Echinoderms are spiny-skinned invertebrates of the phylum Echinodermata, specifically recognized by the pentamerous body (five-pointed) symmetry in adults. The phylum derives its name from the Greek words ‘ekhînos,’ meaning ‘hedgehog,’ and ‘dérma,’ meaning skin. Starfish, sea urchins, brittle stars, sand dollars, and sea cucumbers are common examples of echinoderms.
Found on the ocean bed, they reproduce asexually and are capable of completely regenerating their body parts. They are well-recognized for their unique water vascular system, a fluid-filled canal network that functions in gas exchange.
Although some fossils of sea lilies measure around 20 meters (66 feet), most living echinoderms are relatively small, with diameters of around 10 cm (four inches).
Most adult echinoderms are recognized by a basic pentaradial body (with five arms). However, starfish from the genus Leptasterias and some brittle stars possess six arms. Some crinoids and sea stars have arms in multiples of five, such as the starfish Labidiaster annulatus, which contains up to fifty arms, and the sea lily Comaster schlegelii, which has two hundred of them.
Echinoderms possess a dermal skeleton (derived from the dermis) composed of calcium carbonate ossicles. As found in the hard test shell of sea urchins, these ossicles may fuse or form flexible joints, as found in the arms of sea stars, brittle stars, and crinoids. The skeleton is supported by an outer epidermis, which often displays a range of colors, such as dark red, stripes of black and white, and purple.
Water Vascular System
It is a unique network of fluid-filled vessels derived from the body coelom that assists in locomotion, feeding, and gaseous exchange. This system opens to the exterior through a calcareous opening called the madreporite on the body’s aboral (upper) surface. It functions as a sieve and filters the water moving in and out of the echinoderm’s body. The madreporite is connected to the stone canal, a duct that links to a ring canal encircling the mouth or the esophagus.
The ring canal radiates into five radial canals extending along the arms, branching further into short lateral canals. At the tip of each lateral canal is a swollen bulbar ampulla that protrudes into a tube foot or podium. It is through the expansion and contraction of this foot that the fluid is distributed throughout an echinoderm’s body.
The hemal and perihaemal coeloms form an open and much reduced circulatory system. They lack a true heart and possess a central blood vessel ring that spreads out radially into each arm. Their blood or coelomic fluid does not contain respiratory pigments. Instead, it is filled with immune cells known as coelomocytes, which include various types such as phagocytic amoebocytes, spherule cells, vibratile cells, and crystal cells.
Echinoderms respire through dermal branchiae or papulae, as in starfish, genital bursae in brittle stars, peristomial gills in sea urchins, and cloacal trees in sea cucumbers. The gaseous exchange also takes place through their tube feet.
They possess a simple digestive system consisting of a mouth, esophagus, two-part stomach, intestine, and rectum. The anus, if present, is centrally located on the aboral surface of the body.
In many starfish species, the upper part of the stomach is turned inside out to enable digestion outside the body, whereas brittle stars possess a blind gut without any intestine or anus, thus expelling waste through the mouth.
Echinoderms are ammonotelic, diffusing ammonia out through the respiratory surfaces of their bodies.
They have a simple ring-like nervous system comprising a radial net of interconnected neurons. Multiple nerves radiate from the central ring around the mouth into each arm of the echinoderm. Although they lack a central brain, some species possess ganglia as a substitute.
Starfish have simple eyespots and special sensory cells in the epithelium, while sea urchins have statocysts for maintaining balance.
Currently, the phylum Echinodermata contains three extant subphylums and five extant classes, including almost 7,000 living species. It is the largest marine phylum and the second-largest group of deuterostomes after the chordates.
Echinoderms are traditionally divided into two major groups: Eleutherozoa, comprising the classes Asteroidea, Ophiuroidea, Echinoidea, and Holothuroidea, and Pelmatozoa, comprising the class Crinoidea and the extinct subphylum Blastozoa.
They are cosmopolitan in distribution and are found in almost every depth of the oceans. While adults usually inhabit the benthic zone or seabed, the larvae are pelagic, living in the upper columns of the open ocean.
Most starfish are carnivores, which feed on shellfish, clams, mussels, and smaller starfish. In contrast, sea urchins usually feed on algae, crinoids live off plankton, and sea cucumbers scavenge on waste particles on the ocean floor.
Although most echinoderms use their tube feet for propulsion, sometimes sea urchins use their spines for the same purpose. The tube feet function through cyclic contraction and relaxation along the surface as the animal glides forward. Each tube foot has a tip shaped like a suction pad that helps create suction against the substrate.
Sea cucumbers are generally sluggish and crawl on the seabed or burrow into it using peristalsis (alternate contraction and relaxation of muscles). Some species also drag themselves along the surface using their buccal tentacles.
Sea lilies are almost always sessile, sticking to a surface by holdfasts (the rigid ends of their stalks), and their movement is restricted to bending stems and rolling arms. On the contrary, many sea feathers move across the seabed by lifting their bodies off the substrate or even swimming in water.
Many echinoderms self-amputate or autotomize their body parts and easily regenerate them with time. For instance, sea urchins constantly replace their damaged spines, while sea cucumbers let go of their internal organs when threatened and regenerate them in a few months. However, in species belonging to the genus Sclerasterias, arms are deliberately autotomized to develop into a complete individual (asexual reproduction). Regeneration can occur in any one of the following ways.
Echinoderms may reproduce either sexually or asexually, depending on the species.
In most echinoderms, the male and female sexes are separate, except for a few asteroids, holothurians, and ophiuroids, which are hermaphrodites (contain both male and female sex organs in a single body). Depending on the species and external conditions, these invertebrates typically attain sexual maturity in two or three years.
The eggs and sperm are usually released in the water, leading to external fertilization. However, some species of sea stars, brittle stars, and sea cucumbers undergo internal fertilization within their bodies. The release of gametes is synchronized with the lunar cycle in some species, while in others, the males and females aggregate during the reproductive season to improve chances of fertilization.
Larval Development
After an egg has been fertilized, a juvenile may be produced from it in different ways, depending on the internal composition of the egg.
Small eggs that do not contain much yolk mostly develop into free-swimming larvae that feed on small, floating organisms. On the other hand, those that are yolky form larvae that sustain on their yolk material.
Sometimes, especially in species inhabiting cold regions, large eggs transform directly into juveniles without going through the larval stages (direct development). In such cases, the parents provide extra protection to the brood. For example, in some starfish and brittle stars, the eggs are carried in specialized chambers in different body parts, like the stomach and under the arms, or even in the body cavities.
In most adult asterozoans and sea cucumbers, asexual reproduction occurs by transverse fission, where the echinoderm splits into two from a little in front of the body’s middle. This mode of reproduction is also found in some sand dollars, sea urchins, and sea cucumber larvae. In such rare scenarios, the primary larva may split transversely, autotomize into a secondary larva, or bud off into a new individual.
Under favorable environmental conditions and abundant resources, echinoderms may also reproduce by larval cloning, though it is an energetically expensive process.
Their natural predators include bony and cartilaginous fish, gastropod mollusks, crabs, birds like gulls and eider ducks, and mammals such as Arctic foxes, sea otters, and human beings. Larger echinoderms may prey upon smaller ones, thus exhibiting cannibalism.
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