Sea Anemone

Sea anemones belonging to the phylum Coelenterata and the order Actinaria, these marine predators are so named because their appearance is land-based anemones. As members of the class Anthozoa, sea anemones are characterized by their substantial polyps, which enhance their digestive capabilities and enable them to capture and consume larger prey. Sea anemones do not have a medusa stage. They bear a close resemblance to jellyfish, corals, and other coelenterates. The structure of a sea anemone includes a polyp attached to the substrate by an adhesive foot and a columnar body that supports the oral disc. 

1.0What are Sea Anemones?

The sea anemone transported by the hermit crab gains distribution and food acquisition advantages. Many sea anemone species also derive additional nutrients through symbiotic relationships with single-celled dinoflagellates known as zooxanthellae or green algae called zoochlorellae, which reside within their cells. Additionally, some sea anemones form beneficial associations with hermit crabs, small fish, or other animals, enhancing their survival and well-being.

2.0Sea Anemone Classification

3.0Characteristics of Sea Anemones

• Solitary or colonial polyps that lack a medusoid phase; entirely marine; either attached or fixed to a substrate; cylindrical bodies with biradial symmetry; approximately 6,100 known species.
• The oral end widens to form an oral disc equipped with six to several hundred hollow tentacles encircling the mouth; the ectoderm and endoderm are divided by a robust mesoglea containing fibres and cells.
• Vertical partitions exist within the enteron.
• The presence of a skeleton is variable (for example, absent in sea anemones).
• Coiled mesenteric filaments, which function as gastric filaments, are found on the free edges of the mesenteries.
• The mesenteries consist of a dual layer of endoderm supported by a mesogleal plate.
• The nervous system consists of a diffuse nerve net with no central nervous system.
• The reproductive cells originate from the endoderm, and sexual products are released into the coelenteron; reproduction also occurs asexually through budding.
• The planula larva undergoes metamorphosis to reach the adult stage.

4.0Sea Anemone Body Structure

• The adults are solely polypoids, meaning they lack a medusoid stage, are solitary, and exhibit radial symmetry.
• Their body is cylindrical, featuring a flat pedal disc at the base, a central column, and an oral disc at the top.
• The oral disc is surrounded by a ring of hollow, unbranched tentacles and a prominently positioned central mouth on a raised region.
• A few rounded, conical tubercles are located where the column meets the pedal disc.

Internal Structure 

• The body is columnar, wider than it is tall. The front end features an oral disc, with the mouth positioned centrally.
• Numerous short conical tentacles emerge from the edges of the oral disc, organized into five concentric rings. The outermost circle contains the most tentacles, forty-eight, while each successive ring has fewer tentacles.
• The body's rear is flattened into a basal disc, which is an anchoring mechanism. The oral disc forms a gullet or food tube that hangs down into the main cavity of the body.
• On either side of the gullet are pairs of smooth, heavily ciliated furrows at both ends of its long diameter, allowing water to flow into the enteron via the siphonoglyph. The central cavity inside the body is divided into radial sections by six complete septa, known as primary mesenteries, which stretch from the outer wall to the gullet.
• Among these primary mesenteries are two types of incomplete partitions: the larger ones are called secondary mesenteries, and the smaller ones are tertiary mesenteries. These partitions further divide the gastrovascular cavity into vertical chambers. Beneath the oral disc, each septum features a pair of Ostia that allow for the direct exchange between the compartments separated by mesenteries.
• The thick, free inner edge of each septum forms a mesenteric filament, which continues posteriorly into a thread-like structure called an acontium. This protrudes through the mouth or cinclides (openings in the body wall) to help subdue prey.

 Body Wall 

• A thick, tough layer of mesoglea separates the epidermis and gastrodermis.
• The epidermis and gastrodermis contain long, columnar, ciliated epitheliomuscular cells.
• The mesoglea features a network of delicate fibres and interspersed cells, giving it the properties of an intermediary cell layer.
• A single internal cavity, the coelenteron or gastrovascular cavity, lined by the gastrodermis, is enclosed by the body wall and opens externally only via the mouth.
• Stinging capsules in the epidermis are particularly plentiful within the mesenteric filaments.
• Gland cells are prolific in both the gullet's epidermal layer and the mesenteric filaments.
• Each septum is equipped with three sets of muscles: retractor (vertical), transverse (across), and parietal (oblique).
• A nerve net is found in both the ectoderm and endoderm.
• Gonads, which are endodermal in origin, indicate separate sexes. Reproduction is solely sexual, with sea anemones releasing sperm and eggs into the sea via the mouth. The fertilized eggs evolve into planula larvae that temporarily lead a planktonic life before settling on the seabed and maturing directly into juvenile polyps.
• The development involves a complex metamorphosis starting from the planula stage.

5.0The Digestive System of Sea Anemones 

• Sea anemones feature a simple digestive system with a gastrovascular cavity that serves both as a stomach and has a singular opening functioning as both mouth and anus.
• Excretion of waste and undigested material occurs through this same opening.
• The mouth of sea anemones is generally a slit-like aperture, often accompanied by a groove known as a siphonoglyph at one or both ends. This groove, lined with cilia, assists in drawing food particles inward and facilitates water circulation within the gastrovascular cavity.
• A flattened pharynx leads into the gastrovascular cavity. This extension of the body wall is lined with the animal's epidermis. Typically, the pharynx extends about a third of the body's length before it opens into the gastrovascular cavity, which takes up the rest of the body space.
• The internal structure of the gastrovascular cavity is segmented into several chambers by radially arranged mesenteries that extend inward from the body wall. While some mesenteries form complete partitions that connect at the base of the pharynx, others do not span the entire width. These mesenteries are often found in multiples of twelve and are symmetrically organized around the central space, lined on both sides by stomach tissue separated by a slim mesogleal layer and include tissue filaments specialized in secreting digestive enzymes.
• In certain species, these digestive filaments, known as acontial filaments, dangle below the mesenteries' lower edges within the gastrovascular cavity. Armed with nematocysts, these filaments can be ejected through cinclides—blister-like openings in the column wall—as a defense mechanism.

6.0Nervous System of Sea Anemones 

• The organism possesses a basic nervous system that lacks centralization but coordinates activities related to maintaining homeostasis and reacting to biochemical and physical stimuli.
• Two nerve nets exist, one in the epidermis and the other in the gastrodermis. These nerve nets connect at various key points, including the pharynx, where the septa meet the oral and pedal discs, and across the mesoglea.
• While specialized sense organs are absent, sensory cells such as nematocysts and chemoreceptors allow the organism to detect and respond to environmental changes.

7.0Muscle Movement in Sea Anemones

• The musculature of sea anemones is relatively simple compared to that of most other animals, yet it is more specialized than that found in other cnidarians, like corals.
• In the outer (epidermis) and inner (gastrodermis) layers, cells contain microfilaments that cluster into contractile fibres. However, unlike those in more advanced organisms, these fibres don't constitute true muscles since they aren't freely hanging within the body cavity.
• Longitudinal fibres are present in the tentacles, the oral disc, and the mesenteries, enabling these creatures to contract along their entire body length.
• Circular fibres are located in the body wall and, in certain species, around the oral disc. This arrangement allows the animal to retract its tentacles into a protective sphincter when necessary.
• Lacking a rigid skeleton, sea anemones utilize these contractile cells to exert force against the fluid within their gastrovascular cavity, effectively creating a hydrostatic skeleton. This system allows the anemone to inflate and extend its tentacles for feeding and deflate and retract when it rests or feels threatened.

8.0Movement and Locomotion of Sea Anemones

• A sea anemone can dramatically alter its form because of its ability to manipulate the longitudinal, transverse, and diagonal muscle sheets in its columns and tentacles. This allows it to stretch, contract, bend, and twist. The anemone can also invert its gullet and mesenteries or retract its oral disc and tentacles into the gullet while closing it with a sphincter. The gullet folds across and expels water through the mouth during this process.
• While some sea anemone species embed themselves in soft sediment, most remain primarily fixed in place, adhering to solid surfaces via their pedal disc.
• However, sea anemones can move and slowly glide across their base.

9.0Nutrition in Sea Anemones

• Sea anemones generally act as predators, capturing prey of appropriate size that comes within the grasp of their tentacles.
• They immobilize their catch using nematocysts and transport it to their mouth, pushing it into the pharynx.

10.0Life Cycle of Sea Anemones

• Sea anemones and other anthozoans differ from most cnidarians because they do not have a medusal stage in their life cycle. Instead, all aspects of sexual reproduction are managed by the polyp, which produces eggs and sperm. The resulting fertilized egg then develops into a planula larva, which drifts temporarily before settling on the seabed. It undergoes metamorphosis into a juvenile sea anemone, becoming a new polyp.
• Sea anemones' reproductive strategies vary. Some species have distinct male and female individuals, while others are sequential hermaphrodites, with the sexual organs maturing at different times.
• While asexual reproduction is less common, sea anemones have significant regenerative abilities that allow reproduction longitudinally or transversely, such as budding, fragmentation, or binary fission. For instance, Anthopleura species reproduce by dividing along their length. Pedal laceration is another method, where a ring from the anemone's base breaks off and fragments, with each piece regenerating into new, clonal individuals.
• Additionally, sea anemones can leave behind fragments that regenerate into new individuals as they move across surfaces. In the case of Metridium dianthus, higher fragmentation rates are observed among individuals living among live mussels compared to those among dead shells, with all new individuals developing tentacles within three weeks.

11.0Mutualistic Relationship

• Despite being unable to perform photosynthesis, sea anemones engage in a significant facultative mutualistic relationship with specific types of single-celled algae that reside within their gastrodermal cells, primarily in the tentacles and oral disc. These algae, which may be zooxanthellae, zoochlorellae, or a combination of both, provide the sea anemone with essential nutrients such as oxygen, glycerol, glucose, and alanine produced through photosynthesis. In return, the algae gain consistent access to sunlight and protection from micro-feeders, thanks to the anemone's maintenance. The algae also enjoy protection from herbivores due to the anemone's stinging cells.
• For instance, the appearance of the aggregating anemone (Anthopleura elegantissima) largely depends on the specific algae present within it. Similarly, the hidden anemone (Lebrunia coralligens) features seaweed-like pseudotentacles rich in zooxanthellae. These spread out during the day to facilitate photosynthesis and retract at night, allowing the tentacles to extend and hunt for prey.
• Several fish species and invertebrates also form symbiotic or mutualistic relationships with sea anemones. Notably, clownfish are known for seeking refuge among anemone's stinging cells, which offer protection from predators, while the anemone benefits from the nutrients in the clownfish's waste. Other creatures associated with sea anemones include cardinalfish, juvenile three spot bacillus, incognito gobies, juvenile painted greenlings, various crabs, shrimp, opossum shrimp, and marine snails.
• Some particularly unique interactions involve anemones such as Adamsia, Calliactis, and Neoaiptasia, which reside on the shells of hermit crabs or snails, and the Bundeopsis or Triactis anemones that are carried in the claws of Lybia boxing crabs. These relationships illustrate the complex and diverse nature of symbiotic partnerships in marine environments.