Phylum Echinodermata – Definition, Characteristics, Diversity, Classification Of Echinoderms

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Phylum Echinodermata – Definition, Characteristics, Diversity, Classification Of Echinoderms

Introduction:

Echinoderms derive their name from the spiny or bumpy covering on the external surface of many of them. The term “echino-” from Greek means spiny, while “derm” from Latin means skin. Some examples of echinoderms include sea stars, sea urchins, sea cucumbers, brittle stars, and feather stars.

Echinoderm | Definition, Characteristics, Species, & Facts | Britannica

Despite the differences in appearance, echinoderms share two main characteristics that distinguish them from other animals: a water vascular system and radial symmetry with five sides. Echinoderms inhabit every ocean, including Antarctica, and can survive in depths exceeding 5,000 meters.

Although echinoderms are commonly found in coastal regions, they also exist in deeper waters.

Echinodermata, a phylum of organisms, are only found in marine environments and have not been discovered in freshwater or terrestrial habitats. These are complex organisms with multiple organ systems and share distinct characteristics.

Echinoderms exhibit diverse colours and shapes and are significant in ecological and geological contexts. The water vascular system in echinoderms serves various functions, including nutrient transportation, waste removal, and gas exchange.

Definition:

Any marine invertebrate animal constituting the phylum Echinodermata is characterized by tube feet, a calcite body-covering (test), and a five-part symmetrical body. The group includes starfish, sea urchins, and sea cucumbers.

Characteristics And Diversity Of Echinodermata

Characteristics:

    • Echinoderms are found exclusively in marine environments.
    •  They have a star-like appearance and can be spherical or elongated.
    • These organisms have spiny outer skin.
    • They display a level of organization at the organ system level. Most species have both a circulatory system and a digestive system.
    • Echinoderms are triploblastic and possess a coelomic cavity.
    • Their skeleton is composed of calcium carbonate.
    • They have an open circulatory system.
    • Respiration occurs through either gills or a cloacal respiratory tree.
    • They have a simple radial nervous system and do not possess an excretory system.
    • The body is unsegmented, lacking a distinct head. The mouth is on the ventral side, while the anus is on the dorsal side.
    • Locomotion is facilitated by tube feet.
    • Echinoderms reproduce sexually through the fusion of gametes and can also reproduce asexually through regeneration. Fertilization occurs externally.
    • Development in echinoderms is indirect.
    • They can regenerate damaged body parts.
    • Sense organs in echinoderms, including chemoreceptors, tactile organs, and terminal tentacles, are relatively underdeveloped.

Diversity:

The currently existing echinoderms can be categorized into five different groups or clades, which are the Crinoidea (Sea Lilies), Asteroidea (Starfish), Ophiuroidea (Brittle Stars), Echinoidea (Sea Urchins), and Holothuroidea (Sea Cucumbers).

Read Also: Genetic Drift – Difference Between Genetic Drift And Genetic Flow.

 What Are The Importance Of Studying Echinoderms?

Echinoderms play an important role in the marine food web as they serve as a primary food source for several animals, such as sea otters, while controlling seaweed growth by acting as grazers on the ocean floor. Moreover, echinoderms have multiple uses, including serving as food, medicine, and a source of lime for farmers.

Classification Of The Phylum:

Echinoderms are classified into major groups: Asteroidea, Ophiuroidea, Echinoidea, Crinoidea, and Holothuroidea.

Asteroidea:

The first group, which includes Asterias and Zoroaster, possess a star-shaped and flattened body with five arms. They also have tube feet equipped with suckers, which they use for various functions. Respiration takes place through papulae, and their body is made up of calcareous plates and movable spines. Pedicellaria is also present in these organisms.

Ophiuroidea:

The organisms in the order Ophiderma and Amphuria have flattened bodies with disc-shaped structures arranged in a pentamerous pattern. Their tube feet do not have suckers, and they respire through Bursae. The central disc is clearly distinguished from the long arms.

Echinoidea:

The organism has a body that is shaped like half of a sphere. The tube feet have suckers in them. The organism does not have any arms. Its skeleton is tightly packed and has spines that can move. Examples of such organisms include Echinus and Cidaris.

Holothuroidea:

The organism has a body that is elongated and shaped like a cylinder. It does not have any arms, spines, or pedicellariae. The organism breathes through the cloacal respiratory tree and has tube feet with suckers. Examples of such organisms include Cucumaria and Holothuria.

Crinoidea:

The organism has a star-shaped body with bifurcated arms that lack spines and pedicellariae. Its tube feet do not have any suckers. Examples of such organisms include Neometra and Antedon.

Anatomy And Physiology Of Echinoderms:

 

The illustration shows a sea star, which has a mouth on the bottom and an anus on top, both in the middle of the star. The disk-shaped stomach is sandwiched between the mouth and anus. Two tubes radiate from the stomach to each arm, and many small digestive glands connect to these tubes. Beneath the stomach is a central ring canal that also connects to tubes that extend into each arm. Tube feet are attached to these tubes. Each tube foot resembles a medicine dropper, with a bulb-shaped ampulla at the top and an extension called a podium at the bottom. The bottom of the podium protrudes from the bottom of the starfish. There are many podia along the length of the arm, which allow the sea star to latch onto objects and walk. A structure called a madreporite connects to the central ring, and protrudes from the upper surface of the sea star, next to the anus.

Image Showing the anatomy of a sea star.

Echinoderms, as adults, display pentaradial symmetry and possess a calcareous endoskeleton made of ossicles. However, in the early larval stages, all echinoderms exhibit bilateral symmetry.

The endoskeleton is formed by epidermal cells and may contain pigment cells that give these animals vivid colours, as well as cells that contain toxins. Every arm of echinoderms, such as sea stars, has two rows of tube feet on the oral side.

These tube feet aid in attachment to the substratum. Echinoderms have a true coelom modified into a unique circulatory system known as the water vascular system. An intriguing characteristic of these creatures is their ability to regenerate, even when losing over 75% of their body mass.

Water Vascular System:

Echinoderms have a unique water vascular system, including a central ring canal and radial canals along each arm. This system is responsible for several functions, such as gaseous exchange, nutrition, predation, and locomotion, by circulating water throughout the organism.

The water vascular system also features tube feet extending through holes from the skeleton. These tube feet can expand or contract based on the volume of water in the system, allowing the echinoderm to move using hydrostatic pressure. Water enters the madreporite on the aboral side and then flows into the stone canal, which moves it into the ring canal.

The ring canal connects the radial canals (five in a pentaradial animal), and the radial canals move water into the ampullae, which have tube feet through which the water moves. This unique water vascular system allows echinoderms to move and to pry open mollusc shells during feeding.

Nervous System:

Echinoderms have a relatively simple nervous system consisting of a nerve ring at the body’s centre and five radial nerves extending outward along the arms. Unlike other animals, echinoderms lack structures such as a brain or fused ganglia.

Excretory System:

Echinoderms have podocytes, specialized cells responsible for the ultrafiltration of bodily fluids. These podocytes are situated at the centre of echinoderms and are connected to an opening called the madreporite via an internal system of canals.

Reproduction:

Echinoderms exhibit sexual dimorphism, and their reproduction involves the release of eggs and sperm cells into the surrounding water, where external fertilization occurs. Some species of echinoderms undergo asexual reproduction through larval division and multiplication before reaching sexual maturity.

Additionally, they are capable of asexual reproduction and regenerating lost body parts due to injury.

Unique Adaptations And Regenerative Abilities:

Sea urchins have evolved spines as a defence mechanism against predators. Combined with tube feet, these spines are also useful for movement and obtaining food. Some sea urchin species have sharp, thin spines that can easily penetrate flesh, and some of these spines are coated with toxins, causing a painful sting.

Echinoderms can regenerate lost organs or appendages, including muscle cells, due to their ability for myogenesis. This makes them unique in the animal kingdom.

Echinoderms possess the remarkable ability to regenerate lost body parts or appendages. This process, known as regeneration, varies among species. When a sea star or brittle star loses a body or arm, it initially covers the wound with specialized skin cells to protect it. Eventually, the lost appendage regenerates, resulting in a fully functional body part.

Ecological Role And Habitat:

Echinoderms have multiple vital functions in the ecosystem. Some types, like sand dollars and sea cucumbers, dig into the sand and help to increase oxygen levels in deep seafloor areas.

This can facilitate the survival of a greater number of organisms. Furthermore, starfish can prevent the proliferation of algae on coral reefs. Regular sea urchins are present in many different environments, such as rocky outcrops, rock crevices, coral reefs, sandy lagoons, seagrass beds, and kelp forests.

Reproduction and Life Cycle

Sexual And Asexual Reproduction In Echinoderms:

Echinoderms exhibit both sexual and asexual modes of reproduction. Sexual reproduction involves the fusion of gametes released into the water through gonads in different body parts.

Fertilization occurs externally, followed by larval development. Asexual reproduction occurs primarily through regeneration, where lost body parts can regrow and replace them. Some echinoderms can also reproduce asexually through fragmentation, where each fragment can regenerate into a complete individual.

Larval Stages And Metamorphosis:

Echinoderms have a biphasic life cycle, with free-swimming larval stages that differ in form and symmetry from the adult echinoderm. Common types of echinoderm larvae include Bipinnaria, Pluteus, and Doliolaria.

Metamorphosis is the process by which the larva transforms into the adult form, involving significant morphological changes such as the development of radial symmetry and adult structures like the digestive, nervous, and water vascular systems.

Factors Influencing Reproduction And Population Dynamics:

Various factors can affect echinoderms’ reproduction and population dynamics, just like any other species. Environmental conditions, such as temperature, salinity, water quality, and food availability, can directly or indirectly affect reproductive processes.

The echinoderms exhibit a range of reproductive strategies, including sexual and asexual reproduction, and the timing and frequency of reproductive events can vary among species.

External fertilization is common in echinoderms, and factors such as proximity and timing of spawning events, water currents for dispersal, and predation on the larvae can affect fertilization and larval survival. Predation from other organisms and resource competition can impact reproductive success and population size.

Echinoderms are susceptible to diseases and parasitic infections, affecting their reproductive capacity and overall population health. Human activities such as habitat destruction, pollution, overfishing, and climate change can disrupt echinoderms’ reproductive cycles and population dynamics.

Interactions with Humans

Economic Importance (aquaculture, fisheries, pharmaceuticals):

Echinoderms have various uses in different industries. In aquaculture, some species are raised for their commercial value. For example, sea cucumbers are farmed in Asia for their meat, which is considered a delicacy, while sea urchins are cultured for their roe, which is popular in sushi.

Echinoderms are also harvested from the wild for food and traditional medicine. Some species have potential medicinal properties and are being studied for their applications in drug development, wound healing, and biomedical research.

Echinoderms are also useful in bioremediation as they can break down organic matter and improve water quality. Lastly, echinoderms are widely studied in scientific research, serving as model organisms in developmental biology and regeneration studies and contributing to broader scientific understanding.

Conservation Concerns And Threats To Echinoderm Populations:

Echinoderm populations face multiple threats and conservation issues that can cause harm to their survival and ecosystem health. Key concerns and threats include habitat loss and degradation, overfishing and bycatch, climate change, pollution and contamination, invasive species, and disease and pathogens.

Destruction and degradation of marine habitats can directly damage or remove essential habitats, reducing the availability of suitable areas for echinoderms to live, reproduce, and find food. Overfishing and unsustainable fishing practices can lead to the depletion of echinoderm populations.

Climate change can affect their physiology, reproductive cycles, larval development, and population dynamics. Pollution from various sources poses a significant threat to echinoderms, and invasive species can severely impact native echinoderm populations.

Disease outbreaks can spread rapidly within populations and result in mass mortality events, affecting the abundance and diversity of echinoderms.

Conclusion:

Echinoderms are a diverse group of marine organisms with unique characteristics and adaptations, such as specialized structures, complex reproductive methods, and remarkable regenerative abilities.

They play a significant role in marine ecosystems by contributing to nutrient cycling, sediment regulation, and maintaining biodiversity. Further research is essential to deepen our understanding of these creatures and their potential for medical and biotechnological applications.

Conservation efforts are important to protect echinoderm populations and their habitats from overfishing, habitat degradation, and climate change threats. By appreciating their ecological importance and supporting conservation efforts, we can safeguard these fascinating organisms and the delicate balance of marine.

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