Most sea anemones live attached, catching passing food with their tentacles. Sea anemones can move slowly by gliding on their base. Many are also capable of moving rapidly to avoid predation or competition by detaching, catching a current and re-attaching elsewhere. Snakelocks Anemonia viridis common species in the British south west coast. The diet of most anemones consists of small animals such as plankton, crabs and fish, however a number of bigger sea anemones will eat much larger prey.
For example, dahlia anemones can be greedy feeders that will prey on starfish and jellyfish. Anemones have rings of tentacles surrounding their central mouth. Tentacles have specialised stinging cells called nematocysts. They use these to immobilise their prey so that the tentacles are then able to move the food into the mouth. The extending tentacles can also be used to catch passing food as it drifts past.
Sea anemones vary in size, with some tropical species reaching more than a metre in diameter. One of the largest in British waters is the Horesman anemone Urticina eques , reaching sizes of 35cm across. One of the smallest in Britain is the rare anemone Gonactinia prolifera , which rarely grows more than 5mm tall. Anemones have adapted to a wide range of habitats, from the muddy depths of sea lochs, to seashores, wrecks and offshore reefs.
The gonads are formed from the gastrodermis and gametes are expelled through the mouth. Planula larvae are formed by external fertilization; they settle on a substratum in a polypoid form. These polyps may bud to form additional polyps or begin immediately to produce medusa buds.
In a few species, the planula larva may develop directly into the medusa. The life cycle Figure 3 of most scyphozoans includes both sexual medusoid and asexual polypoid body forms.
Figure 3. Scyphozoan life cycle. The lifecycle of most jellyfish includes two stages: the medusa stage and the polyp stage.
The polyp reproduces asexually by budding, and the medusa reproduces sexually. However, cubozoans display overall morphological and anatomical characteristics that are similar to those of the scyphozoans. A prominent difference between the two classes is the arrangement of tentacles. The cubozoans contain muscular pads called pedalia at the corners of the square bell canopy, with one or more tentacles attached to each pedalium.
In some cases, the digestive system may extend into the pedalia. Nematocysts may be arranged in a spiral configuration along the tentacles; this arrangement helps to effectively subdue and capture prey. Cubozoans include the most venomous of all the cnidarians Figure 4. These animals are unusual in having image-forming eyes, including a cornea, lens, and retina.
Because these structures are made from a number of interactive tissues, they can be called true organs. Eyes are located in four clusters between each pair of pedalia. Each cluster consists of four simple eye spots plus two image-forming eyes oriented in different directions. How images formed by these very complex eyes are processed remains a mystery, since cubozoans have extensive nerve nets but no distinct brain.
Nontheless, the presence of eyes helps the cubozoans to be active and effective hunters of small marine animals like worms, arthropods, and fish. Cubozoans have separate sexes and fertilization occurs inside the female. Planula larvae may develop inside the female or be released, depending on species. Each planula develops into a polyp.
These polyps may bud to form more polyps to create a colony; each polyp then transforms into a single medusa. Figure 4. The a tiny cubazoan jelly Malo kingi is thimble shaped and, like all cubozoan jellies, b has four muscular pedalia to which the tentacles attach.
Two people in Australia, where Irukandji jellies are most commonly found, are believed to have died from Irukandji stings. Hydrozoa is a diverse group that includes nearly 3, species; most are marine, although some freshwater species are known Figure 5. Most species exhibit both polypoid and medusoid forms in their lifecycles, although the familiar Hydra has only the polyp form.
The medusoid form has a muscular veil or velum below the margin of the bell and for this reason is called a hydromedusa. In contrast, the medusoid form of Scyphozoa lacks a velum and is termed a scyphomedusa.
Order Corallimorpharia, Anemone Corals or Mushroom Corals : Members of this order are sometimes called anemone corals or mushroom corals because they resemble anemones more closely than other types of corals due to their large, flat, disc-like shape and short tentacles. They grow like wheel spokes, radiating from a center and forming concentric circles.
The diameter of the circle increases as they grow. This order is extremely popular in home aquariums. Order Zoantharia, Zoanthids : Zoanthids have long, prominent tentacles arranged in two rows. Unlike stony and soft corals, Zoanthids incorporate sand and other substrate into their colonies for structure.
They may live as individual polyps or in colonial groups. Order Actiniaria, Anemones: Larger anemones tend to be solitary while smaller species may use asexual reproduction to propagate and live in large concentrations when there is suitable habitat. Anemones come in a wide range of colors, some owing their coloration to the zooxanthellae, microscopic algae, they host.
Anemones have a disc-shaped bottom they use to attach themselves to rocks, in crevices and on other suitable surfaces, including the shells of other marine invertebrates. Subclass Ceriantharia, Tube-dwelling Anemones: This subclass looks similar to sea anemones, but tube-dwelling anemones are known for being solitary and living buried in soft sediments.
They live inside tubes made of secreted mucus and organelles, and can recede into these tubes for protection. Ceriantharia includes the orders Spirularia and Penicillaria.
Anthozoans are found from intertidal zones to deep ocean trenches, in both warm and cold waters. Reef-building corals are only found in shallow tropical and subtropical waters. Anthozoa are found in the largest numbers in warm, tropical waters in coral reef habitats.
Some corals receive their nutrition from mutualistic zooxanthellae that require sunlight for photosynthesis, the by-products of which are sugars consumed by the hosting coral. Depending on their size, corals may catch prey varying in size from microscopic zooplankton to small fish.
At the Smithsonian's National Zoo, anemones and corals are housed under special lighting fixtures and receive a mixture of minced clams, fish and shrimp. Corals can reproduce sexually and asexually. Asexual reproduction happens in the form of budding that occurs when a parent polyp produced by sexual reproduction divides, creating a colony of genetically identical but individual animals. Corals also fragment during storms and these pieces can regrow into new colonies.
A colony can grow to hundreds of thousands of individual polyps. Corals reach sexual maturity at different ages, varying by species and colony size.
Some can mature and reproduce successfully in as little as four years, while others can take decades to reproduce. Sexual reproduction usually takes place through broadcast spawning of gametes or internal brooding of larvae. When broadcast spawning, corals release both eggs and sperm into the open water in a synchronized spawning event triggered by a variety of environmental cues. Fertilized larvae float in the water column until they settle in a suitable place to attach and begin a new colony.
Sexual spawning allows larvae to be carried to other areas, spreading new genetic material to a different habitat. It is estimated that most reefs are between 5, and 10, years old, with some actually tracing their beginnings back 50 million years. It is unclear how long the life span of an individual coral or anemone is. Coral reefs are threatened by disease, climate change, over-fishing, pollution, over-development of coastal zones and the aquarium trade.
Fishing with cyanide and explosives has reached epidemic levels, and run-off from over-fertilization and insecticides has further injured coral structures by fueling the growth of competing algae. The steady increase of silt loads into waterways from destructive farming and logging practices has also put pressure on already deteriorating reefs.
The trade of both fish and corals for hobby tanks gives low-income communities near reefs a source of profit, which encourages the destruction and collection of these organisms. In the past few decades, one-tenth of all coral reefs have been destroyed and one-third have been seriously degraded.
0コメント