Anthozoa | Flower Animals, Anemones, & Corals Explained

[00:00:06]With four of the five major classes of phlyum  Cnidaria covered, there is just one left, Anthozoa. Anthozoans, or “flower animals”, are  all Cnidarian polyps that lack a medusa stage.

[00:00:22]There are three subclasses of Anthozoans;  Hexacorallia, which includes over 4,300 species of sea anemones, hard corals, and many others,  Octocorallia which includes the horny corals, sea fans, sea pens, and soft corals, and  Ceriantharia, which includes the tube anemones.

[00:00:46]At first glance, the tube anemones  look very similar to the sea anemones, but the two are actually quite different. Unlike  sea anemones, the tube anemones have completely unpaired septa and can withdraw into the fibrous  tubes made of mucus and sticky substances excreted by their glutinant type nematocysts. In addition,  the tube anemones have two crowns of tentacles, an outer crown of long extended tentacles and  an inner crown of smaller erect tentacles.

[00:01:20]These Cnidarians have notoriously  cryptic life histories and some, like Isarachnanthus nocturnus have extensive  larval stages that closely resemble medusae.

[00:01:34]Meanwhile, all members of Octocorallia are  colonial, connected by a system of gastrodermal tubes known as solenia. Each polyp has exactly  eight tentacles arranged around the mouth and eight septa arranged within the gastrovascular  cavity. Though different species of octocorallian polyps may look similar, the structures they  form are incredibly diverse. Compare for example, the venus fan, to the feathery sea pen. Or  compare the bamboo corals, to the blue coral.

[00:02:11]Collectively, these animals are sometimes called  “soft corals” because they lack the distinctive stony skeleton of the reef-building corals. The sea pens, named for their resemblance to old fashioned “quill pens,” are unique  among the anthozoans in that instead of having relatively morphologically uniform  polyps, a sea pen’s polyps are specialized.

[00:02:35]A single polyp develops into a rigid, erect  stalk, or rachis, loses its tentacles, and forms a bulbous root-like structure, or peduncle,  at its base. This polyp then reproduces asexually to produce branching polyps from its central  stalk that form siphonozooids, for water intake and respiration, autozooids with nematocysts for  feeding, and gonozooids for reproduction. The entire colony is fortified by calcium carbonate  in the form of spicules and a central axial rod.

[00:03:16]Some sea pens, like the orange sea pen, survive  best at depths between 14 and 225 meters, while others, like the Umbellulas can survive  at depths of about 200 meters to over 6,100 meters below sea level, where they  feed on planktonic organisms. Others, like the strikingly colored and bioluminescent sea  pansy are found only in shallow coastal waters.

[00:03:46]Now, unlike members of Octocorallia which have  eight-fold symmetry, members of the subclass Hexacorallia have six-fold symmetry. They  are further divided into five extant orders: Actiniaria, the sea anemones, Antipatharia, the  black corals, Corallimorpharia, the false corals, Scleractinia, the stony corals, and  Zoantharia, also referred to as Zoanthidea or Zoanthiniaria, which includes the zoanthids. The sea anemones of order Antipatharia are named after the terrestrial flower of the same name.  They range in size from 5 to 200 millimeters, though some, like Mertens’ carpet sea anemone, can  grow up to 1 meter in diameter. Though anemones are more common in warm water, they are also found  on substrate in coastal seas all over the world.

[00:04:47]In general, sea anemones are cylindrical polyps  that are larger, heavier, and more muscular than hydra polyps. Their small slit-like mouths  are positioned at the top of their oral disk and surrounded by tentacles. Their mouths  lead into a pharynx which empties into the gastrovascular cavity. Their gastrovascular  cavities are further divided by complete septa into six radial chambers, and each chamber  is partially divided by smaller, incomplete septa that further increase the surface area of  the gastrovascular cavity to aid in digestion.

[00:05:26]Sea anemones are some of the most  muscular Cnidarians. Their well-defined, longitudinal retractor muscles stretch from  their pedal disc, which is used for attachment, to their oral disk. Many species also have  longitudinal epidermal fibers within the tentacles and oral disk. These muscles allow them to  actively expand and contract their tentacles when searching for prey, which they paralyze  with nematocysts and drag into their mouths for digestion. Their musculature also  allows them to glide along the substrate, contract and withdraw their tentacles and  oral disks, and even swim, more or less, by utilizing rhythmic bending movements. Anemones are perhaps most famous for their symbiotic relationship with  anemonefish, in particular the clown fish. Anemonefish and their hosts have a  long history of mutualistic coevolution.

[00:06:28]The sea anemone protects the anemonefish from  predators, as well as providing food through the scraps left from the anemone’s meals. In addition,  occasional dead anemone tentacles function as a safe nest site. In return, the anemonefish defend  the anemone from its predators and parasites.

[00:06:51]Anemonefish are highly host-specific, and  different species prefer different anemones.

[00:06:57]The most common anemones to host anemonefish are  the carpet anemones, the magnificent sea anemones, and the bubble-tip anemone. The specific mucus  that the anemonefish secrete does not cause host anemone nematocysts to discharge. Anemonefish  that had their mucus removed experimentally were killed by their host anemones. Other  mutualisms include the anemone shrimp, the boxer crab, and the anemone hermit crab which  appear to pick and choose specific anemones.

[00:07:36]Some anemones are monoecious, that is they are  male and female at the same time, while others are dioecious, meaning either male or female.  The monoecious anemones are also protandrous, they develop first as males that produce  sperm, and later as females that produce eggs.

[00:07:58]Their gonads are arranged within their septa where  they are fertilized externally by species that spawn eggs, and internally by species that allow  fertilization to occur within the gastrovascular cavity. Fertilized eggs develop into ciliated  zygotes that swim or drift to suitable substrate.

[00:08:19]Many sea anemones can also reproduce  asexually by budding, longitudinal fission, transverse fission, or pedal laceration, where  small pieces of the pedal disk break off as the animal moves and regenerate into new anemones. Though the anemones play important ecological roles, they do not rival the ecosystem building  capacity of the hexacorallian corals of Order Scleractinia. These include the “true,” or  “stony,” corals also known as hermatypic, or reef-building corals. A close-up of an  individual coral polyp resembles a miniature sea anemone. And like the sea anemones, their  gastrovascular cavity is divided into multiples of six. However, unlike sea anemones, they have no  pedal disk and instead live in secreted calcareous cups that project into the polyp between its true  septa. Superficially, stony corals resemble the soft corals. But they are unique in their calcium  carbonate skeletons, rather than chitinous, which give them the capacity to build some of the most  productive of all ecosystems, the coral reefs.

[00:09:38]In many corals, the secreted calcareous skeleton  can become massive, and individual coral polyps form a living, connected tissue  sheet that covers the surface.

[00:09:50]The ectodermal cells of hermatypic corals  contain high concentrations of symbiotic, unicellular, photosynthetic dinoflagellates  known as zooxanthellae, up to 5 million per square centimeter in fact. Up to 50% of the  organic compounds produced through photosynthesis by the dinoflagellates are utilized as food  for the coral polyps. The dinoflagellates also supplement their hosts with oxygen as a byproduct  of photosynthesis. This oxygen, combined with additional energy derived from sugars produced  by zooxanthellae enables these hermatypic corals to grow at a rate that is up to three times  faster than similar species without symbionts.

[00:10:40]The black corals, or horn corals, from the  order Antipatharia, are deep-water soft corals generally distinguished from other corals  by their polyp-filled black or dark-brown skeletons lined with small spines. Black  corals are found around the world from subtidal to abyssal depths, though they are most common  in tropical and subtropical waters at depths greater than 50 meters, where competition for  space is less fierce. Within suitable habitats, sessile organisms must compete for space, and as  such, have evolved a wide range of adaptations to hold off invaders. Black corals, on the  other hand, escape such competition by occupying deep-sea habitats with strong currents that are  generally avoided by other sessile organisms.

[00:11:33]They survive at such depths as suspension feeders,  with strong holdfasts, that mainly prey upon passing zooplankton they capture with their  nematocysts. In addition, deep-sea black corals, in particular, may be some of the oldest colonial  organisms on the planet. At least one sample of Leiopathes glaberrima found at 450 meters  below sea level, was over 4,200 years old.

[00:12:03]The false corals, commonly known as the mushroom  anemones of order Corallimorpharia, look similar to anemones since they have no skeletons. However,  they are more closely related to the stony corals.

[00:12:18]Though the false corals do not build reefs,  they are often found among stony corals where they sometimes overgrow reefs in a carpet-like  formation. Most false corals grow as colonies that resemble encrusting corals, like the elephant  ear anemone. Others like the strawberry anemone are solitary. Most false corals are, like the  black corals, suspension feeders that prey on tiny planktonic organisms common in coral  reefs, though some, like Ricordea yuma, supplement their diet with symbiotic  zooxanthellae, much like the stony corals.

[00:12:57]Finally, the zoanthids, of order Zoantharia,  sometimes called Zoanthidea or Zoanthiniaria, are some of the most commonly collected corals  used by hobbyists in reef aquaria due to their high tolerance and ease of reproduction  in captivity. Many also fluoresce under a black light, and others have been bred to  produce a wide range of distinctive colors.

[00:13:23]They generally grow in small colonies and are  distinguished from other anthozoans by their exclusively marginal tentacles and ability  to incorporate sand and other small pieces of material into their tissues to help make  their structure. Like all anthozoans, they are capable of asexual reproduction, though they  generally remain attached to the original polyp.

[00:13:47]Like the false corals, zoanthids are often found  living among the reef-building corals, where they feed by capturing plankton and supplementing  their diet with symbiotic zooxanthellae.

[00:14:00]And that’s it for the general characteristics  and phylogeny of class Anthozoa. However, since the reefs constructed by the  hermatypic corals, or order Scleractinia, form the basis of entire ecosystems, we are  going to want to discuss those in more detail.

[00:14:18]So let’s move forward and investigate  the structure and types of coral reefs.