Echinoderms are a unique group of marine animals found in all oceans, from shallow intertidal zones to the deepest abyssal plains. This article explores the meaning of their name, their defining characteristics, diverse forms, and ecological contributions to marine environments.
Unpacking the Name
The term “echinoderm” originates from Ancient Greek, combining “echinos” (spiny or hedgehog) and “derma” (skin). This etymology literally translates to “spiny-skinned,” highlighting the rough, often prickly, outer texture common to many members of this phylum.
Defining Features
Echinoderms possess unique biological characteristics. As adults, they typically exhibit pentaradial symmetry, with body parts arranged in five radiating sections around a central axis. This contrasts with the bilateral symmetry observed in their larval stages.
A unique hydraulic system within echinoderms is the water vascular system. This internal network of fluid-filled canals and external tube feet aids in locomotion, feeding, and gas exchange. Water enters through a sieve-like plate (madreporite) and circulates to extend and retract the tube feet, allowing movement and prey manipulation.
Their internal skeleton, or endoskeleton, is composed of small, calcium carbonate plates called ossicles. These ossicles are embedded within the dermis and can be fused, as seen in the rigid tests of sea urchins, or articulated for flexibility, like in sea stars. These skeletal components often bear external projections such as spines or granules.
Echinoderms lack a centralized brain, relying instead on a decentralized nervous system. This system, with a nerve ring and radial nerves, coordinates movements and responses to stimuli. Many species exhibit regenerative abilities, regrowing lost limbs or internal organs. Sea cucumbers, for example, can expel parts of their internal organs as a defense and regenerate them within months.
Diversity in the Ocean
The phylum Echinodermata encompasses approximately 7,600 living species, with diverse forms adapted to various marine habitats. They are generally classified into five main extant classes. Each class has distinct body plans and ecological roles.
Sea stars (Asteroidea) are known for their five or more arms radiating from a central disc. They use tube feet for movement and are often predators, consuming bivalves and other invertebrates. Sea urchins and sand dollars (Echinoidea) have spherical or flattened bodies covered in spines. Sea urchins primarily graze on algae, maintaining balance in coastal ecosystems.
Sea cucumbers (Holothuroidea) have elongated, leathery bodies, resembling their namesake. They lack visible arms and move using rows of tube feet, often burrowing in sediment. Brittle stars (Ophiuroidea) possess long, slender, flexible arms distinctly separated from a central disc. These agile creatures use their arms for rapid movement across the seafloor, unlike sea stars that rely on tube feet.
Finally, sea lilies and feather stars (Crinoidea) are filter feeders. Sea lilies are often sessile, attached to the seafloor by a stalk, while feather stars can be free-swimming. Both use feathery arms to capture particles from the water.
Their Role in Marine Ecosystems
Echinoderms play important roles in maintaining the health and balance of marine ecosystems. Their diverse feeding strategies contribute to nutrient cycling and regulate populations of other organisms. Many echinoderms act as grazers, predators, or detritivores, influencing the structure of their communities.
Some echinoderms are considered keystone species, meaning their presence significantly impacts the ecosystem. For example, sea urchins can prevent the overgrowth of kelp forests by grazing on algae, while sea stars can control mussel and barnacle populations in intertidal areas. Disruptions to these populations, such as sea star wasting disease, can lead to significant ecological shifts.
Sea cucumbers function as bioturbators, processing large amounts of seafloor sediment. Their feeding and burrowing activities help to aerate and mix the sediment, which releases nutrients and supports microbial activity. Echinoderms also serve as a food source for various marine animals, including fish, sea birds, and marine mammals, integrating them into complex food webs. Their sensitivity to environmental changes also makes some species indicators of ocean health, providing insights into the impacts of pollution and climate change.