The Asteroids are free-living echinoderms, with radial symmetry and moving on their oral surface. Asteroids consist of a central disc with the mouth in the middle of the undersurface (oral side) and anus in the centre of the upper surface (aboral side). Ray-like extensions, called rays or arms (usually five, sometimes many) radiate laterally from the disc, though not always distinct (Anseropoda, Culcita and others are shaped like pentagons). The outer surface is rough, warty, tuberculate or spiny and the arms may be fringed with spines.
Pentamerous symmetry is the norm and five arms are common, though the number differs depending on species. The arms define five radii of symmetry. With the starfish held oral surface uppermost and with the arm opposite the madreporite labelled A, the other arms/radii are labelled clockwise in alphabetical order. Interradii are between the arms. Dimensions vary, the largest starfish are some 60 cm from arm tip to arm tip, and the smallest are about 1 cm when fully grown.
The arms may be separate for much of their length (e.g. the Common Starfish, Asterias rubens) or joined for most of their length as in Cushion-stars or Starlets (e.g. Asterina gibbosa) or arms may be absent (as in the pentagonal Culcita). The colour of sea stars varies from yellow to orange, red, green, blue, gray and brown and they may be patterned. The aboral surface is generally more intensely coloured while the oral surface is generally paler.
The body wall
The outer surface is covered by monociliated (flagellated?) and non-ciliated epithelial cells, mucus cells and ciliated sensory cells. The mucus traps detritus which is swept away by the cilia so keeping the animal’ surface clean. Minute pincer-like movable pedicellariae assist in this function. These often surround the spines (and are themselves modified spines) and may be stalked or non-stalked (sessile). These jaw-like pincers remove small animals and larvae that settle on the starfish. At the base of the single-layered epithelium is a layer of nerve cells forming the subepidermal plexus. The epithelium rests upon a basement membrane.
Below the integument is the thick dermis, which contains skeletal plates, called ossicles. Each ossicle is a single crystal of magnesium-rich calcite (6(Ca,Mg)CO3) and together the ossicles form an endoskeleton. In burrowing starfish the centre of each aboral ossicle may be raised in a parasol-shaped paxilla, which may be crowned by small movable spines. Adjacent paxillae create a protected space above the integument through which respiratory and feeding currents may flow even when the animal is buried. Ossicles are bound together by connective tissue.
A single groove, the ambulacral groove, runs down the oral surface of each arm. Rows of tube-feet lie within these grooves. The ambulacral grooves are supported by a definite arrangement of ossicles: two rows of rod-shaped ambulacral ossicles form the V-shaped ambulacral groove itself. Where these meet they form the prominent ambulacral ridge on their inner surface. Each ambulacral ossicle forms half of a pore through which a tube-foot protrudes between each serial pair of ossicles in each row resulting in two rows of tube-feet per arm, one on each side of the groove. (The pores may zigzag giving the impression of four rows of tube-feet per ambulacral groove). Lateral to the ambulacral ossicles are the adambulacral ossicles bearing movable spines. These spines may be lowered across the ambulacral groove or raised by pairs of antagonistic muscles.
Most asteroids are carnivorous and predate slow-moving or sedentary animals and also weak fish. They will also scavenge from carcasses. In those with long, flexible arms the prey is held by the arms while the stomach is everted onto the prey, releasing enzymes that digest the soft tissues, which are then sucked into the digestive tract. When feeding on bivalves (e.g. muscles, oysters) these seastars will prize the valves apart (using their tube-feet suckers to gain a hold) until they open by as little as 0.1 mm and then they will evert their stomach through this gap and digest their prey! Of course the bivalve will resist opening using its adductor muscle to try and keep the valves shut, however, the starfish usually gains an opening within 5-25 minutes, though the whole process from opening the bivalve to complete digestion has been seen to take up to 10 hours. The force required to open a bivalve is considerable and it is debatable whether or not the starfish can win by brute force, or whether it utilises toxins. Stomach contents of some starfish are known to have cardio-toxic effects on oysters.
Many starfish swallow their prey whole rather than everting their stomach (cardiac stomach). They generally eat small animals, but may have very distensible mouths and may consume bivalves, snails, crustaceans, polychaetes, and other echinoderms, including young starfish.
Non-predaceous starfish may feed by everting their stomach over the sea-bottom, digesting any organic matter encountered. Some species catch small fish and crustaceans with their pedicellariae, if these animals come to rest on top of the seastar.
Some starfish are ciliary mucous feeders: plankton, detritus or mud that contacts the body surface is trapped in mucus then transported to the ambulacral grooves by the epidermal cilia and then along the mouth. In non-ciliary mucous feeders the same mucus-ciliary mechanism serves to remove debris from the animal.
Whilst some starfish have very restricted diets, others are generalists and feed on whatever is available, though they may have preferences. They detect and locate prey by chemicals released into the water. Some can detect buried prey and then burrow down into the substratum to reach it. Finally some starfish feed using a combination of the above methods.
The Digestive System
The alimentary canal is short and straight connecting the ventral mouth to the dorsal anus. The mouth opens in the centre of the peristomial membrane and is provided with a sphincter. The mouth leads into a short, wide oesophagus that connects to the stomach. The stomach is often divided by a constriction into the oral voluminous and folded cardiac stomach and the smaller flattened aboral pyloric stomach. Connected to the pyloric stomach, via pyloric ducts, are ten glands: the pyloric caeca (digestive glands, brachial caeca, hepatic caeca), two of which run, more or less, the length of each arm). Each pyloric caeca is attached to the aboral wall of each arm by two longitudinal mesenteries. Two mesenteries also attach the cardiac stomach to each ambulacral ridge (gastric ligaments). Other mesenteries connect the stomach to the disk walls and to the interbrachial septa. A very short intestine connects the pyloric stomach to the anus. Rectal or intestinal caeca may be attached to the intestine. The intestine distal to the caeca is sometimes called the rectum. One or more of the anus, intestine and intestinal caeca are absent in some families.
Digestion is largely extracellular, the stomach wall and pyloric caeca secreting enzymes. Ciliary currents carry digested particles from the stomach into the pyloric ducts and into the pyloric caeca where they are further digested (extra- and intracellularly) and absorbed. Products of digestion may be stored in the caeca or passed into the coelom for distribution around the seastar. Waste is passed from the pyloric caeca to the rectum, via the pyloric ducts, and expelled through the anus. If rectal caeca are present, then these aid expulsion by pumping.
The general body cavity: the perivisceral coelom of the disc is a single cavity continuous with the arm cavities. The tubular coelomic systems comprise the well-developed water-vascular, haemal and perihaemal systems. Coelomic fluid is similar to sea water, but has a slightly higher K+ content and lower Mg+ content and contains protein and coelomocytes (phagocytic amoeboid cells) and is less alkaline (pH 6 - 8.1) than sea water (pH 8.2+).
Coelomic fluid is kept circulating by the ciliated lining of the coelom. Generally it may flow towards the arm tips aborally and then back to the disc along the ventrolateral surfaces. These currents ensure thorough mixing of the coelomic fluid.
The water-vascular system is a system of water-carrying tubes that function to supply fluid to the hydraulically operated tube-feet (podia, sing. podium). This system is unique to echinoderms. The internal fluid is similar to sea water except that it contains coelomocytes, some protein and has an elevated potassium content. Cilia drive water around this system, but are assisted by ampullae or sac-like contractile pumps.
The madreporiteis a circular, grooved plate situated on an interradius and the only external structure breaking the radial symmetry. The bottom of each groove contains many pores by which the madreporite connects the water-vascular system to the external environment, but its specific function is uncertain. It may simply function to allow external and internal hydrostatic pressures to equilibrate. A cranny in its inner surface contains the madreporic ampulla and the dorsal sac. The madreporite leads vertically down into the stone canal (so-called because its walls are strengthened with calcareous spicules). The stone canal contains a scroll-shaped projection into its lumen from one of its inside walls, which facilitates water circulation (towards the mouth inside the rolls of the scroll and away from the mouth outside the scroll). At its oral end the stone canal opens into the circum-oral water ring and connects to the madreporic ampulla at its aboral end. This ampulla contracts cyclically and acts as a pump assisting fluid flow. From the circum-oral canal one radial canal passes along each arm, giving off side-branches to each tube-foot (via an ampulla at the base of each podium) and terminating in a modified terminal tube-foot, which lacks an ampulla and is sensory in function.
Tiedemann’s bodies Five pairs of interradial glands arising from the wall of the circum-oral ring (one may be missing where the stone canal joins the circum-oral ring, leaving 9 bodies). It is thought that these glands may synthesise coelomocytes.
Polian vesicles Interradial muscular sacs born on the circum-oral ring in some Asteroids (absent, for example, in the common Asterias). Probably function to maintain pressure in the system.
Mode of Operation of the Tube-feet
Tube feet (podia) are supported by a hydraulic and connective-tissue skeleton under the control of antagonistic muscle pairs systems. The essential components are:
A fluid-filled cavity. Contraction of the ampulla pump by ampulla muscles increases the pressure in the ampulla cavity, which forces its fluid into the fluid-filled cavity of the podium. The resultant increase in pressure within the podium causes tube-foot extension.
Connective tissue skeleton. Tube-foot extension under pressure is permitted and limited by radial hoops of connective tissue fibres arranged in series down the length of the podium, and connected by a longitudinal or axial series of hinges. The hinges allow the hoops to move together when the podium retracts and to separate by a finite amount when the podium extends. The hoops prevent wasteful radial extension of the podium when it is under pressure.
Retractor muscles. These longitudinal muscles shorten the podium by contracting, when the ampulla muscles relax. The resultant pressure moves fluid back into the ampulla. The retractor muscles and ampulla muscles are antagonistic.
Orienting or postural muscles. These work in antagonistic pairs to move the tube foot forwards and backwards. When the podia are attached to the substrate, co-ordinated movements forwards or backwards will propel the echinoderm.
Sucker. The terminal sucker of the podium enables the podium to attach to the substrate and hence to apply force to the substrate.
Disc levator muscles. Contraction of these muscles breaks the sucker seal, enabling the adhered tube foot to detach from the substrate.
Terminal plate. A skeletal plate in the centre of the disc, to which the disc levator muscles attach.
This fluid-transport system is enclosed in coelomic spaces (perihaemal spaces/sinuses) and is not readily apparent except in serial sections. The oral haemal ring (enclosed in a septum in the perihaemal or hyponeural ring sinus) gives off a radial haemal sinus (enclosed in the septum in the hyponeural radial sinus) into each arm. These radial sinuses run oral to the radial water canals. The aboral haemal ring (running around the rectum inside the aboral or genital coelomic sinus) gives off branches to the gonads within each arm (inside the coelomic branches to the gonads). The pyloric haemal ring surrounds the pyloric stomach and gives off branches, called the gastric haemal tufts, to the walls of the cardiac stomach and the hepatic haemal strands to the walls of the hepatic caeca of each arm. Perihaemal sinuses do not enclose these parts of the haemal system. Products of digestion enter the haemal system.
Axial gland Contains the axial haemal sinus and terminates in the contractile dorsal sac, which acts as a pump for the haemal system. The axial gland, the gastric haemal tufts and the aboral haemal ring are also reported to be contractile. The axial gland is rich in coelomocytes.
Between the ossicles sac-like or wart-like vesicles protrude from the external surface of the starfish. These are called papulae and their fluid-filled interiors are continuous with the coelom. These are formed from two ciliated epidermal layers – the external ciliated epidermis covering the starfish and the internal ciliary epithelium lining the coelom cavities – with a thin layer of connective tissue sandwiched in-between. As fluid flows through the coelom, driven by ciliary currents, it gives rise to eddies inside the papulae. Coelomocytes trapped inside the eddies accumulate inside the papulae, where they may form a clot. Coelomocytes ingest foreign materials and non-soluble waste products and then collect in the tips of the papulae, which are pinched off. Other coelomocytes migrate to the outside through the epidermis, especially on the tube feet, and hence remove waste from the starfish. Waste-laden coelomocytes also exit via the pyloric caecae and madreporite. The pyloric caeca may also directly absorb and expel waste from the coelomic fluid. Nitrogenous ammonium diffuses out through tube feet and papulae.
The coelomic fluid is similar to sea water and there is no power of osmoregulation and the body wall is permeable to salts and water. Starfish can adapt to a range of salinities, however.
Gas exchange occurs across the podia and papulae. This is aided by ciliary currents on the outer and inner epithelia of the papulae. In burrowing starfish, branched papulae are protected by the paxillae and ventilating currents flow through the channels underneath the paxillae.
Nervous and Sensory Systems
The nerve centre consists of a pentagonal circumoral nerve ring, in the peristomial membrane just beneath the peristomial epidermis. This gives off five sensory radial nerve cords which travel the length of each arm in the bottom of the ambulacral groove just interior to the epidermis and separated from the hyponeural sinus on its interior side by a thin dermis and the coelomic epithelium. Each cord terminates in a sensory cushion aboral to the base of the terminal tentacle. The radial nerves are V-shaped in cross-section. The radial nerves are continuous with the subepidermal plexus, which covers the whole surface and is concentrated around body-wall appendages, which it innervates. These appendages include the podia. In the outer margins of each ambulacral groove the subepidermal plexus is thickened into marginal nerve cords. A pair of these marginal nerve cords innervates each arm with motor neurons. They give off a pair of lateral motor nerves to each ambulacral ossicle, innervating the lateral transverse interambulacral muscles.
Lange’s nerve is a nervous sheet in the lateral part of the oral wall of the hyponeural sinus. These nerves are primarily motor and extend to the peristomium. They are separated from the radial nerves by a thin connective tissue layer.
At the end of each arm is a modified podium, the terminal tentacle, which has a sensory function. At the oral base of the terminal tentacle is the optic cushion (a red spot): a cluster of pigment-cup ocelli, which may be covered by lenses. Up to 200 ocelli may cluster in one optic cushion. Some species lack ocelli and some deep water species lack photoreceptors altogether. When starfish move they often curve the tips of their arms upwards to expose the ocelli to the light.
Sensory cells are scattered over the epidermis and concentrated on the surface of the podial suckers, the bases of spines and pedicellariae and along the adambulacral region (up to 7 x 104 per mm2) and on the terminal tentacles.
Most starfish are negatively phototactic: avoiding light and preferring shade. Many however prefer light, though this may depend on the light intensity: moderate light may be favoured, but direct sunlight avoided. Burrowing seastars may emerge under suitable moderate levels of light.
Starfish will right themselves if placed upside-down. When inverted, the starfish will be still for a moment and will then curve its arm-tips aborally until the podia gain a grip on the substratum. Usually two of the arms will then walk underneath the animal, recruiting more podia and raising the disc, which eventually flips over and is lowered (a slow somersault, which takes from 20s up to 90 min). Whether the stimulus is loss of podial contact, gravity or some other stimulus is uncertain. Isolated arms are also capable of righting. At least some starfish are known to be responsive to gravity, though this may be a response to the direction of pull on the podia.
Starfish, and isolated arms of starfish, respond to touch. The podia retract if touched, and the retraction may spread along the arm and then to the whole animal, followed by podia re-extension. Touching the aboral surface may evoke the dorsal reflex: dorsal flexure of one or more arms. If the side of an arm is touched, podia may extend towards the stimulus.
The coelomic side of the body wall contains an outer circular and an inner longitudinal muscle layer.
The longitudinal muscle is thickened into a median aboral line that runs from the disk along each arm.
An upper transverse muscle and a lower transverse muscle connect each pair of ambulacral ossicles. Contraction of the upper causes the ambulacral groove to widen, whilst contraction of the lower narrows the groove.
Upper and lower longitudinal ambulacral muscles connect adjacent ambulacral ossicles, contraction of which shortens the ambulacral groove.
Longitudinal muscles between adjacent ambulacral ossicles aid in sideways movements of the arms.
Dorsolateral arm muscles in Benthopectinids may cause thrashing movements of the arms allowing these starfish to swim.
Coordinated action of the tube feet brings about slow creeping locomotion. One arm temporarily dominates and leads the way, according to which arm receives the strongest positive stimulus (induced arm dominance). Alternatively, in some starfish one particular arm may dominate most of the time (intrinsic arm dominance).
Asexual reproduction occurs in some species. Most starfish seem to have great regenerative powers and will regrow lost arms and repair damage to the disc. Often only one arm with a small piece of disc attached to it is all that is required for complete regeneration. Regeneration may require a year to complete, however. Uniquely Linckia is able to reproduce by forcibly casting off whole arms (autotomy): the arm regenerated into a complete starfish. (Regenerating forms are known as comets when they have only small regenerating arms at the base of the old arm). Spontaneous fission is common in some genera: the disc splits in two along a pre-determined line that leaves the arms intact. Each half subsequently regenerates into two new starfish.
Sexual reproduction. Most asteroids are dioecious and have ten gonads: two in each arm. These normally occupy a small volume near the base of the arm, but almost completely fill the arm when full of eggs or sperm. Each gonad opens via a gonopore (or cluster of gonopores) usually located between the bases of the arms (sometimes on the oral surface). Some seastars are hermaphrodite, this depends on species and also varies within a species. Some individuals of a normally dioecious species may have one or more mixed gonad. Once a year the gametes are shed into the sea. A single female may shed 2.5 x 106 eggs. Fertilisation occurs in the sea. Starfish will aggregate together prior to shedding gametes, so maximising the chances of fertilization. Some may pair off, with the male sitting on top of the female with his arms alternating with hers. The presence in nearby water of gametes of the opposite sex will stimulate gamete shedding. Gamete shedding generally occurs in spring in the Northern Hemisphere and may be a response to rising temperatures.
Sexes are generally visibly indistinguishable, though there may be slight colour-differences in some species and minor morphological differences (statistical differences in body shape and/or size). In many hermaphrodites the sex may change as the starfish grows.
Generally parental duties end once the gametes are shed into the sea; however, some mother starfish brood their eggs. The strategy depends on species and the different modes of brooding are as follows:
The starfish may arch upwards on its arms to form a brooding chamber.
Eggs may be kept in the pouches of the cardiac stomach.
The paxillae of some cushion stars may support a supradorsal membrane with the enclosed space being ventilated as water enters through incurrent spiracles and exits through the excurrent osculum in the membrane. Eggs may be brooded in this chamber.
The bases of the arms may swell and their ossicles interlock to form a basket. Each such interradial container may hold 5-9 eggs.
Brooding species produce fewer eggs (a few hundred at most) and larger, yokey eggs.
Radial cleavage produces a hollow blastula. Gastrulation leads to a gastrula. The embryo becomes free swimming at some point between the blastula and gastrula stage. Initially the entire embryo surface is ciliated, but later the cilia become defined to a locomotor band as the gastrula develops into a dipleurula larva. The dipleura larva has a circumoral ciliary band, which distinguishes it from trochophore larvae possessing an equatorial ciliary tract (the prototroct). Arms develop from the body surface and the ciliary bands extend into these arms, thereby increasing their effective surface area. The larva is now a bipinnaria larva. The ciliary bands are used in locomotion and feeding. The bands transport fine suspended particles and phytoplankton to the mouth.
Three additional short flexible arms develop at the anterior end and the larva becomes a brachiolaria larva. The coelom is continuous with these three arms and the arm tips contain adhesive cells. These adhesive arms temporarily anchor the larva to the substrate when it settles out of the water column about two months after the beginning of the brachiolaria stage. Between the bases of these three arms is an adhesive sucker, which subsequently forms a more permanent adhesion.