Starfish (Asterias): Introduction, Classification, History, Habit and Habitat, External Features, Aboral Surface, and Coelom
Echinoderms in the Asteroidea class also referred to as Starfish or Sea Stars, have a Star-like shape. Given that they belong to the Asteroidea class, starfish are also referred to as Asteroids. On the seafloor of all the world’s oceans, from warm, tropical zones to icy, polar regions, there are about 1,900 kinds of starfish. They can be found 6,000 m (20,000 ft) below the surface, from the intertidal zone to abyssal depths.
The French naturalist de Blainville gave starfish the scientific name Asteroidea in 1830. It comes from the Greek words aster (a star) and eidos (form, likeness, appearance). The phylum Echinodermata contains the class Asteroidea. The echinoderms also include sea urchins, sand dollars, brittle and basket stars, sea cucumbers, and crinoids in addition to starfish. The subphylum Asterozoa, which includes starfish, is characterised by its mature members having a flattened, star-shaped body with a central disc and numerous radiating limbs.
Asterias is commonly known by the name of starfish. The name starfish is somewhat misleading suggesting an organism to be like a star and fish but as Asterias lacks in both the characteristics, therefore, recently it is renamed sea star. There occur about 150 species of Asterias all of which have different geographical distributions. On the shores of English and North Europe, Asterias rubens can be found. A. vulgaris is found on the North Atlantic coast of North America, A. forbesi occurs on the eastern sea shore from Maine to the Gulf of Mexico, A. amurensis is found in the Behring sea, Japan and Korea, and A. tenera occurs on the sea shore from Nova Scotia to New Jersey. Pentaceros, Astropecten, Asterina, Heliaster, Solaster, Luidia, and others are some additional common sea stars. The description that follows will provide a comprehensive overview of the Asterias genus’ anatomical structure.
Habit and Habitat
Asterias is a mainly marine, benthonic animal that lives in many types of bottom, primarily in the littoral zone. They crawl around on these bottoms, sometimes in the open and other times more or less hidden. While some species have been shown to prefer rocky sea bottoms, Asterias forbesi can be found in similar abundance on hard, rocky, sandy, or soft substrates. The majority of Asterias species are solitary creatures, but under specific ecological circumstances such as avoiding direct sunlight or extreme drying, many individuals may band together for protection in one location. The majority of them are nocturnal; they are calm during the day and become active at night. They often move fairly slowly while crawling on the ground. All sea stars are carnivorous and feed voraciously on almost any available slow-moving or sessile animals, chiefly on polychaetes, crustaceans, molluscs and other echinoderms and even corpses. Numerous Asterias species interact biologically with members of various zoological groups in a variety of ways, including commensalism and parasitism. In general, sea stars show a remarkable capacity for autotomy and regeneration.
External Features of Asterias
(i) Shape, Size and Colour: Asterias possesses a pentamerous body that is radially symmetrical. The body is made up of a pentagonal centre disc from which five long, tapered, evenly spaced extensions, called rays or arms, protrude outward. There may be more than five arms in some genera, such as the 7–14 arms in Solaster and the more than 40 arms in Heliaster. Though certain versions may be much smaller or longer, the size ranges from 10 to 20 centimetres in diameter. Variable hues of yellow, orange, brown, and purple are present. The aboral or abactinal surface is the higher convex and significantly darker of the body’s two surfaces. The bottom surface, also known as the oral or actinal surface, is flat and has less pigmentation. The left and right sides of the bilaterally symmetrical larva are represented by the oral and aboral surfaces, not the ventral and dorsal surfaces. The areas of the centre disc between the arms are referred to as inter-radii, and the axes that the arms occupy are known as radii. There isn’t even a head with definite features.
(ii) Oral Surface: The oral or actinal surface is the flat, dark orange to the purplish-coloured side of the body that, in its natural state, faces the substratum and contains the mouth or oral orifice.
The oral surface bears the following structures:
The actinosome or mouth is an orifice on the oral surface located in the centre of the pentagonal central disc. Each of its five angles, which are pentagonal and pointed at an arm, is present. The mouth is protected by five clusters of oral spines, or mouth papillae, and is encircled by the peristomial membrane, a fragile and soft membrane.
2. Ambulacral Grooves
A little groove known as the ambulacral groove extends from each angle of the mouth and goes down the centre of the oral surface of each arm.
3. Tube Feet or Podia
Tube feet, also known as podia, are a group of four rows of locomotor, food-capturing, respiratory, and sensory organs that are located in each ambulacral groove. The tube feet are retractile structures with terminal discs or suckers that are soft, thin-walled, tubular, and retractile. For secure adhesion to the surface they are placed on, the suckers act as suction cups.
4. Ambulacral Spines
Two or three rows of moveable calcareous ambulacral spines that can close over the grooves surround and protect each ambulacral groove on the side. These spines frequently grow thicker and larger as they get closer to the mouth. They group into five groups, one at each disc interradius, and are known as mouth papilla. Three rows of thick, immovable spines can be found outside the ambulacral spines, after which another series of marginal spines can be found along the edges of the arms, separating the oral from the aboral surface.
5. Sense Organs
Five unpaired eyespots and five unpaired terminal tentacles make up the sense organs. The terminal tentacle, a little midline, non-retractile, hollow protrusion, is present at the tip of each arm. It functions as an olfactory and tactile organ. A brilliant red photosensitive eye spot made composed of many ocelli appears at the base of each tentacle.
(iii) Aboral Surface
Aboral or abactinal surface refers to the convex, pale orange to the purplish-coloured side of the body that continues to face upward or toward the upper surface. The following structures are seen on the aboral surface:
1. Anus: The anus, a tiny circular orifice, is located at the centre of the central disc of the aboral surface.
2. Madreporite: A flat, sub-circular, asymmetrical, and grooved plate known as the madreporite plate or madreporite is located between the bases of two of the five arms at the aboral surface of the central disc. On the surface of madreporite, numerous radiating, pore-filled grooves are either thin, straight, or somewhat wavy. Thus, the madreporite is a porous plate like a sieve that connects to the water circulatory system’s stone canal. The existence of more than one madreporite in some species is caused by an increase in the number of arms over the typical five. The number of madreporites per individual remains at one. A bivium is the collective name for the two arms that have madreporite between their bases, and a trivium is a name for the other three arms. As a result, Asterias’ radial symmetry is transformed into bilateral symmetry by the symmetrical position of the madreporite.
3. Spines: Numerous short, robust, blunt, calcareous spines or tubercles cover the entire aboral surface. Variable in size, the spines are placed in haphazard rows parallel to the long axis of the arms. The calcareous plates or ossicles, which are buried in the integument and make up the endoskeleton, provide support for the spines.
4. Papulae or Gills: There are a lot of tiny dermal pores located between the integument ossicles. The dermal branchia, gill, or papula is a very tiny, fragile, tubular, conical, finger-like, or thread-like, thin-walled, membranous, and retractile projection that emerges from each dermal pore. The lumen of the papulae, which are hollow evaginations of the body wall, continues along with the coelom. Coelom lines the inside of them. They provide both respiratory and excretory activities.
5. Pedicellariae: Pedicellariae, which are tiny, modified pincers or jaws with a whitish colour and a spine-like pattern on them, cover the entire aboral surface in addition to the spines and gills. Pedicellariae can be found on the oral surface as well. Each pedicellaria consists of a long or short, sturdy, flexible stalk that lacks internal calcareous support. The stalk has three calcareous ossicles or plates, a basilar piece or plate at its extremity, and jaws or valves that are movably articulated with the basilar piece and serrated along their apposed edges. Forcipulate pedunculate pedicellariae are characterised by three calcareous parts and a stalk.
Asterias possess two types of forcipulate pedunculate pedicellariae. viz.:
(i) Straight type
(ii) Crossed type
(i) Straight type Pedunculate Pedicellariae: These pedicellariae are simple. More-or-less straight and linked basally to the basal portion are their two jaws. When shut, they remain parallel and come together at the end of their length. Three muscle pairs help the two jaws push against one another like the blades of a force. The jaws are closed by two pairs of adductor muscles and opened by two pairs of abductor’s muscles.
(ii) Crossed type Pedunculate Pedicellariae: The basal ends of the two jaws of the crossing kind of pedicellariae cross one other like the mandibles of a crossbill, enclosing the basal piece between their crossed sections. Two sets of adductor muscles and one pair of abductor’s muscles also control the jaws in this species of pedicellariae. On Asterias’ body, you can also find other pedicellariae known as sessile pedicellariae since they have no stalk. By preventing detritus and creatures like sponges and coelenterates from adhering to the body surface, they act as both offensive and defensive organs and protect the gills and general body surface.
Body Wall of Asterias
The body wall of Asterias consists of the following tissue layers:
(i) Cuticle: A distinct cuticle made up of two layers, an inner delicate layer and an outside thick homogeneous layer, covers the body’s surface.
(ii) Epidermis: A layer of ciliated epithelium that covers all of the body’s exterior appendages, including the spines, pedicellariae, tube feet, and gills, is found just below the cuticle. The epidermis is made up of a variety of cells, including common flagellated or ciliated columnar cells, neurosensory cells, mucous gland cells or goblet cells with finely granular contents, muriform gland cells filled with coarse spherules, and pigment granules that give the animal its distinctive external colour.
(iii) Nervous Layer: The thickness of the neural layer under the epidermis varies depending on the area and is penetrable by the attenuated bases of the epidermal cells on their elastic filaments.
(iv) Basement Membrane: A fragile basement membrane, located just below the nervous layer, divides the epidermis and nerve layer from the dermis underneath.
(v) Dermis: Fibrous connective tissue that was produced from the mesoderm makes up the dermis. It has two parts, the outer and the inner, and is the thickest layer of the body wall. The endoskeletal ossicles are secreted, housed, and bound together by the outer dermal region, while the inner dermal region has multiple blood-containing spaces known as perihaemal spaces.
(vi) Muscular Layer: Smooth muscle fibres make up the layer of muscle. An outer circular muscle layer and an interior longitudinal muscle layer are distinguished. The majority of these muscle layers are underdeveloped, except for those in the aboral wall, where stronger longitudinal bundles radiate forth from the disc’s centre along the mid-dorsal line of each arm to bend it aborally.
(vii) Coelomic Epithelium: The coelom is lined by the body wall’s deepest layer, which is made up of flagellated cuboidal cells with mesodermal ancestry. The peritoneum or coelomic epithelium is the term for the body wall’s deepest layer.
Coelom in Asterias
A coelomic epithelium of ciliated cuboidal cells lines the real and roomy coelom of Asterias. It has several compartments, viz
(i) A perivisceral coelom that surrounds the gonads and other visceral organs like the digestive system and extends in a central disc and rays,
(ii) Coelom of water vascular system,
(iii) Axial sinus,
(iv) Perihaemal sinus and canals and,
(v) Genital sinuses, etc.
A colourless, alkaline fluid called coelomic fluid fills the coelom, and it contains dissolved nutrients such as amino acids, fatty acids, glycerol, and glucose. Along with nutrition, the coelomic fluid also contains two different forms of phagocytic amoeboid corpuscles known as coelomocytes: those with petaloid pseudopodia and those with regular, slender pseudopodia. Similar to the haemolymph of arthropods, the coelomic fluid bathes the body’s tissue and serves as the circulatory system. It carries out the respiratory and excretory activities as well as the distribution of nutrients to various bodily cells.
Starfish: Interesting Facts
Sea stars, also known as starfish, are exquisite creatures that come in a variety of colours, forms, and sizes. The majority of starfish only have five arms, but certain species can have up to 40 arms, giving them all a starlike appearance. The fascinating marine organisms, which are echinoderms, use their tube feet to move about. With the help of their unique stomachs, they can devour huge prey and regrow damaged limbs.
1. A Sea Star Is Not A Fish
Sea stars are not actual fish, even though they are generally known as “starfish” and live underwater. Like fish, they lack fins, gills, and scales.
2. Echinoderms Are Sea Stars
3. There Are Numerous Species of Sea Stars.
4. Sea stars come in roughly 2,000 different species.
5. A Sea Star Does Not Always Have Five Arms
6. Arm Regeneration in Sea Stars
7. The Armor That Guards Sea Stars
8. No Blood Is Found in Sea Stars
9. Moving with their tube feet, sea stars have eyes, and they eat by turning their stomachs inside out.
10. True starfish are all members of the Asteroidea class.