Echinoderms are invertebrates found exclusively within marine environments. The name is derived from the Greek words echinos (“spiny”) and dermos (“skin”), referencing their characteristic spiny surface. This phylum encompasses approximately 7,000 living species, ranging from the familiar sea star to the less-recognized sea cucumber. Echinoderms inhabit ocean depths from the shallow intertidal zones down to the abyssal plains, possessing unique anatomical features.
Defining Characteristics of Echinoderms
A defining feature of adult echinoderms is their penta-radial symmetry, meaning their body parts are arranged in five or multiples of five around a central axis. This structural organization is a secondary adaptation, as the larval stages of all echinoderms display bilateral symmetry. The shift to radial symmetry allows the adult to interact with its environment equally from all directions, which is advantageous for slow-moving or sessile organisms.
Echinoderms possess an internal skeleton, or endoskeleton, composed of calcified plates known as ossicles. These plates are made of calcium carbonate and are developed by the epidermal cells. In many species, these ossicles fuse to form a hard, internal shell called a test, while in others, they project outward as the spines that give the phylum its name. The endoskeleton provides protection and structure.
The water vascular system functions as a hydraulic network for locomotion, feeding, and gas exchange. Water enters this system through a sieve-like plate called the madreporite, then circulates through a central ring canal and radial canals that extend into the body sections. This fluid-filled system powers the hundreds of tiny, suction-cup tipped appendages known as tube feet. By altering the water pressure within bulb-like structures called ampullae, the animal can extend and retract the tube feet, allowing for movement, gripping surfaces, and capturing prey.
The Five Major Classes of Echinoderms
The Class Asteroidea, or sea stars, are characterized by a star shape with five or more thick arms radiating from a central disk. Sea stars are predominantly predators, and many species have the ability to evert their stomach out of their mouth to digest prey, such as bivalves, externally. They are also known for their impressive ability to regenerate lost arms or even an entire body from a single arm and a portion of the central disk.
Brittle stars belong to the Class Ophiuroidea and are distinct from sea stars because their slender, flexible arms are sharply demarcated from a small, central body disk. They use these arms for rapid movement across the seafloor, lacking the suckers on their tube feet that sea stars use for gripping. Brittle stars function as scavengers or detritivores, using their arms to capture food particles.
Sea urchins and sand dollars comprise the Class Echinoidea, characterized by a lack of arms and a body encased in a rigid, spherical or flattened test. Sea urchins use a complex, jaw-like feeding structure called Aristotle’s lantern, which consists of five continuously growing teeth used to scrape algae from rocks. Sand dollars generally feed by sifting organic particles from the sediment.
The Class Holothuroidea, or sea cucumbers, have an elongated, cylindrical body, which represents a functional shift toward bilateral symmetry as they lie horizontally on the seafloor. They have reduced ossicles, giving them soft, leathery skin, and they lack the pronounced spines of other echinoderms. Sea cucumbers are notable for a defense mechanism where they can expel their internal organs, which are later regenerated.
Sea lilies and feather stars make up the Class Crinoidea, considered the most primitive group. Sea lilies are typically sessile, attached to the seafloor by a stalk, while feather stars are free-swimming. Both are filter feeders, using numerous feather-like arms to capture suspended food particles and direct them toward an upward-facing mouth.
Essential Roles in Marine Ecosystems
Echinoderms occupy diverse trophic levels, functioning as grazers, predators, and detritivores, which gives them a substantial influence on the structure of marine communities. Some sea star species are recognized as keystone organisms because their predatory habits prevent the overgrowth of prey, such as mussels, which maintains species diversity on rocky shores. If these sea stars are removed, the ecosystem can be dramatically altered by the unchecked proliferation of their prey.
Sea urchins, as herbivores, play a significant role in controlling the distribution of marine plant life, particularly in kelp forests. By grazing on algae, they prevent dense kelp populations from becoming monocultures. However, an overabundance of urchins can lead to the formation of “urchin barrens,” completely denuded of kelp.
Sea cucumbers, along with sand dollars, act as deposit feeders, consuming large quantities of seafloor sediment to extract organic matter. This process, known as bioturbation, cleans the ocean floor, recycles nutrients back into the water column, and enhances oxygenation deeper into the sediment.