Phylum Echinodermata
Introduction
The organisms belonging to the phylum Echinodermata are exclusively marine. Till date, there have been no traces of any terrestrial or freshwater Echinoderms.
These are multicellular organisms with well-developed organ systems. All the animals belonging to this phylum share the same characteristics features. They are colourful organisms with unique shapes. They are ecologically and geologically very important.
The Echinoderms are found in sea-depths as well as in the intertidal zones. An interesting feature of the phylum Echinodermata is that all the organisms belonging to this phylum are marine. None of the organisms is freshwater or marine.
The water vascular system present in echinoderms accounts for gaseous exchange, circulation of nutrients and waste elimination.
Characteristics of Echinodermata
1.They have a star-like appearance and are spherical or elongated.
2.They are exclusively marine animals.
3.The organisms are spiny-skinned.
4.They exhibit organ system level of organization. Most members have a circulatory system as well as a digestive system.
5.They are triploblastic and have a coelomic cavity.
6.The skeleton is made up of calcium carbonate.
7.They have an open circulatory system.
8.They respire through gills or cloacal respiratory tree.
9.They have a simple radial nervous system and the excretory system are absent.
10.The body is unsegmented with no distinct head. The mouth is present on the ventral side while the anus is on the dorsal side.
11.The tube feet aids in locomotion.
12.They reproduce sexually through gametic fusion and asexually through regeneration. Fertilization is external.
13.The development is indirect.
14.They possess the power of regeneration.
15.They have poorly developed sense organs. These include chemoreceptors, tactile organs, terminal tentacles, etc.
Classification of Echinodermata
Asteroidea
They have a flattened, star-shaped body with five arms.
They have tube feet with suckers.
They respire through papulae.
The body comprises of calcareous plates and movable spines.
Pedicellaria is present.
Eg., Asterias, Zoroaster
Ophiuroidea
The body is flat with pentamerous discs.
The tube feet are devoid of suckers.
They respire through Bursae.
The long arms are demarcated from the central disc.
Eg., Ophiderma, Amphuria
Echinoidea
The body is hemispherical.
The tube feet contains suckers.
The body does not have arms.
The body has a compact skeleton and movable spines.
Eg., Echinus, Cidaris
Holothuroidea
The body is long and cylindrical.
The arms, spines, and pedicellariae are absent.
They respire through the cloacal respiratory tree.
They possess tube feet with suckers.
Eg., Cucumaria, Holothuria
Crinoidea
The body is star-shaped.
The tube feet have no suckers.
The arms are bifurcated.
Spines and pedicellariae are absent.
Eg., Neometra, Antedon
Starfish
Starfish, also known as sea stars are echinoderms that belong to the class Asteroidea. They are marine invertebrates that are found in seabeds in oceans all over the world.
Starfish are star-shaped echinoderms; they are also referred to as sea stars. These marine invertebrates typically have a central disc and five arms; some other species possess a greater number of arms. Generally, the species are vibrantly coloured in different shades. They have tube feet that operate by a hydraulic system and a mouth towards the centre of the lower surface.
Starfish are opportunistic feeders; different species have special feeding behaviours. They can reproduce sexually and asexually. Most of them have the capability to regenerate lost or damaged parts of the body and can also shed arms in defense.
Classification
Kingdom : Animalia
Phylum. : Echinodermata
Class. : Asteroidea
Order. : Forcipulatida
Family. : Asteriidae
Genus. : Asterias
Here is a detailed diagrammatic representation of the genus Asterias (Starfish) along with features explained.
Features
There are a total of eight species found in the genus Asterias.
They have five arms that are broad at the base and taper at the end and curl up a little too.
The tube feet present on the lower surfaces of their arms are tubular projections that are a part of the vascular system. These projections help in locomotion and feeding.
The aboral (upper) surface is usually orange to brick red in colour and the oral (lower) surface is of paler shades.
The madreporite is a pressure-equalising valve like structure that is used to filter water into the vascular systems of the sea stars.
It is a calcareous opening that looks like a wart or button-like structure on the aboral surface of the organism.
Ampulla are small sac-like structures that are connected to stone canals. The stone canals together form a circular ring canal.
The ampulla are also connected to sucker-like podia. As the ampulla contracts, the podia is stretched, water is moved in and the whole process helps in their movement.
The starfishes can grow up to a maximum diameter of 20 inches but are commonly found in the range of 4 – 12 inches.
It feeds on molluscs and other benthic invertebrates.
Different parts of a starfish
External Morphology
They show pentaradial symmetry wherein most of the species possess five arms that radiate from a central disk. Towards the end of each of the arm is a microscopic eye called the ocellus which aids them to distinguish between dark and light and detecting movement. Its body comprises ossicles constituting calcium carbonate forming the endoskeleton. These can take different forms externally such as granules and spines. Their top surface is called abactinal or the aboral side while the base side is called the actinal or oral surface.
The madreporite is a faint circle seen somewhat off-centre present on the central disk. It is a porous plate involved in providing excess water for different requirements. Pedicellariae are small valve-like structures seen in some groups. It can be seen all through the body or in tufts at the bottom of every spine which are supposedly functional in feeding or defense or even capturing and killing prey.
Circulatory system of Sea stars
Sea stars do not pump blood through their bodies. They use a seawater and a complex water vascular system for things to move. Its tube are vital in the circulatory system which are also used in movement. On its underside, their have hundreds of tube feet.
Hard body plates
Sea stars lack a backbone; they have a skeleton under their skin. The endoskeleton comprises a complex network of hard bony plates made of calcium carbonate bound by flexible tissues.
Tube feet
Comparatively, they move slowly and are mobile. They locomote using special structures called tube feet that are somewhat like tentacles having small suckers towards the terminal. With tentacles, starfish can have a grip of rock walls etc. in a coordinated manner.
Stomach
They have 2 stomachs – the pyloric and the cardiac stomach. To engulf prey, it pushes the cardiac stomach out of the stomach in the centre of the underside. Then it secretes a digestive enzyme to disintegrate its prey. When the cardiac stomach comes to the back, food gets transferred to the pyloric stomach.
Nerve net
They lack a central nervous system and have a nerve net wherein all nerves are spread over its body. They have sensory parts in the body that detect a stimulus.
Life Cycle Of A Starfish
Starfish exhibit a complex lifecycle, in which the young pass through two developmental stages before settling down on the ocean floor and becoming mature adults. While certain starfish species adapt successfully as aquarium pets, very few starfish are propagated in captivity. Accordingly, aquarists are only likely to observe adults in captive environments, although you may notice your starfish -- also called sea stars -- releasing eggs or sperm into the water column.
Gamete Fertilization
Starfish engage in external fertilization through a process known as spawning. To spawn, male and female starfish release eggs and sperm from their bodies. Some of the sperm and eggs contact each other, initiating fertilization and development. When spawning, many starfish adopt arched body postures, with the center of their bodies raised high off the substrate. You may see the released gametes in the water, as they will form a milky, white cloud in the water. Some species exhibit seasonal breeding habits, although it may be possible to observe spawning at any time of year.
Early Life
Various starfish exhibit different developmental patterns, yet most follow the same general theme. After the gametes fertilize each other, they quickly become bilaterally symmetrical larvae, called bipinnaria. These tiny creatures feed on smaller planktonic organisms and float about for a few days or weeks before transforming into non-feeding, larvae called brachiolaria, which settle on the ocean floor.
Metamorphosing to Maturity
Once the brachiolaria settle in an appropriate habitat, they develop radial symmetry and begin metamorphosing into adults. There is some evidence to suggest that the larvae seek out areas with adult pheromones, which may signal to the larvae that the habitat is suitable. After assuming their characteristic shape, the starfish begin growing to attain adult size.
Asexual Reproduction
In addition to sexual reproduction, some starfish species can reproduce asexually. This is often a response to an unfortunate situation, rather than a deliberate method of reproduction. Many species can regenerate arms lost to battles with predators. And in some cases, detached arms can regenerate their bodies and become fully functioning organisms.
Hermaphroditic Species
A few starfish species possess both male and female reproductive organs. Such specimens are capable of self-fertilization. These bisexual species sometimes brood their developing young, which may be related to the fact that most such species live in areas in which the conditions do not favor the development of free-floating larvae.
Starfish are resourceful creatures that can reproduce in more than one way. Starfish can either spawn together, or they can asexually reproduce- though this isn't the ideal way to produce new starfish because it doesn't promote genetic diversity within the species. While sexual reproduction is a naturally occurring process, asexual reproduction for starfish is usually only the result of dismemberment that results in the formation of two whole starfish with the same DNA.
Spawning
Starfish reproduce sexually by spawning. Spawning means that the sex cells are released into the water. Starfish gather in groups to reproduce, which increases the likelihood the sperm and eggs will find each other. Starfish have sexual organs, or gonads, in each arm. During a breeding season the males' gonads fill with sperm and the females' gonads fill with eggs. When starfish spawn, the males release sperm and the females release eggs in great numbers. Female starfish may release millions of tiny eggs into the water during a spawning session.
Egg Stage
When the eggs become fertilized they develop into zygotes. Some species of starfish brood their eggs after they're fertilized. They may sit over their eggs, or form a brooding basket in which the eggs are sheltered. The eggs may be held on the starfish's central disk or in sacs between the arms. Once the eggs hatch they are released into the open ocean to drift with the current and search for food.
Larval Stage
In the larval stage the starfish is a miniscule, jelly-like blob that floats around feeding on microscopic flora and fauna. These larva are too small to be seen with the naked eye. The starfish larva metamorphosizes through several stages until it develops into a juvenile. The larva have bilateral symmetry, but in their adult form they'll have radial symmetry. Bilateral symmetry means the left and right halves resemble each other, where as in radially symmetrical animals the limbs radiate from a single point in the center of the body.
Juvenile Stage
In the last stage of larval development the starfish abandons its free-floating planktonic lifestyle for a benthic (surface-dwelling) existence. The larva anchors itself to a surface and transforms into the juvenile stage. A juvenile starfish looks like a small version of an adult. Juvenile starfish spend most of their time in seclusion, hiding in crevices and under rocks to protect themselves from predators. After doing some growing they explore the ocean floors and reefs eating algae and other particulate matter. At about two years old the starfish is grown enough to be considered an adult starfish and can begin the reproductive stage of its life.
Asexual Reproduction
If a starfish's arm is removed along with part of the central disk (the center of the body from which the arms radiate) a whole new sea star can form from that detached piece. The amputated starfish can also regenerate a new arm to replace the lost limb. Because two starfish have been formed from one, this is considered asexual reproduction. If you see a starfish that has some arms significantly longer than the rest, this is a sign the animal has regenerated some of its limbs.
