Echinoderms are a distinctive group of marine invertebrates found across all oceans. The name “echinoderm” originates from Greek words meaning “spiny skin,” describing the outer surface of many members. This diverse phylum includes well-known creatures such as sea stars, sea urchins, sea cucumbers, brittle stars, and sea lilies. These organisms inhabit various marine environments, from shallow coastal areas to deep abyssal zones. They are a significant component of marine biodiversity, playing various roles within their ecosystems.
Key Characteristics
A defining feature of adult echinoderms is their pentamerous radial symmetry. This means their body parts are arranged in a pattern around a central axis, often with five radiating areas. While adult echinoderms exhibit this radial symmetry, their larval stages are typically bilaterally symmetrical, suggesting an evolutionary origin.
Echinoderms possess a water vascular system, a hydraulic system. This complex network of fluid-filled canals extends throughout the body, connecting to external projections called tube feet. Water enters this system through a sieve plate called the madreporite, which is often visible on the upper surface of sea stars. The water vascular system facilitates locomotion, aids in feeding, and contributes to gas exchange and sensory perception.
Echinoderms possess an endoskeleton composed of small calcareous plates called ossicles. These ossicles are made of calcium carbonate and are embedded in the dermis of the body wall, providing rigidity and protection. In some echinoderms, like sea urchins, these ossicles are fused to form a rigid structure known as a test. In others, such as sea stars, they articulate to allow flexibility. External projections contribute to the “spiny skin” appearance.
Echinoderms lack a distinct head, a characteristic known as cephalization. They have a simple nervous system. Their internal organs are distributed symmetrically around the central body axis within a large body cavity called the coelom.
A World of Diversity
The phylum Echinodermata encompasses a wide array of forms, categorized into several main classes. Sea stars, belonging to the class Asteroidea, are perhaps the most recognizable, featuring a central disc and five or more radiating arms. They are found globally in diverse habitats, from shallow tide pools to deep-sea environments.
Sea urchins and sand dollars fall under the class Echinoidea. Sea urchins are globular and covered with movable spines, while sand dollars have a flattened, disc-shaped body often covered in fine spines. Their hard, internal shell, or test, is made of fused plates.
Sea cucumbers, members of the class Holothuroidea, have an elongated, worm-like body with a soft, muscular skin, lacking the prominent spines of other echinoderms. Their skeletal structures are reduced to microscopic spicules embedded in their skin. They possess tentacles around their mouth, used for feeding.
Brittle stars, classified as Ophiuroidea, share a star-like appearance with sea stars but have a clearly defined central disc separate from their long, slender, whip-like arms. Unlike sea stars, they primarily move by wriggling their flexible arms rather than relying solely on tube feet for locomotion.
Sea lilies and feather stars, belonging to the class Crinoidea, are considered ancient echinoderms. Sea lilies typically attach to the seafloor with a stalk, while feather stars are free-moving. Both have a cup-shaped body with feathery, branched arms used for filter feeding.
How They Live
Echinoderms exhibit diverse strategies for movement, feeding, and survival. Locomotion in most echinoderms is primarily achieved through their numerous tube feet. These tube feet extend and contract using water pressure from the water vascular system, allowing for slow but deliberate movement across surfaces. Some, like brittle stars, also use muscular movements of their arms for more rapid crawling.
Their feeding strategies are varied. Many sea stars are predators, feeding on mollusks by everting their stomach to digest prey externally. Sea urchins are grazers, scraping algae from rocks and other surfaces. Sea cucumbers are deposit feeders, consuming organic particles from sediment on the ocean floor. Crinoids and some brittle stars are filter feeders, capturing suspended food particles from the water column with their arms.
Echinoderms have the ability to regenerate. They can regrow lost or damaged body parts, and in some cases, a complete new organism can regenerate from a single detached arm. This capability is a defensive mechanism, allowing them to escape predators by shedding limbs.
Reproduction in echinoderms is sexual, involving external fertilization. Males and females release their eggs and sperm directly into the surrounding seawater, where fertilization occurs. This synchronized release increases the likelihood of successful fertilization. The fertilized eggs develop into larval stages before metamorphosing into the radially symmetrical adults.
Their Place in the Ecosystem
Echinoderms hold positions within marine ecosystems, contributing to their balance and health. Some species, such as sea stars and sea urchins, are considered keystone species because their presence or absence disproportionately affects their environment. For example, sea urchins play a role in controlling algal populations, and their overpopulation can lead to the destruction of kelp forests.
Echinoderms also contribute to nutrient cycling in marine environments. Sea cucumbers and sand dollars, through their burrowing and feeding activities, mix and aerate sediments, which helps in the breakdown and recycling of organic matter. This process, known as bioturbation, enhances nutrient availability. The calcium carbonate from their skeletons contributes to the marine carbon cycle upon their death. Their sensitivity to environmental changes also makes them potential indicators of ocean health.