What Are Echinoderms?
Echinoderms are varieties of invertebrate marine animals belonging to the phylum Echinodermata. They include starfishes (sea stars), brittle stars, sea cucumbers, sea urchins, and sea lilies. These marine invertebrates exhibit brilliant colors such as red, green, orange, and purple. However, many tropical echinoderm species are dark brown to black. Though lighter colors like yellow are common among echinoderm species that are not normally exposed to strong sunlight.
Echinoderms are an ancient group of animals. They extend back to at least the Cambrian period. An enigmatic fossil Arkarua, from the Vendian about 560 million years ago has been identified as the earliest known echinoderm. Though many authorities have considered this identification inconclusive. However, it seems clear that the echinoderms descended from bilateral ancestors. This is because their larvae are bilateral but later become radially symmetrical as they developed. It is believed by many scientists that early echinoderms were sessile and evolved radiality as an adaptation to sessile existence.
Echinoderms exhibit a great diversity of body forms. Their shape may be like that of a star that possesses arms extending from a central disk. Also, they can appear with branched and feathery arms extending from a body that is often attached to a stalk. Their shape too may be round to cylindrical. However, the body form of the echinoderm ranges from that of the wormlike burrowing sea cucumbers to the stemmed flowerlike sea lilies, and to the heavily armored intertidal sea urchin or starfish. Moreso, the plates of their internal skeleton may articulate with each other like in sea stars or be sutured together to create a rigid test, like in sea urchins.
Echinodermata (Phylum of Echinoderms)
Echinodermata represents a bizarre group sharply distinguished from all other animals. The name Echinodermata is derived from the external spines or protuberances of echinoderms. The projections from their skeleton, which sometimes resemble spikes give the phylum its name. Echino in greek meaning spiny and derma meaning skin. However, all members of this phylum have a calcareous endoskeleton either in the form of plates or represented by scattered tiny ossicles. The most noticeable characteristics of members of the phylum Echinodermata are:
- Spiny endoskeleton of plates
- Water-vascular systems
- Dermal branchiae
- Basic Penta-radial symmetry in the adults
There is no other group with such complex organ systems that has radial symmetry. However, the phylum Echinodermata is divided into several classes. It consists of 5 distinct classes.
Classes of Echinoderms
- Class Asteroidea (Sea stars, often called starfishes)
- Class Ophiuroidea (Brittle stars or serpent stars, basket stars)
- Class Echinoidea (Echinoids such as sea urchins)
- Class Holothuroidea (Sea cucumbers)
- Class Crinoidea (Crinoids include species of feather stars and sea lilies)
Where do Echinoderms live?
Echinoderms live in all oceans of the world and are found in the coldest and warmest seas of the world. The most common animals in deep oceans are echinoderms and they exist at all depths, from intertidal to abyssal regions. They rarely venture into brackish waters and only a few of them tolerate brackish water. Most of them cannot tolerate changes in temperature, salinity, and light intensity. Hence, they tend to move away from water bodies where these factors are not optimal. The vertical or horizontal distribution of many echinoderm species is governed by water temperature.
Echinoderms generally are bottom dwellers, though there are a few pelagic species. However, a large population of echinoderms can be seen in mud and ooze offshore. Thus, echinoderms are the dominant organism in some marine areas. Echinoids and asteroids are mostly found along the seashore and many species associated with coral reefs range across the entire Indian and Pacific oceans. However, echinoderms tend to have a fairly limited depth range as species that occur in near-shore environments do not usually reach depths more than 328 feet. Some deep-sea species of echinoderms may be seen often from 3,280 feet to more than 16,400 feet. Moreso, one sea cucumber species has a known range of 121–17,077 feet and only sea cucumbers reach ocean depths of 32,800 feet and more.
Additionally, echinoderms usually use other animals as homes. For instance, thousands of brittle stars may live in some tropical sponges. Another example is the sea cucumbers that may attach themselves to the spines of sluggish Antarctic echinoids. Also, one sea cucumber may attach itself to the skin of a deep-sea fish. Echinoderms on the other hand, also host a wide variety of organisms. For example, Sea urchins host various crustaceans and barnacles. Also, crinoids host parasitic worms. There is also commensalism among tropical sea cucumbers and pearlfish.
What do echinoderms eat?
- Brittle stars eat small organisms that float in the water or lie on the bottom.
- Basket stars feed on zooplankton.
- Starfishes are hunters and prey on shellfishes and other starfishes. They also feed on sponges, corals, sea anemones, detritus, and algal films.
- Many sea urchins feed on algae and may feed on dead animals matter or fish too.
- Crinoids feed on particles that float past and they can be fed in the laboratory with diatoms.
- Sea cucumbers ingest large amounts of sediments and absorb the organic matter. Then through their gut, they pass out the indigestible mineral particles.
What echinoderms eat varies greatly amongst them. Sea urchins are grazers, whereas crinoids and some brittle stars are passive filter feeders. Also, Sea stars are active hunters whereas sea cucumbers are deposit feeders.
How do echinoderms eat?
Echinoderms eat and feed in several ways and their feeding habits range. However, their feeding could be scavenging, active selective predation or nonselective mud swallowing. Nevertheless, echinoderms can survive apparent starvation for several weeks at a time under artificial conditions such as the aquarium.
Many ophiuroids feed on small organisms lying on the bottom or floating in the water. Hence, they capture them with their arms and tube feet and pass them towards the mouth.
Most asteroids are active predators while others are mud swallowers. They prey on shellfish and even other starfishes. As some asteroids feed, they extrude their stomach through the mouth onto the prey. Hence, partial digestion occurs externally, after which the stomach is retracted. Then digestion is completed inside the body.
The crinoids are suspension feeders. They capture planktonic organisms in a network of mucus produced by tube feet (soft appendages) contained in grooves on the arms or tentacles. The arms are then spread at right angles to the prevailing current. Thus by the activity of the cilia and the tube feet, small prey animals are then passed to the mouth of the crinoids along the grooves.
The sea urchins are omnivorous or vegetarian browsers. With their hard teeth, they either scrape algae and other small organisms from rocks or eat seaweed. Many deep-sea echinoids feed mainly on plants that are carried into the sea from land. Whereas those echinoids that usually lack teeth are burrowers. With the aid of spines and tube feet, they pass small organisms to the mouth.
A huge number of holothurians feed by actively swallowing mud and sand. They digest the organic material and egest the waste in the form of characteristic castings. This is done in a manner similar to that of earthworms. Some sea cucumbers capture plankton in a network of branching sticky tentacles. Whereas other sea cucumbers select food from the seafloor and with their tentacles, push it into their mouths.
Echinoderms digestive system
Echinoderms have a simple digestive system that varies based on their diet. Generally, their digestive system consists of the mouth, intestine, stomach, and anus. In many echinoderm species, the anus is on the top surface and the mouth is on the underside of the animal.
Sea stars are mostly carnivorous and have a mouth, esophagus, intestine, two-part stomach, and rectum. Their anus is located in the center of the aboral body surface. They can push their large cardiac stomach outside of their body onto their prey, allowing them to digest the food externally. This feeding habit allows sea stars to hunt prey that is much larger than their mouth would otherwise allow. Whereas in other species, the whole food must be ingested.
Crinoids catch plankton with their outstretched arms as passive suspension feeders. The food is passed to the mouth. The mouth is linked by a loop that consists of a short esophagus and a longer intestine to the anus.
Sea urchins use their specialized mouthparts to tear, graze, and chew algae and other vegetable or animal material. They are herbivores and have an esophagus, a large stomach, and a rectum. Their anus is at the apex of the test.
On the other hand, brittle stars have a blind gut with no intestine or anus. Their diets vary and they expel food waste products through their mouth. Sea cucumbers are detritivores and sort through the sediment with their buccal tentacles. They have a simple gut that has a long coiled intestine and a capacious cloaca.
How do echinoderms move?
Swimming is seen to occur in ophiuroids, crinoids, and holothurians. Ophiuroids tend to move by thrashing the arms in a rowing motion in which two pairs of extended arms take strokes. The fifth arm is either extended forward in the direction the ophiuroid is traveling or trails behind.
Asteroids and echinoids use spines and tube feet in locomotion. They may move forward with any area of their body and without turning around, reverse direction. The feet may be used either as levers, which the echinoderm steps along a surface, or as attachment mechanisms that pull them. Presumably, sea daisies move in the same way.
Holothurians movement is being carried out by the tube feet with contraction and expansion of the body. A slug-like movement is common amongst them. Also using their sticky tentacles as anchors, holothurians of the family Synaptidae are able to pull themselves across a surface. Moreso, some holothurians are capable of efficient swimming.
Stalked crinoids such as sea lilies have stems and generally are firmly fixed to a surface by structures called holdfasts. Some of them release themselves to move to new attachment areas. Whereas the unstalked crinoids such as feather stars generally swim. This is done by beating their numerous arms down and up in a coordinated way against the water. However, feather stars that do not swim, pull themselves using their arms across a surface.
- Echinoderms are free-living taxa.
- They have a unique water-vascular system of coelomic origin that extends from body surfaces as series of tentacles-like projections (podia or tube feet) protracted by an increase of fluid pressure within them.
- Echinoderms live in marine habitats.
- Their body is unsegmented with Penta-radial symmetry.
- Echinoderm body is rounded, cylindrical, or star-shaped with five or more radiating areas or ambulacra, alternating with interambulacral areas.
- They have no head.
- The nervous system is of the circumoral ring and radial nerves.
- Autotomy and regeneration of lost parts are conspicuous.
- They have a triploblastic body.
- There is asexual reproduction by fragmentation in some echinoderms.
- The coelom is extensive, forming a perivisceral cavity.
- Echinoderms have an endoskeleton of dermal calcareous ossicles with spines or of calcareous spicules in the dermis.
- The digestive system is usually complete. It can be axial or coiled.
- Echinoderms have no brain but have few specialized sensory organs.
- They possess a sensory system of tactile and chemoreceptors, photoreceptors, podia, statocysts, and terminal tentacles.
- Echinoderms lack excretory organs.
- Their sexes are separate with large gonads.
- They possess simple ducts with no elaborate copulatory apparatus or secondary sexual structures.
- Fertilization is usually external.
- Eggs are brooded in some echinoderms and development is through free swimming, bilateral, larval stages.
- Respiration is done by tube feet, papulae, bursae, and respiratory tree.
- Their skeletal elements are connected by ligaments of mutable collagenous tissue under neural control.
- They locomote by tube feet which project from ambulacral areas by the movement of spines or by the movement of arms which project from the central disc of the body.
- The blood-vascular system is much reduced in echinoderms.
Echinoderms have been said to have evolved from animals that have bilateral symmetry. An adult echinoderm possesses five-sided or Penta-radial symmetry. Whereas the larval stage of echinoderm is ciliated, free-swimming organisms that organize in bilateral symmetry. This makes them look like embryos of chordates. However, later on, the left side of the body grows at the expense of the right side. The left side grows in a Penta-radially symmetric way, such that the body is arranged in 5 parts around a central axis.
Water Vascular System in Echinoderms
The water vascular system in echinoderms is a hydraulic system used by them for food and waste transportation, respiration, and locomotion. Ambulacral system and aquiferous system are terms sometimes used to refer to the water vascular system. This system is made up of canals that connect numerous tube feet.
The water vascular system is a coelomic compartment that is unique to Echinoderms. Echinoderms possess a unique ambulacral or water vascular system which consists of a central ring canal and radial canals that extend along each arm. Furthermore, water circulates through these structures and facilitates respiration, nutrition, locomotion, and predation.
This water vascular system also projects in the form of tube feet, from holes in the skeleton. These tube feet can contract or expand depending on the volume of water present in the system of that arm. Therefore, the organism can either protrude or retract the tube feet by using hydrostatic pressure.
Water then enters the madreporite (sieve-like structure on the upper surface) of the echinoderm. From the madreporite, the water passes into the stone canal, which moves it into the ring canal. Attached to the ring canal are 4 or 5 pairs of folded pouches (Tiedemann’s bodies) and from 1 to 5 Polian vesicles. Polian vesicles are for fluid storage and regulation of the internal pressure within the water vascular system. Tiedemann’s bodies may produce coelomocytes, which are somewhat similar to the blood of vertebrates.
The radial canals diverge from the ring canal, one into the ambulacral groove of each ray. A series of small lateral canals connect the radial canal to the tube feet or cylindrical podia, along the sides of the ambulacral groove in each ray. However, each tube foot has a muscular sac or ampulla at its inner end which lies within the body coelom. Thus, the radial canals move the water into the ampullae. The ampullae have tube feet through which the water moves. Hence, echinoderms can move and force open mollusk shells during feeding by moving water through the water vascular system. The water vascular system contents are essentially seawater. Aside from coelomocytes, the fluid also contains high levels of potassium salts and some protein.
The water vascular system operates hydraulically and is an effective locomotor mechanism. Its main design applies muscular pressure to the coelomic fluid in tube feet to stiffen them for walking. However, the structure of the water vascular system varies between the 5 classes of echinoderms. Therefore, the system is part of the coelomic cavities of echinoderms, together with the haemal coelom (or haemal system), gonadal coelom, perihaemal coelom, and perivisceral coelom.
Examples of Echinoderms
- Sea urchins
- Sea cucumbers
- Brittle star or Serpent Star
- Basket star
- Sea Daisies
Starfish are also called sea stars and are common examples of echinoderms. They have a star-shaped appearance and belong to the class Asteroidea. From the tropics to frigid polar waters, about 1,500 species of starfish live on the seabed in all the oceans of the world. From the intertidal zone down to abyssal depths of 20,000 ft below, Sea stars are seen.
These marine invertebrates usually have a central disc and five arms. However, some starfish species have a larger number of arms. Their upper surface may be spiny, smooth, or granular. The aboral is covered with overlapping plates. Many of them are brightly colored in various shades of orange or red. Whereas others are grey, blue, or brown.
They have tube feet and a mouth at the center of the lower or oral surface. Usually, they are opportunistic feeders, preying on benthic invertebrates. Starfishes have complex life cycles and can reproduce asexually and sexually. Most of them can regenerate damaged parts or lost arms. Also as a means of defense, they can shed arms. These sea stars are sometimes collected as curios and in some cultures, they are eaten.
Water vascular system of starfishes
In starfishes, water enters the system through the madreporite connected to the stone canal. The stone canal runs to the circular ring canal, from which radial canals diverge outwards along the ambulacral grooves. However, each arm of the sea star has one such groove on its underside. Whereas in echinoderms like sea urchins, they run along the outside of the body.
Each side of the radial canals bears a row of ampullae. These ampullae are connected by lateral canals. In starfishes, these are always staggered. This is so, for an ampulla on the left to follow one on the right, and so on down the length of the radial canal.
Suckerlike podia connect the ampullae. This entire structure is what is referred to as a tube foot. Therefore, contraction of the ampullae eventually causes the podia to stretch as water is brought into them. Hence, this whole process allows movement and even though it is extremely slow, it is quite powerful.
Type of Starfish
- Crown-of-thorns Starfish
- Cushion star
- Sunflower Sea Star
- Biscuit star
- Horned Sea Star or Chocolate Chip Sea Star
- Pink Sea Star
Sea urchins are spiny globular echinoderms that belong to the class Echinoidea. They have fivefold symmetry as adults whereas their pluteus larvae have bilateral (mirror) symmetry. Urchins inhabit all oceans and depth zones from the intertidal regions to deep oceans. Approximately 950 species of sea urchins occur on the seabed.
They have hard shells (tests) which are round and spiny. However, some of them have greatly reduced tests. Echinoids lack arms but their tests reflect a typical pentamerous plan of echinoderms in their five ambulacral areas. Usually, Sea urchins are seen moving slowly and crawling with their tube feet. Sometimes, they push themselves with their spines. Urchins feed mainly on algae, although they also eat sessile or slow-moving organisms. Animals like Starfish, sea otters, triggerfish, wolf eels, including humans prey on sea urchins.
The majority of sea urchins are regular and others are irregular. Regular urchins have a hemispherical shape, radial symmetry, and medium to long spines. Whereas irregular urchins like sand dollars and heart urchins are secondarily bilateral and their spines are usually short. Regular urchins move by means of their tube feet with some assistance from their spines. Whereas irregular urchins move mainly by their spines. Urchins are quite colorful. Soft-tested urchins usually have bright warning coloration. Moreso, their pedicellariae deliver painful toxins.
Water vascular system of Sea urchins
In sea urchins, the madreporite is located on the upper surface within one of the plates surrounding the anus. The stone canal descends to the ring canal from the madreporite. In sea urchins, the ring canal lies around the esophagus and has a number of Polian vesicles. Since sea urchins possess no arms, the 5 radial canals run simply along the inside of the solid skeletal “test”. These radial canals arch upwards towards the anus.
The ampullae branches off from either side of the radial canals and give rise to 10 rows of tube feet. These tube feet penetrate to the outside through holes in the test. However, the tube feet of sea urchins are often modified for different roles. Eventually, the radial canal ends in a small water-filled tentacle that protrudes through the uppermost plate of the ambulacral region.
Sea cucumbers are examples of echinoderms with elongated bodies and leathery skin. They are marine invertebrates and belong to the class Holothuroidea. Sea cucumbers are named due to their resemblance to the cucumber plant fruit. Their elongated body contains a single branched gonad. They are found globally on the seafloor with a great number of them in the Asia Pacific region.
Furthermore, their endoskeleton is just below the skin. The calcified structures are usually reduced to isolated microscopic ossicles which are joined by connective tissue. However, these can sometimes be enlarged to flattened plates in some species of sea cucumbers forming armor. Nevertheless, in pelagic species like Pelagothuria natatrix, there is no calcareous ring and the skeleton is absent. Many sea cucumbers are gathered for human consumption and some of their species are cultivated in aquaculture systems. These echinoderms serve a useful role as they help recycle nutrients, break down detritus and other organic matter in marine environments.
Water vascular system of Sea cucumbers
In sea cucumber, the water vascular system has no connection to the outside. Thus, instead of seawater, it is filled with internal coelomic fluid. The madreporite is just below the pharynx lying within the body cavity. Sea cucumber’s stone canal is relatively short and the ring canal usually has 1 to 4 Polian vesicles. Although, in the order Apodida, the Polian vesicle may be as many as 50.
The radial canals run through notches surrounding the mouth and then run along with the ambulacral areas. The ambulacral areas are along the length of the body. The lateral canals run to both the large oral tentacles and the tube feet. Both of which possess ampullae. However, the order Apodida, which has no tube feet, also has no radial canals. The canals run to the tentacles and branch off directly from the ring canal.
Crinoids are echinoderms that make up the class Crinoidea. However, there are crinoids in their adult form, that are attached to the sea bottom by a stalk. Such crinoids are commonly called sea lilies. Whereas the unstalked forms are called feather stars. The class Crinoidea was much more diverse and abundant in the past but currently, there are only about 600 living species of crinoids.
Adult crinoids are distinct by having the mouth located on the upper surface. The mouth is surrounded by feeding arms which are linked to a U-shaped gut. Near the mouth, on the oral disc, the anus of crinoids is located. Furthermore, in most crinoids, the five arms are subdivided into ten or more which have feathery pinnules. However, at some stage, most crinoids possess a stem that is used to attach themselves to the substrate. Although, many of them live attached only as juveniles and as adults become free-swimming.
Water vascular system of Crinoids
Uniquely, crinoids have no madreporite, rather the oral surface is dotted with several minute ciliated funnels. These ciliated funnels run into the main body cavity. The ring canal has numerous small stone canals which are located between the arms of the crinoids. The ring canals are open into the body cavity. Hence they are only indirectly connected to the outside.
The 5 radial canals run into the arms, branching several times. Thus to supply all of the pinnules and individual branches lining the arms. Eventually, the radial canals give rise to lateral canals, like in other echinoderms. However, there are no ampullae and clusters of 3 tube feet branch from the ends of each canal. Though they are found singly around the mouth. Additionally, the ring canal is surrounded by contractile muscle fibers and water pressure is maintained by it in the absence of ampullae.
Brittle Star and Basket Star
Brittle stars are also called serpent stars, or ophiuroids. They are echinoderms that belong to the class Ophiuroidea. This class contains two large clades- Euryalida (basket stars) and Ophiurida (brittle stars).
Brittle stars are the largest major group of echinoderms and are probably the most abundant, with over 2000 extant species. They abound in all types of benthic marine habitats and crawl across the seafloor using their flexible arms for locomotion. Generally, they have five long slender whip-like arms.
Brittle stars live on hard substrates in marine habitats where little or no light penetrates. They are usually negatively phototrophic. Hence they insinuate themselves into crevices between rocks and become more active at night. They feed either by browsing food from the bottom or by suspension feeding. Some brittle stars extend their arms into the water to catch suspended particles in mucous strands between arm spines. However, basket stars perch on corals and extend their branched arms to capture plankton.
Water vascular system of Brittle stars and Basket stars
Ophiuroids which include basket stars and brittle stars have a somewhat different water vascular system from sea stars, even though they have a similar appearance. The madreporite is usually located in one of the jaw plates on the underside of the organism. Located in a circular depression on the upper(internal) surface of the jaws, the stone canal runs upwards to the ring canal. However, the ring canal has 4 Polian vesicles.
Furthermore, ophiuroids have no ambulacral groove. Instead, the radial canals run through the solid bone-like ossicles of the arms. The tube feet are paired rather than staggered, and there are no ampullae. A simple valve at the upper end of the foot, in addition to the contraction of the associated canals, helps to control water pressure in the tube feet.
Some strange little disc-shaped animals less than 1cm diameter were discovered in water over 1000 m deep off New Zealand. They were originally described as a new class of echinoderms called Concentricycloidea. Sometimes they are called sea daisies and only two species are known thus far.
Sea daisies have no arms but are Penta-radial in symmetry. However, their tube feet are located around the periphery of the disc instead of along ambulacral areas like other echinoderms. One species has a shallow saclike stomach with no intestine or anus. Whereas the other species have no digestive tract and its oral surface is covered by a membranous velum by which it absorbs nutrients.
Blastoids are echinoderms that belong to the class Blastoidea. Often referred to as Sea buds, they are an extinct type of stemmed echinoderm. They first appear in the Ordovician period with many other echinoderm classes and reached their greatest diversity in the Mississippian subperiod of the Carboniferous period. However, blastoids persisted until their extinction about 250 million years ago, at the end of Permian.
Cystoids belong to the class Cystoidea. This is a class of extinct Crinozoan echinoderms that lived attached by stalks to the seafloor. They existed during the Paleozoic Era until their extinction in the Devonian Period.
Cystoids are distinguished by triangular pore openings from other echinoderms. Superficially, they resembled crinoids. Although they had an ovoid instead of a cup-shaped body. Their mouth was at the upper pole of their body, with the opposite end attached often by a stalk to the substratum. However, some stalkless species of Cystoids did exist. Thus, depending on the species, a number of small tentacles either projected outwards in a row from the ambulacral areas or surrounded the mouth.
More so, the most distinctive characteristic of cystoids was the number of pores present in the rigid skeleton encasing the body. These, allowing fluid to flow in or out of the body were most probably respiratory in nature. However, the pores were clustered in distinct regions of some species. Whereas, in other species, they were distributed widely over the body surface.
The Eocrinoidea are an extinct class of the earliest known group of stalked, arm-bearing echinoderms. They were the most common echinoderms during the Cambrian that lived between the Early Cambrian and Late Silurian periods.
The earliest genera of this group had short holdfast and irregularly structured plates. Then the later forms had a fully developed stalk with regular rows of plates. These echinoderms were benthic suspension feeders. They had five ambulacra on the upper surface, surrounding the mouth which extended into a number of narrow arms.
Reproduction in echinoderm is usually sexual, although several species of starfishes, sea cucumbers, and brittle stars can reproduce asexually. In most echinoderm species, the sexes are separate, having males and females.
Echinoderms release their sperm cells and eggs into the water. They are sexually dimorphic and fertilization occurs externally. The spermatozoa from males and up to several million eggs from the females are shed into the water (spawning). There, the eggs are fertilized.
Before spawning, several echinoderms aggregate. Hence, increasing the probability of the fertilization of the eggs. Also, some display a distinct behavior during the spawning process. For instance, holothurians may raise the front end of the body and wave it about. Whereas some ophiuroids and asteroids raise the center of the body off the seafloor. Presumably, these movements are meant to prevent sperm and eggs from becoming entrapped in the sediment.
Several species of echinoderms are capable of spawning throughout the year. Whereas most of them spawn on an annual cycle, with the spawning period usually lasting 1 or 2 months during summer or spring. Nevertheless, there are spawn-inducing factors such as light, temperature, or salinity of the water.
Using the Japanese feather star (Crinoidea) for instance. Its spawning is correlated with phases of the moon, taking place during early October when the moon is in the 1st or last quarter.
In some echinoderm species, the larvae divide asexually. Thus, they multiply before reaching sexual maturity. Hence, asexual reproduction in echinoderms usually involves fragmentation and regeneration. Fragmentation involves the division of the body into 2 or more parts and regeneration is the regrowth of lost, missing, or destroyed body parts in the animal. The regeneration ability is well developed in echinoderms. Brittle stars, starfishes, and sea lilies especially, can regenerate new arms once the existing one is destroyed.
Some species of holothurians, asteroids, and ophiuroids use fragmentation as a method of reproduction and sexual reproduction is not known to occur in some of these species. For a successful regeneration and fragmentation, a body wall that can be torn is required, together with an ability to seal wounds that will occur as a result of the tear.
Fragmentation occurs in some asteroids when 2 groups of arms pull in opposite directions. Hence, tearing the animal into 2 pieces. For regeneration to be successful, it requires that in the lost pieces, certain body parts be present. For instance, many ophiuroids and asteroids can regenerate a lost portion, if and only if some part of the disk is present. Also, considerable reorganization of tissues occurs in both regenerating parts of sea cucumbers that divide transversely. Moreso, some sea cucumbers under certain conditions, can expel their internal organs. They can expel their internal organs on a seasonal basis, or when the environment is unfavorable, and if attacked. Eventually, within several weeks, a new set of internal organs regenerates. Echinoderms like sea urchins regenerate their pedicellariae, lost spines, and little areas of the internal skeleton or test.
Several echinoderms develop from larvae stage to adolescence by direct or indirect development. The development of the resulting embryo into a juvenile may occur in a variety of ways after the fertilization of the egg.
Small eggs with less yolk develop into free-swimming larvae. These larvae actively feed on small organisms until they metamorphose into juvenile echinoderms. Thus, beginning their life on the seafloor. Whereas, larger eggs with greater amounts of the yolk may develop into a larval form that instead of feeding upon small organisms, subsists upon its own yolk material before transforming into a juvenile echinoderm.
However, echinoderm larvae undergo a complex metamorphosis after few days to several weeks in a free-swimming form that results in the juvenile echinoderm. The fundamental bilateral symmetry is overshadowed by radial symmetry during metamorphosis which is dominated by the formation of five water-vascular canals. However, the larvae may metamorphose as they float among ophiuroids, echinoids, and holothurians and the young then sink to the seafloor. Among crinoids and asteroids, before metamorphosis, the larvae firmly attach to the seafloor. Generally, the average life span of echinoderms is about 4 years and some may live as long as 8 to 10 years.
Indirect development in echinoderms refers to the development involving an egg, planktonic larval stages, and a juvenile form. The fertilized egg divides many times to produce blastula during indirect development. Then, the blastula invaginates at one end, forming a primitive gut. The cells continue to divide, forming grastula (a double-layered embryo). Thus, the hole (blastopore) through which the gut opens to the outside marks the position of the future anus of the echinoderm. Then, at the opposite end of the body from the blastopore, the mouth arises anew.
The gastrula eventually develops into a basic larval type that is characterized by bilateral symmetry called a dipleurula larva. In front of the anus and mouth and on each side of the body, a single band of hairlike projections or cilia is seen. However, the characteristic larvae found among the living classes of echinoderms are modifications of the basic dipleurula pattern.
The dipleurula larva of holothurians is known as an auricularia larva because the ciliated band of the dipleurula larva becomes sinuous and lobed. Dipleurula larvae of asteroids develop into bipinnaria larvae. Moreso, the larval form in the next developmental stage is called a brachiolaria.
Crinoids lack the dipleurula larval stage. Nevertheless, they have a barrel-shaped larva called a doliolaria larva. Ophiuroids and Echinoids have complex advanced larvae named pluteus. Hence the echinoid larva is named echinopluteus and the ophiuroid larva is named ophiopluteus.
Direct development in echinoderms refers to development in which large eggs with abundant yolk transform into juvenile form without passing through a larval stage. The young are usually reared by the female parent in Direct development.
However, brood protection or parental care varies. It could involve the actual retention of the young on the outer surface of the body or the retention of the young one inside the female body until they are born as juveniles. Hence, brood protection is best developed among Arctic, Antarctic, and deep-sea echinoderms. The young one may be held on the underside of the parent’s body or around the mouth. Such is seen commonly in some sea cucumbers and starfishes. In some sea cucumbers, asteroids, and sea urchins, the young one is held in special pouches on the upper surface of the body.
Defense mechanisms in Echinoderms
Echinoderms are prey to many organisms like crabs, sea birds, sharks, and other echinoderms. Hence their defense mechanism or antipredator defenses include the presence of toxins and spines. Toxins that can be delivered through the tube feet.
Sea cucumber discharge sticky entangling threads. They have a cluster of cuvierian tubules that can be ejected from their anus as long sticky threads. These threads entangle and permanently disable a predator. Sea cucumber also ruptures the body wall discharging its gut and internal organs. This is another defensive mechanism adopted by sea cucumbers sometimes as they have a great regenerative capacity. Eventually, they will regrow the missing organs.
The spines of most echinoderms are blunt but the spines of the crown-of-thorns starfish are long and sharp. Hence they can cause a painful puncture wound as there is a toxin in the epithelium covering them.
Also when attacked, Sea stars and brittle stars may exhibit autotomy. Brittle stars and starfishes may detach an arm to distract the predator long enough, for them to escape the predator. This defense mechanism is the reason why it is not unusual to see sea stars (starfishes) in various stages of regrowth with arms of different sizes. However, some Sea star species can swim away from the danger instead of detaching an arm. Hence, foregoing the regrowth by not losing limbs.
Why are echinoderms important to the marine environment?
- Echinoderms are important to the marine environment because they are efficient scavengers of decaying matter on the seafloor.
- Some tropical species of sea urchins permit the corals to flourish by controlling the growth of seaweeds in coral reefs. Therefore, the removal of the sea urchins would cause devastation to the coral reef habitat as a result of the overgrowth of seaweeds.
- Echinoderms produce large numbers of larvae in the marine habitat that provide food for other planktonic organisms.
- Sea urchins in large numbers can devastate sea-grass beds in the tropics. Hence adversely affecting the organisms that dwell within.
- Echinoderms also help regulate the number of small organisms in the marine habitat as they prey upon a variety of them.
- Sea urchins that burrow along a shore and into rocks can increase the erosion of shorelines.
- Echinoderms can alter the structure of seafloor sediments as many sea cucumbers feed by swallowing large quantities of sediment. So a large population of sea cucumbers in an area can greatly alter the chemical and physical composition of the sediments in the marine habitat.
- Also burrowing heart urchins, starfish, and sand dollars disturb surface and subsurface sediments.
- Sea cucumbers serve a useful role as they help recycle nutrients, break down detritus and other organic matter in marine environments.
- The calcareous shells or tests of echinoderms are used as a source of lime. This is used mainly by farmers in areas where limestone is unavailable. Also, some of these shells are used in the production of fish meals.