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Taking their name from a terrestrial flower, sea anemones are sessile polyps most often found in the coral reefs common to warm, tropical waters. A predatory, column-shaped marine animal, sea anemones can range in height from just a few millimeters to well over six feet and may possess as many as several hundred tentacles.
A relative of the corals that make up the reef environments they often inhabit, sea anemones benefit from a symbiotic relationship quite similar to the one found in their coral relatives. Inside the cells of sea anemones are single-celled dinoflagellates, which are typically zooaxanthellae -- also found in the cells of corals -- and zoochlorellae.
Belonging to the order Actiniaria, the scientific classification of sea anemones posits that, as part of the phylum Cnidaria, they are relatives of not just the corals that make up the reef systems they inhabit but also of the jellyfish and the Hydra. The taxonomic classification for sea anemones is as follows:
• Kingdom - Animalia
• Phylum - Cnidaria
• Class - Anthozoa
• Subclass - Hexacorallia
• Order - Actiniaria
There are two suborders that fall under Actiniaria: Enthemonae and Anenthemonae. As for the diversity of sea anemones, there are currently 46 families officially recognized for scientific classification purposes.
Ecology and Symbiotic Relationships
Due to the symbiotic relationship that exists between the single-celled dinoflagellates that inhabit its cells, the sea anemone is actually able to benefit from the process of photosynthesis despite its non-plant status. This is because the algae -- zooaxanthellae and zoochlorellae -- generate the products associated with photosynthesis and makes them readily available for use by the sea anemone. It is therefore possible for the sea anemone to yield a tremendous benefit from the symbiotic relationship, as it has ample access to oxygen as well as access to critical sources of energy in the form of alanine, glucose, and glycerol.
Since this is a symbiotic relationship, there has to be a benefit for the algae as well. Residing within the cells of the sea anemone, the algae is exposed to all the light it needs to engage in photosynthesis without simultaneously being exposed to predators in the form of micro-feeders and herbivores. The sea anemone offers protection to the algae in a variety of ways: its nematocysts -- the stinging cells of the tentacles -- ward off herbivores while the anemone actively protects the algae from the micro-feeders it maintains.
Sea anemones also engage in a number of other symbiotic or commensal relationships with a wide variety of fish and invertebrates, many of which are quite fascinating. Perhaps the most interesting of all the many symbiotic relationships involves the boxing crab aka pom pom crab, which carries a relatively small species of sea anemone in its claws. The boxing crab is not the only type of crab to engage in an interesting symbiotic relationship, as several species of hermit crabs and some snails permit anemones to live on its shell.
Of course, the relationship most observers are most likely to immediately recognize is the one that exists between sea anemones and clownfish. Similar relationships involving sea anemones also exist with a number of other marine animals, including cardinalfish, Bucchich’s goby, juvenile threespot dascyllus, and juvenile painted greenlings, not to mention a wide variety of shrimp, crabs, and snails.
Sea anemones are considered sessile polyps, but it is worth noting that while most are indeed sessile, there are also a number of species that are pelagic and possess a gas chamber within their pedal disc that enables the anemone to float in the water while inverted. Sea anemones are column-shaped, but they exist across such a wide spectrum of sizes that some species are small enough to live on the backs of hermit crabs while others are large enough that they far exceed the height of an average man. At the small end of the spectrum, sea anemones can be just four millimeters and may possess as few as 10 tentacles; at the larger end, they can be up to two meters in height and possess hundreds of tentacles.
There are several methods through which sea anemones secure food, which often differs from species to species. Some anemones rely on tentacles to render prey immobile, but others favor small particles that can be caught through the secretion of mucus in conjunction with the currents generated through the movement of the tentacles. Those that favor the former method possess stinging nematocysts that deliver an immobilizing toxin into its prey.
These stinging nematocysts are located within cnidocytes, which also serve a defensive purpose in addition to its predatory function. When either prey or predator makes contact with one of the sea anemone’s tentacles, an automated response is triggered that releases the nematocysts, which in turn delivers the actinotoxins contained within the small venom vesicle. The neurotoxins then render the prey immobile so the sea anemone can then begin the process of moving the prey into its mouth for digestion.
Given the highly toxic nature of these tentacles, it is not at all surprising that the many species of marine animals immune to the actinotoxins take up residence inside the sea anemone’s tentacles for protection from its own predators. One of many symbiotic relationships involving sea anemones, fish immune to the toxin benefit from the protection against predators and reciprocate by attracting potential prey for their host.
In what might be best described as an essentially incomplete gut, the digestive system of the sea anemone features a gastrovascular cavity that includes just one opening to the outside. This single opening therefore functions as the mouth of the sea anemone as well as its anus, so that its prey is ingested through the mouth and any undigested matter or waste is simply excreted through the same opening. This opening also features a ciliated groove -- which is known as a siphonophore -- that carries water in and out of the gastrovascular cavity.
Most sea anemones are predatory in nature and secure their prey with the help of their tentacles. Containing stinging nematocysts capable of delivering a toxin that either kills or immobilizes it prey, sea anemones then move the prey into its mouth and eventually into the gastrovascular cavity for digestion. Inside the mouth is a flattened pharynx that opens into the gastrovascular cavity, which is divided by mesenteries that form chambers or partitions that are symmetrically arranged in multiples of 12 and oriented around the pharynx.
The chambers or partitions formed by the mesenteries within the gastrovascular cavity also feature a stomach lining that secretes the digestive enzymes needed to break down the food the sea anemone ingests. The lining can be found on both sides of the mesenteries, but it is typically separated by a layer of mesoglea that tends to be quite thin.
The nervous system of the sea anemone lacks centralization and exists without any specialized sensory organs. As a result, this primitive nervous system simply exists to maintain homeostasis within the sea anemone by coordinating a variety of its necessary processes and generating a physical or biochemical response to specific stimuli.
Although the nerves and muscles of the sea anemone are more advanced and specialized when compared to corals -- its cnidarian relative -- it is nonetheless the case that the sea anemone possesses much simpler systems of nerves and muscles than most other animals. Microfilaments are grouped into contractile fibers in both the epidermis (the outer layer of the sea anemone) and the gastrodermis (the inner layer), but the fibers are not freely suspended and are therefore not considered “true” muscles.
Sea anemones possess longitudinal and circular fibers that aid in a variety of its necessary processes. The longitudinal fibers in the mesenteries, for example, are capable of contracting across the entirety of the sea anemone’s body and can therefore serve as a hydrostatic skeleton. Longitudinal fibers also exist within the sea anemone’s tentacles as well as the oral disc, and circular fibers exist within the body wall of the sea anemone. Although not present in all species, circular fibers in the oral disc enable the sea anemone to retract its tentacles for protective purposes.
As previously mentioned, the contractile fibers can create a hydrostatic skeleton that allows the sea anemone to stabilize itself despite lacking an actual skeletal system. The sea anemone is able to accomplish this by closing its mouth to maintain a constant volume within its gastrovascular cavity and therefore becomes far more rigid than would otherwise be the case.
As a member of the class Anthozoa, the lifecycle of sea anemones does not include a medusa stage and reproduces through asexual means as well as through the production of eggs and sperm from the polyp. In the latter method, males release sperm through their mouths, which then stimulate the release of eggs from the females in the same manner. Once fertilized, the egg ultimately develops into the planula that in turn develops into another polyp.
Asexual reproduction is also common among sea anemones, with budding, binary fission, and pedal laceration all existing as different methods of possible reproduction. The latter method, pedal laceration, occurs when material from the pedal disc fragments after breaking off in a ring. The fragmented material then regenerates into individual polyps, which, like all sea anemones, tend to grow quite slowly and typically goes on to enjoy the lengthy average lifespan of a sea anemone.
The majority of sea anemones are sessile, but they are able to relocate if the environmental conditions are no longer ideal or if they become subjected to sudden predation. Using its contractile fibers, anemones facing these circumstances can swim to a new location through the flexion produced by the fibers after it has freed itself from the substrate on which it was previously located. Pelagic anemones are obviously able to relocate as needed given their unfixed position.
Sessile anemones do not always use the flexion of its contractile fibers to swim to a new location when necessary, as some will simply attach themselves to hermit crabs or a submerged object until it finds a suitable new location; others will temporarily burrow in the sand or even engage in a parasitic relationship with another marine animal.
Practical Considerations in Marine Aquaria
Sea anemones have long been popular in saltwater reef aquaria, but this popularity has enjoyed a sharp and sustained increase over the past decade or so. Often sought in conjunction with clownfish and other marine animals immune to its tentacles, sea anemones are considered an attractive addition to a saltwater reef tank and create a unique aesthetic when paired with all manner of corals.
There is some concern over the potential for the rising demand for sea anemones to result in overexploitation, particularly in terms of the number of juvenile and adult sea anemones being collected from the wild. With advances in aquaculture -- along with the practice of post-larval capture and rearing -- marine aquaria enthusiasts can still include sea anemones within their contained marine ecosystems without contributing to the potential for further overexploitation.