#anthozoan

Medusa Anemone Peachia neozelanica

A sea anemone that attaches itself to medusa jellyfish, found near Aotearoa New Zealand.

Anthozoa is a class under Cnidaria which includes marine invertebrates like sea anemones, sea pens, stony coral, and soft corals, it has 10 orders and thousands of species!! Anthozoans completely lack a medusa stage they are only ever polyps throughout their life. They’re one of the most popular Cnidarians due to some being important reef building coral species. There is many subclasses and orders under Anthozoa. 

Sea anemone

Even though Anthozoans are mostly stuck as sedentary polyps they still make up about two-thirds of cnidarian species. Stony corals, tube anemones, sea pens, sea fans and of course the marvelous sea anemones are all anthozoans. Most of the times the evolutionary relationship between different types of anthozoans is based on things like:

 Colony morphology and life history

The shape and number of tentacles

The arrangements of divisions in the gastrovascular cavity (mesenteries, which will make a comeback later….)

Nematocyst structure

Skeletal structure

 Most anthozoans are colonial and basically our original polyp buds off into smaller polyps—vegetative propagation style! But there are some anthozoans that reproduce sexually. So, what does an anthozoans look like in general? They tend to have a tubular body with one mouth and a halo of horrifying tentacles surrounding said mouth. The tentacles are actually hollow (like my soul) and they connect to the collection (AKA the gastrovascular cavity). The layerbetween the endoderm (inner layer) and ectoderm (outer layer) is pretty thick in anthozoans. The muscles of the mesenteries allow the poly to retract and empower some anemones to swim in really dorky ways. 

The mesenteries have little extensions known as mesenterial filaments which basically increase surface area for digesting food respiring and killing whatever poor prey is still trapped inside and alive within the coelenteron. The mesenteries are packed with cilia, gland cells and the incredible cnidae.

One of the ways in which sea anemones are similar to their close bestie the anemone fish is in terms of how some anthozoans can change their sex during their life time. Not only that but some anthozoans are hermaphrodites. We can continue to split up Anthozoans into sub categories such as Octocorallia (e.g soft corals, gorgonians, blue corals, and sea pens) and Hexacorallia (e.g. anemones, stony corals). How are these creatures divided into these two major subclasses? It’s based on the number of branching tentacles and complete mesenteries. Octocorallia has eight pinnately branched tentacles and complete mesenteries on the other hand Hexacorallia has six pinnately branched tentacles and complete mesenteries. However many anthozoans don't fall into these two categories. Some are classified as Antipatharia and Ceriantharia, but the quest to iron out the phylogeny of Anthozoans continues.

Understanding anthozoans and cnidarians in general is important from an evolutionary perspective, because they are some of the oldest creatures. Studying cnidarians and anthozoans could give insight into things that are common in a lot of animals (e.g. biological clocks, immunity,  nervous system etc.)

Next time I’ll dive into the anatomy of an evil mastermind.

Citations

Berntson, E. A., Bayer, F. M., McArthur, A. G. & France, S. C. Phylogenetic relationships within the Octocorallia (Cnidaria: Anthozoa) based on nuclear 18S rRNA sequences. Mar. Biol. 138, 235–246 (2001).

Jouiaei, M. et al. Ancient venom systems: A review on cnidaria toxins. Toxins (Basel). 7, 2251–2271 (2015).

Parazoantus | ©Marcello Di Francesco

Parazoanthus is a genus of anemone-like zoanthid coral, known as Cluster anemone.

While most corals grow as colonies in tropical waters, the Devonshire cup coral (Caryophyllia smithii) is solitary and lives in temperate parts of the ocean. It grows with its cup-shaped skeleton attached to a rock or even a shipwreck. When the tentacles are expanded, these tiny corals look just like anemones, with each tapering, transparent tentacle ending in a small knob. Devonshire cup coral often occurs in a variety of corals from white or orange.