Science necessarily needs to define and classify what it studies. Prey or predator? It was surely one of the first questions the human being asked to survive…
Taxonomy is the scientific practice aiming at the classification of live beings in ranks previously defined by Carl Von Linnaeus: kingdom > phylum > class > order > family > genus > species. This is an essential step for our understanding of biodiversity as well as to identify species with economic value or direct human significance such as anticancer properties, a topic widely developed at Coral Biome! Coral taxonomy began with the description of Tubipora musica (organ-pipe coral), the first coral species named by Linnaeus in 1758. Nowadays, while the taxonomy of some coral genera and species seem well established, it remains very difficult to name and classify many others. Indeed, numerous species descriptions are based on morphological criteria that are often subject to intraspecific plasticity. However, in the past few decades, biologists are more and more using genetic tools and recent classifications based on these molecular analyses frequently challenge the traditional views based on morphological data. This is the topic of the present article dealing with the hundreds of distinct Zoanthus colour morphs venerated by aquarists but likely belonging only to a few genetically distinct species.
Who loves Zoanthus?
Zoanthus make us crazy because of their amazing fluorescent colour patterns. This diversity of colour associated with a few anatomical variations was traditionally considered by the scientific community as “morph diversity” representing different species. Consequently, more than 100 Zoanthus species were described (Fautin, 2003) on the basis of their colour patterns and morphological measurements such as counting the number of septa in the gastric cavity, or the form and shape of the polyp sphincter muscle. More recently, Dr James Reimer, a world expert on Zoanthus taxonomy, systematically used DNA markers and genetic analyses. Thanks to his results, he reconsidered the validity of some Zoanthus species: one of his main conclusions is that there are not so many and that you surely know all of them by their scientific name! Indeed, the researcher and his team showed that some described species were just redundant names of the same species (Reimer et al., 2004, Reimer et al., 2012). Results also highlighted the existence of sibling species pairs with members living either in the Caribbean or in the Indo-Pacific basin.
So, the two species belonging to each pair can only be distinguished thanks to their collection location and to a little extent their genetic features. Even if DNA-based approaches cannot unambiguously separate them so far, they are (or will be) distinct species. Since Panama’s isthmus split up the Atlantic and Indo-Pacific basins, populations of many marine species have been separated from either side. This geographic isolation induces allopatric speciation of these populations because they are genetically isolated one from the other. On each side of Panama’s isthmus, Zoanthus species are following their own evolutionary history without any genetic mixing. For now, they are called « sibling species pairs » because we are not able to unambiguously identify them with any molecular techniques, but only with their collection location.
How many Zoanthus species are there?
Definitely not as much as we thought! Results thus far indicate perhaps less than ten Zoanthus species are valid so far. A lot of colour morphs are only local populations or variants of the same species. The situation is similar when considering Palythoa species…
Which species does my Zoanthus belong to?
Calm down! We are not going to identify the hundreds of Zoanthus colour morphs known in the hobby and list their scientific names, but only focus on a few examples to give you clues for identification of your own specimens. Let’s try to identify the four known sibling species pairs listed in the table 1 below.
Table 1: Distribution of Zoanthus sibling species
between Caribbean sea and Indo-Pacific bassin
Zoanthus sociatus and Zoanthus sansibaricus sibling species pair constitute one of the most commonly kept groups in the hobby. There are many colour morphs in both species such as ‘Alien eye zoa’, ‘Superman zoa’, ‘LA Lakers zoa’, ‘Bambam zoa’ or ‘Rasta zoa’. Polyps are interconnected and are about 6 to 10 mm in diameter when open and expanded (Reimer et al. 2004). Closed polyps often show pink ring around the body. Here are some examples:
Zoanthus solanderi and Zoanthus gigantus are another famous sibling species pair, aka ‘People Eaters zoa’. Indeed, they are large sized, about 10 to 25 mm in oral disk diameter. They often exhibit a neon green mouth and generally present a striped pattern on the closed polyps (Reimer et al. 2006a). They also show this striped pattern on open polyps, and when you look closely at these gorgeous polyps, you will often see a half radial pattern from the mouth. Too many people refer to these large Zoanthus polyps as Palythoa… But of course THEY ARE NOT! You surely saw the usual Red/Purple/Green People Eaters but a lot of other colour morphs exist. Some examples in the pictures below:
Zoanthus pulchellus and Zoanthus vietnamensis are also very appreciated by hobbyists. Within this species pair are ‘Armor of god zoa’, ‘Rainbow incinerator zoa’, ‘Shazam zoa’, ‘Utter Chaos zoa’, and a lot of other insane colour morphs. Polyps are 4-6 mm in diameter with ‘a slightly larger oral disk’ according to Reimer et al., 2012. You can easily find some specimens equivalent in size to the ‘People Eater’ species. But their size is generally in the middle between the two previous species pairs. They usually have white mouths with a radial strip along the mouth axis. Some colour morphs from this species pair:
Zoanthus aff. pulchellus and Zoanthus kuroshio are a sibling species pair which especially holds our attention at Coral Biome. Indeed, Z. aff. pulchellus is the iconic Vice Zoa from Florida with the famous ‘Miami Vice’ zoa (Blue with Pink oral discs). This is an undescribed species for now but it is likely phylogenetically very close to Z. pulchellus. They occur in a large variety of colour morphs while Z. kuroshio are often pink. Both show a radial strip along the mouth axis. Vice Zoas are so named because they create the tightest mat of any Zoanthus species, creating gorgeous polygonal colonies. Some of our favorite colour morphs from this species pair:
Take care not to confuse Zoanthus and Palythoa!
Zoanthus do not incorporate sand (and other debris) into their polyps as Palythoa do. That provides a rough texture to Palythoa while Zoanthus feel smooth to the touch. In addition, Palythoa are capable of secreting large amount of mucus-like substances that confer a slimy body surface. The confusion in the hobby is mostly due to incorrect use of ‘Paly’ to refer to large-sized Zoanthus species. We strongly discourage this mistake because it creates some confusion between the otherwise easily distinguished genera. By the way, you can also forget the genus Protopalythoa, which was challenged by Reimer et al. (2006a) and formally died with Low et al. (2016).
Considering all these statements mainly based on the most up-to-date scientific knowledge, we will try to include in our e-shop the right species names in the Zoa category. Feel free to contact us with pictures of your own specimens if you need help in identification (Zoanthus or Palythoa).
Fautin DG (2003) Hexacorallians of the world.http://hercules.kgs.ku.edu/hexacoral/anemone2/index.cfm
Low, M. E. Y., Sinniger, F., & Reimer, J. D. (2016). The order Zoantharia Rafinesque, 1815 (Cnidaria, Anthozoa: Hexacorallia): supraspecific classification and nomenclature. ZooKeys, (641), 1–80. Advance online publication.
Reimer JD, Ono S, Fujiwara Y, Takishita K, Tsukahara J (2004) Reconsidering Zoanthus spp. diversity: molecular evidence of conspecifity within four previously presumed species. Zoological Science, 21(5): 517-525.
Reimer, J. D., Ono, S., Takishita, K., Tsukahara, J., & Maruyama, T. (2006a). Molecular evidence suggesting species in the zoanthid genera Palythoa and Protopalythoa (Anthozoa: Hexacorallia) are congeneric. Zoological science, 23(1), 87-94.
Reimer, J. D., Ono, S., Iwama, A., Takishita, K., Tsukahara, J., & Maruyama, T. (2006b). Morphological and molecular revision of Zoanthus (Anthozoa: Hexacorallia) from southwestern Japan, with descriptions of two new species. Zoological science, 23(3), 261-275.
Reimer, J. D. (2008). Implications for different diversity levels of Symbiodinium spp.(Dinophyceae, Suessiales) within closely related hosts: zoanthids (Cnidaria: Hexacorallia: Anthozoa) as a case study. Galaxea, Journal of Coral Reef Studies, 10(1), 3-13.
Reimer, J. D., & Todd, P. A. (2009). Preliminary molecular examination of zooxanthellate zoanthids (Hexacorallia, Zoantharia) and associated zooxanthellae (Symbiodinium spp.) diversity in Singapore. Raffles Bulletin of Zoology, 22, 103-120.
Reimer, J. D., Foord, C., & Irei, Y. (2012). Species diversity of shallow water zoanthids (Cnidaria: Anthozoa: Hexacorallia) in Florida. Journal of Marine Biology.