The number of arms that radiate from a central disc varies by group, although most starfish have five. Some species have six or seven arms, while others have tens of thousands of them. Labidiaster annulatus (Antarctic Labidiaster) can have up to fifty.
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A thin cuticle, an epidermis made up of a single layer of cells, a thick dermis made up of connective tissue, and a thin coelomic myoepithelial layer that provides longitudinal and circular musculature make up the body wall. Ossicles are calcium carbonate components that form an endoskeleton in the dermis. These are honeycombed structures made up of lattices of calcite microcrystals. They come in a variety of shapes and sizes, with some having external granules, tubercles, and spines, but the majority are tabular plates that fit together neatly in a tessellated pattern to form the aboral surface’s main covering. The madreporite (the water vascular system’s entry), pedicellariae, and paxillae are examples of specialized structures. Pedicellariae are compound ossicles with jaws that resemble forceps. They clean trash from the body’s surface and, in reaction to physical or chemical stimuli, wave about on flexible stalks, biting constantly. They frequently form groups around spines. Paxillae are umbrella-like structures found on buried-in-sediment starfish. The edges of adjacent paxillae join to form a fake cuticle that protects the madreporite and fragile gill structures under a water cavity. The epidermal layer covers all ossicles, even those that protrude externally.
Pedicellariae are found in a variety of starfish families, including the Valvatida and Forcipulatida. They appear in pompom-like tufts at the base of each spine in the Forcipulatida, such as Asterias and Pisaster, whereas they are distributed over the body surface in the Goniasteridae, such as Hippasteria phrygiana. Some are supposed to help with defense, while others are thought to help with feeding or removing creatures that try to settle on the starfish’s surface. Labidiaster annulatus, Rathbunaster californicus, and Novodinia antillensis are examples of species that use their enormous pedicellariae to catch small fish and crustaceans.
Papulae, thin-walled protrusions of the body cavity that reach through the body wall and into the surrounding water, may also be present. These are used to help in breathing. Collagen fibres positioned at right angles to each other and arranged in a three-dimensional web support the structures, with ossicles and papulae in the interstices. This structure allows the starfish to easily flex their arms while also allowing for the rapid onset of stiffness and rigidity required for stress-related movements.
Water vascular system
Locomotion, adhesion, food manipulation, and gas exchange are all part of the starfish’s water vascular system, which is a hydraulic system made up of a network of fluid-filled canals. Water enters the system through the madreporite, a porous, sieve-like ossicle on the aboral surface that is often visible. It is connected to a ring canal surrounding the mouth opening by a stone canal, which is commonly lined with calcareous material. This leads to a series of radial canals, one of which travels down the ambulacral groove in each arm. Short lateral canals break off from the radial canal on either side, each ending in an ampulla. Short connecting canals flow through ossicles in the ambulacral groove to connect these bulb-shaped organs to tube feet (podia) on the animal’s exterior. Tube feet are normally found in two rows, but in some species, the lateral canals alternate between being long and short, giving the appearance of four rows. The canal system’s whole interior is lined with cilia.
When the ampullae’s longitudinal muscles contract, the lateral canals’ valves close, and water is driven into the tube foot. These reach out to make touch with the substrate. Despite the fact that the tube feet look like suction cups, the gripping motion is caused by adhesive chemicals rather than suction. Release from the substrate is enabled by the presence of other substances and the relaxation of the ampullae. The tube feet adhere to surfaces in a wave, with one arm portion connecting to the surface while the other releases. While moving, certain starfish raise the tips of their arms, exposing the sensory tube feet and the eyespot to the most external stimuli.
Starfish, having evolved from bilateral organisms, may move in a bilateral manner, especially when hunting or in danger. Certain arms operate as the leading arms when crawling, while others trail behind. The leather star (Dermasterias imbricata), which can only travel 15 cm (6 in) in a minute, is an example of a starfish that cannot move swiftly. Some burrowing species from the genera Astropecten and Luidia have points on their long tube feet instead of suckers, allowing them to move significantly faster and “glide” across the ocean floor. The sand star (Luidia foliolata) may reach speeds of up to 2.8 meters (9 feet 2 inches) per minute. When a starfish becomes upside down, two adjacent arms are bent backwards to offer support, the opposite arm is used to stamp the ground as the two remaining arms are elevated on either side, and the stamping arm is ultimately released as the starfish rolls over and returns to its regular stance.
The tube feet serve as supplementary gills in addition to their motility function. The water vascular system transports oxygen and carbon dioxide from the tube feet to the tube feet, as well as nutrients from the gut to the motility muscles. Cilia initiate fluid movement in both directions. Other gills, known as papulae, are thin-walled bulges on the aboral surface of the disc and arms that allow gas exchange. These deliver oxygen to the coelomic fluid, which serves as a gaseous transport medium. The fluid in the major body cavity distributes oxygen dissolved in water throughout the body; the circulatory system may also play a modest role.
Digestive system and excretion
The starfish stomach takes up the majority of the disc and extends into the arms. The mouth is situated in the middle of the oral surface, bordered by a tough peristomial membrane and sealed by a sphincter. The stomach is divided by a constriction into a bigger, eversible cardiac portion and a smaller pyloric portion. The mouth opens through a short oesophagus into a stomach separated by a constriction into a bigger, eversible cardiac portion and a smaller pyloric portion. The glandular and pouched cardiac stomach is held by ligaments attached to ossicles in the arms, allowing it to be pulled back into position after being everted. The pyloric caeca is one of the two expansions of the pyloric stomach into each arm. These are hollow tubes that are elongated and branched, with a system of glands that produce digesting enzymes and absorb nutrients from food. A tiny anus at the apex of the aboral surface of the disc connects the pyloric stomach to the short intestine and rectum.
Astropecten and Luidia, for example, swallow their prey whole and begin to digest it in their cardiac stomachs. Through their mouths, shell valves and other inedible debris are discharged. The partially digested fluid is then transferred into their pyloric stomachs and caecas, where digestion and absorption continue. The cardiac stomach can be everted from the organism’s body to engulf and digest food in more sophisticated species of starfish. When the starfish eats a clam or another bivalve, it pulls the two valves apart slightly with its tube feet and inserts a little bit of its stomach, which releases enzymes to breakdown the meal. The stomach is withdrawn into the disc, along with the partially digested prey. The meal is then transferred to the pyloric stomach, which is always contained within the disc. A neuropeptide known as NGFFYamide activates the cardiac stomach’s retraction and contraction.
Starfish can hunt animals much larger than their mouths because of their capacity to digest food outside the body. Clams and oysters, arthropods, tiny fish, and gastropod mollusks are all part of their diet. Some starfish aren’t pure carnivores, and may eat algae or organic waste to augment their meals. Some are grazers, while others capture food particles in sticky mucus strands that are carried towards the mouth in ciliated grooves.
Ammonia is the most common nitrogenous waste product. Waste ammonia is eliminated by diffusion through the tube feet and papulae, as starfish lack specific excretory systems. Coelomocytes, which are also found in the hemal and water circulatory systems, are phagocytic cells found in bodily fluid. These cells consume garbage and travel to the papulae’s tips, where a piece of the body wall is nipped off and expelled into the surrounding water. The pyloric glands may also exude some waste, which is then passed through the feces.
Starfish do not appear to have any osmoregulation mechanisms and maintain the same salt concentration in their bodily fluids as the surrounding water. Although some species may withstand low salinity, starfish are not found in fresh water or even many estuarine habitats due to the lack of an osmoregulation system.
Nervous and sensory systems
Starfish are sensitive to touch, light, temperature, orientation, and the state of the water around them, despite their lack of well-defined sense organs. Touch is painful to the tube feet, spines, and pedicellariae. Chemical sensitivity is also found in the tube feet, particularly at the tips of the rays, allowing the starfish to sense scent sources such as food. At the tips of the arms are eyespots, each made up of 80–200 simple ocelli. These are made up of light-responsive pigmented epithelial cells that are protected by a thick, transparent cuticle that also serves to focus light. Many starfish have unique photoreceptor cells in different areas of their body, allowing them to respond to light even when their eyespots are hidden. The species determines whether they advance or retreat.
While a starfish lacks a centralized brain, it does have a complex neurological system that includes a nerve ring around the mouth and a radial nerve that runs parallel to the radial canal along the ambulacral portion of each arm. Two nerve nets make up the peripheral nervous system: a sensory system in the epidermis and a motor system in the coelomic cavity lining. The two are linked by neuronal fibers that travel through the dermis. The sensory and motor components of the ring and radial nerves coordinate the starfish’s balance and directional systems. The sensory organs provide information to the sensory component, while the tube feet and musculature are controlled by the motor nerves. The starfish lacks the ability to plan ahead of time. When one arm senses a desirable odor, it takes precedence and momentarily overrides the other arms to commence moving towards the prey. The mechanism behind this is still a mystery.
The circulatory or haemal system is housed within the bodily cavity. The vessels create three rings: one around the mouth (hyponeural haemal ring), one surrounding the digestive system (gastric ring), and one near the aboral surface (aboral surface ring) (the genital ring). The heart beats around six times per minute and is located at the apex of the axial vessel, which joins the three rings. There are paired gonads at the base of each arm, and a lateral vessel runs from the genital ring past the gonads to the tip of the arm. There is no continuous circulation of fluid within this vessel because it has a blind end. Although this liquid lacks colour and has little or no respiratory function, it is most likely used to transfer nutrients throughout the body.
Starfish create a number of secondary metabolites in the form of lipids, such as steroidal cholesterol derivatives and sphingosine fatty acid amides. The majority of the steroids are saponins and their sulphated derivatives, also known as asterosaponins. They are made up of up to six sugar molecules (commonly glucose and galactose) linked by up to three glycosidic chains and vary by species. Sphingosine long-chain fatty acid amides are common, and several of them have known pharmacological action. Starfish also produce a variety of ceramides, as well as a modest number of alkaloids. Although the activities of these compounds in starfish have not been extensively researched, they are thought to play a role in defense and communication. Some are predation deterrents used by starfish to keep predators away. Antifoulants are added to the pedicellariae to prevent other organisms from settling on the aboral surface of the starfish. Alarm pheromones and escape-inducing compounds, for example, are released to elicit responses in conspecific starfish while also eliciting escape responses in potential prey. Worldwide, scientists are studying the efficacy of these chemicals for potential pharmaceutical or industrial use.
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