
Β A WORD OF CAUTION
There is a need to address the potential toxicity of these corals. Some can contain a compound called palytoxin that is a very dangerous poison. Palytoxin is associated more with Palythoa than Zoanthus, but it is possible for zoas to have it. I donβt know if those zoas produce it themselves or acquire it from neighboring Palythoa. In the wild, palytoxin shows up in organisms that donβt actively produce it such as sponges, mussels, and starfish but live in close proximity to palythoa. It can even be found in other species up the food chain such as crabs and fish through biomagnification. Iβll touch on this again when we discuss pests, because as lethal as palytoxin is, there is no shortage of organisms that are more than happy to consume them with no ill effects.
The scientific community has been aware of palytoxin since the 1960βs but it has been used as a means of biological warfare by Polynesian cultures for much longer. There is a legend of a sacred seaweed that grew in a special pool that when applied to a warriorβs spear would bring sure death to his enemies. In 1961 researchers tracked down the fabled pool in Maui and found colonies of Palythoa toxica.
It wasnβt until the 1990βs that the mechanism of action was identified. Palytoxin binds to the sodium potassium pump on the membrane of cells. If you donβt remember what this is from high school biology, the sodium potassium pump is REALLY important to ion movement in and out of the cell and affects all sorts of biological processes so any interruption of that royally screws things up. It is why the symptoms of palytoxin poisoning are seemingly all-inclusive. Initially there was some skepticism regarding this mechanism of action but it was confirmed in 1995 in a pretty clever study. They exposed yeast cells to palytoxin because yeast cells donβt have a sodium potassium pump. Those yeast cells were unaffected but when given to yeast cells that researchers genetically modified to encode a sheepβs sodium potassium pump, all the cells died.
In practice you are not likely to get palytoxin poisoning just by having zoas or palythoa in your home aquarium. It resides in the flesh of the coral so it becomes an issue when the colony is damaged. People have gotten sick from scrubbing palythoa off of rocks or propagating a colony with a band saw that would send the particles airborne and then inhaled. People have also gotten sick by boiling rocks with colonies on it because that too sends the compounds airborne and unlike many other proteins, palytoxin does not lose its toxicity when heated.
Β ACCLIMATION
Proper acclimation is extremely important considering the stress imposed on the animals by the shipping process.