Starfish: Characteristic Features, Body wall, Water vascular system, Digestive system, Excretion System, Nervous systems, Circulatory system
Introduction
The starfish is a member of the phylum Echinodermata’s class Asteroidea. Since the beginning of time, people have been aware of starfish, and the Greeks gave them the name Aster, which means star. One of the earliest to see and document the starfish attacking and opening a clam was Aristotle. All of the stellate echinoderms were lumped together by Linnaeus under the name Asterias, and this confusion continued for a long time despite names being elevated to family or even ordinal rank. The Asterias were included in the family Stelleridae by Lamarck in 1792. Burmeister gave the combined sea stars and snake stars the name Asteroidea in 1837.
Anatomy and Physiology
Characteristic Features
Asterias forbesi exhibits radial symmetry by having the same set of organs on each of its five limbs. However, a closer look at the starfish reveals that it is constructed using the principle of bilateral symmetry, indicating that this radial symmetry was not the initial design of the structure. The calcerous madreporic plate, which is located between two of the arms on the dorsal surface of the central disc, dictates this layout. Therefore, the anus, which is located in the center of the dorsal surface of the central disc, the arm or radius opposite the madreporic plate, and all three are cut by the plane of symmetry. The bivium is made up of the two arms where the madreporous plate is located, while the trivium is made up of the remaining three.
The grooves on the madreporic plate’s outside surface extend outward from the plate’s middle. These grooves interact with pores within the madreporite’s matrix, which then break open creating tiny flagellated canals that band together to form collecting canals. These collecting canals discharge into the ampulla, a dilated region that gives rise to the stone canal of the water-vascular system. Water enters the water-vascular system at a constant flow thanks to the flagella in the canals.
The mouth, which serves as a point of reference for establishing the animal’s ventral surface, is located on the bottom, or oral, surface of the central disc. Five clusters of oral spines are located next to the peristome, a soft membrane that surrounds the mouth. The ambulacral groove, which has four rows of tube feet or poda, extends radially from the area of the peristome along the ventral surface of each ray. Each podum’s distal end has been transformed to become a terminal disc or sucker, which functions like a suction cup and allows the animal to establish a secure adhesive grip on the surface it is in contact with.
A single terminal tentacle with the eyespot, a little red patch, appears at the distal end of each ambulacral groove. An essential component of this animal’s neurological system is its eyespot, which is an organ of photoreception. The ambulacral spines, a double row of spines that can be pulled together to form a protective latticework over the tube feet in the ambulacral grooves, cover the borders of the grooves.
Due to calcified plates or ossicles in its inner layer, the body surface is extremely thick and hard. The connective tissue and muscle fibers that hold the dorsal surface’s ossicles together result in their shape being somewhat amorphous. The animal’s exterior is covered in a profusion of white spines that are an elevation of the calcerous ossices in the dermis underneath. Numerous tiny pincer-like structures called pedicellariae can be found between and around the bases of these spines. Two blades or jaws with serrated surfaces on opposite sides make up each pedicellaria. In general, the pedicellariae, which are all stalked, keep the animal’s surface free of detritus and clean of organisms that are likely to settle on the body wall’s softer regions.
Each arm has four rows of articulated ossicles that make up the ventral surface. The ambulacral ossicles form the two middle rows. The ambulacral pores, through which the ambulacral feet protrude, are located in between these. A pentagon-shaped group of oral ossicles encircles the peristome. Numerous branchiae may be seen sprouting through the spaces between the ossicles on both the ventral and dorsal surfaces outside of the adambulacral plates. They are little, coelomic cavity protrusions that resemble tubes and are used for breathing.
Even the exterior surfaces of such appendages as the tube feet, pedicellariae, spines, and branchiae are covered in cilia on the outside of starfish. This ciliation helps to remove particles from the surface and may facilitate the transit of food to the mouth.
Water Vascular System
The water vascular system is a modified component of the coelom and consists of a network of canals filled with seawater and equipped with specific corpuscles. It is extremely important for animal mobility and includes the lateral canals, Tiedman’s body, radial canal, ring canal, and tube feet.
(1) Madriporite
The madreporite is a circular calcareous plate that is located in an inter-radial position on the aboral surface of the central disc. There are numerous radiating, narrow, straight, or wavy grooves or furrows on its surface. At the bottom of each furrow are several tiny pores. Each pore enters a pore-canal that is exceedingly narrow, delicate, and tubular. It permeates the madreporite’s composition from the inside out. There could be 200 holes and pore canals in total. The collecting canals are created by joining the pore canals. which beneath the madreporite have an ampulla-like opening.
(2) Stone Canal
An “S”-the shaped stone canal is where the ampulla exits. Orally, the stone canal extends downward and exits into the ring canal that surrounds the mouth. A system of calcareous rings serves to sustain the stone canal’s walls. Very tall flagellated cells line the stone canal’s lumen. In embryonic stages and young Asterias, the stone canal is still just a basic tube. However, in adult Asterias, the stone canal lumen has two spirally rolled lamellae and a noticeable ridge.
(3) Ring Canal
The Ring canal, also known as the Water Ring, is situated directly above (aboral) the Hyponeural Ring Sinus and on the inner side of the Peristomial Ring of Ossicles. It has five sides and is pentagonal, or broad.
(4) The Bodies of Tiedmann
Nine small, yellowish, atypical, or spherical glandular structures, also known as racemose or Tiedmann’s bodies, emerge from the inner edges of the ring canal. The body of the Tiedmann is supported by the peristomial ring of ossicles. Tiedmann’s bodies are thought to be lymphatic glands that produce the amoebocytes for the water vascular system, while their precise role is yet unknown.
(5) Pollian Vesicles
One, two, or four tiny, pear-shaped, thin-walled contractile bladders or reservoirs with long necks known as pollian vesicles exit the ring canal’s outer side in the interradial position. They are accountable for producing amoeboid cells and controlling pressure inside the ambulacral system.
(6) Radial Canal
A radial water canal that extends the entire length of each arm and ends as the lumen of the terminal tentacle is released by the ring canal from its outer surface into each arm. The radial water channel in the arm connects right away to the oral side of the ambulacral muscles.
(7) Lateral Canal
The lateral or podial canals are two series of small, narrow, transverse branches that emerge from the radial canal in each arm. Each lateral canal has a valve to stop fluid from flowing backward into the radial canal and is connected to the bottom of a tube foot.
(8) Tube feet
Each ambulacral groove contains four rows of tube feet. A tube foot is a closed, hollow, elastic structure with thin walls that resembles a sac and has an upper sac-like ampulla, a middle tubular platform, and a lower disc-like sucker. The ambulacral pore, a space between the neighboring ambulacral ossicles where the podium can pass, is where the ampulla is located within the arm. It protrudes into the coelom above the pore. The primary locomotor and respiratory organ of Asterias are its tube feet.
The function of the Water Vascular System:-
Three primary purposes are served by the water vascular system.
(1) Locomotion:- The water vascular system is mostly utilized for locomotion. The water vascular channels’ interior walls are covered in cilia. The seawater enters through the madreporite due to the cilia’s beating. The tube feet’s ampullae are finally reached by the seawater.
The valves at the intersection of the lateral canals and tube feet stop water from flowing into the radial canals as the ampullae contract. The podia are flooded with water. The ambulacral groove protects the podia, which is extended. The substratum is then covered with suckers. The body is now propelled forward as the tube feet constrict. The ampulla is pressed with water from the tube foot. The tube feet shorten as a result. The sucklings are let go. The ampulla then closes, and the procedure is repeated.
(2) Food Capture:- The prey is captured using the tube feet. The suckers are employed to crack open the molluscans’ shells.
(3) Attachment:- The Starfish’s tube feet can be used to secure it to the rocks.
Body Wall and Endoskeleton
The body wall is made up of the muscle layer, parietal peritoneum, dermis, and epidermis from the outside in. The mucous gland cells and muriform cells that make up the epidermis are interspersed with spindle-shaped neurosensory cells and ciliated columnar epithelium. The pigment granules that give the skin its outward color are found in the epidermis. A nerve layer, which varies in thickness, can be found at the base of the epidermis. The epidermis and dermis underneath it are separated by a thin basement membrane.
The thickest layer of the body wall, the dermis, is composed of fibrous connective tissue. A network of canalicular spaces that form a ring space around the base of each papula is permeated throughout the dermis. The connective tissue found in the dermis holds the calcerous plates or ossicles, which make up the skeleton, together. As was already indicated, the ossicles are more or less dispersed elsewhere but are regularly organized around the mouth, in the ambulacral grooves, and frequently around the sides of the arms. The ossicles are shaped in a way that when they are joined together, they create a reticulate skeleton that has openings for the papulae to emerge from. The primary supporting structure of the endoskeleton, which is embedded in the dermis, is separated from the more superficial skeleton of projecting spines.
Muscular System
The muscular system is made up of a narrow layer of smooth muscle that is separated into an inner longitudinal layer and an outer circular layer. Because of their weakness and relative thinness, these layers produce slightly flexible, non-rigid rays. The ambulacral grooves are manipulated by a network of muscles. An upper and a lower transverse muscle connects each pair of ambulacral ossicles. The muscles’ ability to contract and relax controls how deep the ambulacral grooves are. Between the radial water vessel and the hyponeural canals is where the lower transverse muscle is located. Along the entire length of each row, there are additional short upper and lower longitudinal muscles located between neighboring amlunlacral ossicles. The ambulacral grooves tend to shorten as a result of their contraction. Each ambulacral ossicle’s outer end is connected to the nearby adambulacral ossicle via the lateral transverse ambulacral muscles. The ambulacral grooves are widened by their contraction.
The rays can also be moved laterally by longitudinal muscles that run between neighboring adambulacral ossicles.
The Coelom
The coelomic epithelium that surrounds the body cavity makes it a real body cavity, or coelom. The epithelium is ciliated and of the simple cuboidal form. The cilia of the coelomic epithelium keep the albuminous fluid that fills the coelom in a continual state of circulation. This fluid transfers soluble nutrients to the tissue it bathes after receiving them from the digestive system. Additionally, it gets nitrogenous wastes and carbon dioxide from the tissues. The visceral peritoneum refers to the peritoneum that covers the organs, and the parietal peritoneum refers to the peritoneum that lines the body wall. The bodily cavity, which surrounds the viscera and is frequently called the perivisceral cavity, is quite large. However, this coelom is separated into several coelomic canals or sinuses. These are significant because they house the majority of the animal’s haematological or circulatory system. The vertical axial sinus, which encloses the water-vascular system’s stone canal, is one of these sinuses. The atoral sip.us, which is located just under the dorsal body wall of the central disc, and the axial sinus communicate dorsally. The inner division of the oral ring sinus connects to the axial sinus below; the oral ring sinus is separated into an outer and an inner division by a septum. The hyponeural radial sinuses, which each extend into a ray and subtend the radial nerve cord as it runs through the arm, are created by the outer division of the oral ring. Additionally, the coelom serves an excretory purpose that will be covered later.
Digestive System
A sphincter muscle controls the mouth, which is located in the middle of the perioral membrane. A short esophagus that emerges from the mouth leads to a roomy cardiac stomach that fills the majority of the central disc. Two mesenteries coming from the peritoneum on the ambulacral ridges stabilize this stomach. Ten pouches, two of which extend briefly into each arm, are evaginated from the lateral wall of the cardiac stomach. During feeding, the stomach is accessible through the mouth and can be drawn in by the sphincter muscle. The pyloric stomach is located dorsal to the cardiac stomach. The pyloric ceca, a pair of glandular appendages, protrude from this part into each arm. Mesenteries that protrude from the upper part of the arm suspend them in each arm. Each pyloric cecum is a hollow glandular structure with significantly evaginated walls that is brownish or greenish in appearance. A portion of the ciliated columnar epithelium that lines each cecum is thought to secrete digestive enzymes. Each cecum has a duct that develops into the primary pyloric duct, which opens into the pyloric stomach, after joining with the ducts from the other cecae. The currents produced by the beating of the cilia in the pouches control the passage of the enzymes. Food is broken down by enzymes in the pyloric and cardiac stomachs before being sent to the pyloric cecae for absorption. Protease, amylase, and lipase are among the other enzymes secreted by the pyloric cecae.
The pyloric stomach gives birth to a short, narrow intestine that travels to the anus on the dorsal surface of the central disc above. A bilobed intestinal cecum lined with glandular and mucous cells results from the diverticulation of the gut.
The digestive system is made up of ciliated columnar epithelium, a layer of nerve tissue, a layer of connective tissue, and a layer of muscular tissue. The layers separating one organ from another differ in thickness.
The starfish grabs its prey, which is often bivalve mollusks so that the free edges of the shell are brought into close contact with the starfish’s mouth. The starfish’s tube feet apply pressure that causes the valves to open, and at the same time, the coelomic fluid and cavity, as well as the muscles in the arms and body, apply pressure that causes a portion of the cardiac stomach to evert through the starfish’s mouth. The stomach is then put into the mollusc’s valve-to-valve hole. The cardiac stomach then allows the cardiac stomach’s powerful proteolytic enzymes to enter the bivalve’s soft viscera. The viscera almost completely dissolve as a result of these enzymes. The starfish’s cardiac and pyloric stomachs’ cilia create currents that carry the digested materials upward to the starfish’s digestive tract, where more digestion takes place. The starfish then retracts its stomach by tightening the muscles in its retractor, and it closes its mouth by tightening the muscles in its oral sphincter.
The starfish also feeds on fish, sea urchins, snails, worms, and small crustaceans in addition to bivalve molluscs. However, bivalve mollusks like clams and oysters appear to be its favorite food.
Nervous System
Three interconnected systems make up the nervous system, according to one description. The oral or ectoneural system, which is found immediately below the epidermis, makes up the majority of the system. Radial nerves, the general subepidermal plexus, and the nerve ring make up this structure. In the peristomal membrane, close to its periphery, is where the circumoral nerve ring, which has a pentagonal shape, is located. At the bottom of the ambulacral groove, it emits radial nerve fibers that span the length of each arm, supplying them to the peristomal membrane and the esophagus. It eventually comes to an end in the terminal tentacle’s eye site. Fibrils are organized in layers and make up the radial nerve. It is continuous, with a general subepidermal plexus supplying all of the animal’s appendages and spanning the entire body. In the necks of the tube feet’s ampullae, the radial nerve’s fibers make connections with the cell bodies. These, as previously mentioned, coordinate the numerous tasks performed by the feet during locomotion and feeding.
A large subepidermal neuronal network made up of sensory, association, and motor neurons exist. The radial nerves, in particular, connect to the central nervous system. It has been demonstrated that the subepidermal plexus is capable of localized reflex activity. The marginal nerve cord, which runs the length of each side of the plexus and is thicker, is present. the lateral motor nerves, which are a longitudinal group of motor nerves, are then produced. These extend to each ambulacral ossicle in pairs. These ultimately form a plexus when they reach the coelornic lining. This plexus regulates the gonads and the muscular layer of the body wall. The entoneural system is the name given to this system.
The third section of the nervous system is the hyponeural system, which is located underneath the coelomic epithelium lining the hyponeural sinus in the lateral region of the oral uall. Only a tiny layer of dermal connective tissue divides this neural layer, also known as Lang’s nerve, from the lateral portion of the radial nerve. The neighboring lower transverse muscle, which extends between the ambulacral ossicles in the roof of the hyponeural sinus, receives a series of nerves from Lang’s nerve along the arm. Mostly motor, the hyponeural nervous system.
In the starfish’s neurological coordination, the circumoral ring is crucial. This is especially true of how the tube feet move when moving. The rays behave independently of one another when the ring and radial nerve cords are severed.
Neurosensory cells that operate as chemoreceptors and tactile receptors are present throughout the epidermis. The terminal discs of the tube foot and the terminal tentacles both have a significant number of tactile receptors. With a nucleus, a distal thread-like process that extends to the cuticle, and a proximal fibre that enters the subepidermal nerve plexus, these neurons are thin, roughly spindle-shaped cells. The eyespot, which is located at the base of the terminal tentacle, is the most intricate sense organ. Numerous concavities in the shape of cups make up each eyespot. These concavities have changed the epidermis into alternating pigmented and retinal cells. Each pigmented retinal cell contains a distal knobbed visual rod that extends into the concavity, while the pigmented cells themselves contain an orange pigment.
Reproduction
The starfish reproduces both sexually and by regeneration. The starfish has a pair of gonads in each arm that are used for sexual reproduction. They are found aborally in the ray’s proximal areas. Except for attachment near the genital pores, which are each placed close to the interradial line, they are free in the perivisceral coelom. The aboral sinus gives rise to the gonads at the beginning of their development. Each gonad extends from its place of attachment at the base of the ray where it lays lateral to a pyloric cecum, resembling a cluster of grapes. The gonad typically covers the entire length of the ray just before spawning. The female gonads range in hue from pink to orange, while the male gonads are typically a light grey tint. Although hermaphrodite examples have been discovered, the sexes are distinct. Except for females who are brooding, the sexes are typically not distinguishable from the outside.
The gametes of both male and female starfish are released into the water. After that, the ova are fertilized. Then, each zygote is completely and equally divided into a bipinnaria ( bilaterally symmetrical larval organism). It typically takes 1-3 years to metamorphose into an adult. After a year of growth, the starfish can frequently reproduce. It has been noted that starfish can live up to 25 years.
Starfish occasionally reproduce asexually through a procedure called autotomy. This procedure involves separating the disc along a roughly predetermined line that avoids the ossicles and preserves the arms. Then, the component components combine to form a whole creature. Environmental and physiological factors have an impact on sexual reproduction.
Regeneration, the third phase of reproduction, and autotomy are related in that a lost portion is replaced. This component may be lost through shedding (autotomy) or due to other factors. The ray is the bodily part that is lost and regenerates most frequently. The production of the terminal tentacle, the terminal ossicles, and the tissue of the eyespot precedes the formation of the pyloric cecae, the radial canal of the water-vascular system, and the radial nerves in the process of ray regeneration. In a few rare instances, the entire center disc has recovered every component that was lost.
Haemal (Circulatory) System
The path of the ambulacral system, under which the circulatory system is located, is followed by the entire system. Along with the axial organ, it is made up of a system of sinuses and a system of lacunae. The oral ring sinus, which is separated into an exterior and an interior oral sinus by a septum, is located just beneath the ambulacral ring. Five radial sinuses originate from the external sinus. The radial lacuna-enclosing septum divides each radial sinus longitudinally. The left and right channels of the radial sinus join in the tentacle as it passes underneath the radial canal of the ambulacral system to reach the end of the arm. The ambulacral feet get transverse sinuses from the radial sinuses. The axial sinus, which follows the hydrophoric canal, is created by the internal oral sinus. The dorsal end of the axial sinus links with the aboral ring-sinus, which gives birth to five genital sinuses. A glandular component of the axial lacuna, where the lacunae form a plexus, is the axial organ. It has been demonstrated that the dorsal part of the axial gland is contractile. This is thought to be the power behind the fluid and coelomocyte movement in the haemal system. An axial complex is made up of the axial sinus, axial lacunae with the axial organ, and the hydrophoric canal, which are all enclosed in the peritoneum. As was already established, the coelomic fluid transfers soluble nutrients to the tissues after receiving them from the digestive system. It also helps to clean these tissues of trash.
Excretory System
One of nature’s biological mysteries is the starfish’s lack of a real excretory system. It is unusual that a creature so far along the evolutionary tree, with a complex neurological system and many other complex features, would still have just a trace of an excretory system. The coelomic fluid’s amoebocytes perform the excretion function with the help of the rectal cecae. The metabolic wastes are carried by these amoeba-like creatures to the dermobranchiae, where they diffuse into the surrounding water. These phagocytic cells develop by budding from coelomic epithelial cells. There are two primary categories of amoebocytes: those with petaloid pseudopods and those with regular, thin pseudopods. These cells consume the particles to be expelled since they are extremely phagocytic.
Respiratory System
The dermal branchiae, which are responsible for absorbing oxygen, perform the respiratory function. The water currents that flow into the starfish also help them take in oxygen. The coelomic epithelium’s flagella maintain these currents in motion. With a return current running along the inner surface of the ray sides, there is a general internal flow that is directed toward the ray tips. It presumes to pick up oxygen from the dermal branchiae along the way, which it then distributes to the various tissues and organs.
ECOLOGY AND BEHAVIOR
Asterias forbesi can be found from low tide to a depth of about 50 meters along the majority of the eastern seaboard, from Maine to the Gulf of Mexico. This species integrates with closely related species, as is typical of the more specialized groupings of Asteroidea. On rocky or shelly bottoms, specimens are found alone or in sizable groups. Because it is a predaceous oyster-eating creature, this starfish is economically significant and has been the subject of in-depth research. This animal causes the oyster business to lose a lot of money every year. Along with oysters, the starfish also consumes clams, mussels, sea snails, dead fish, worms, and in some rare instances, other starfish.
It is known that starfish migrate at certain seasons, however, it is unclear what this migration looks like or how far it goes. This organism moves quite slowly, on average 6 inches per minute, according to observations of its locomotion. The isolated geographic distribution of this species, however, seems to suggest that the range of these creatures’ wanderings is quite small.
Like all other living things, starfish have natural adversaries. Cold and fresh water, different fish, gulls and crows, and parasites are a few of these harmful agents and natural enemies. The menhaden is undoubtedly the foe that causes the most damage to starfish. This fish only consumes the tiny aquatic organisms that float or swim in the water. Only once the sun has set are the starfish larvae safe from this fish. The scene involves a transformation procedure and attachment to a bottom-level object. The tiny algae and other small types of plankton make up the nutrition of the free-swimming larvae.
The starfish is vulnerable to parasite attack. The parasite organisms assault the starfish’s gonads, damaging the tissue and partially or completely rendering it sterile.