Echinoderms are marine invertebrates of the phylum Echinodermata. Echinoderms are found in every ocean of the world, mostly in the benthic zone, although they may reach their highest diversity in the coral reef ecosystem. There are a little over 7,000 species found in the ocean floor of intertidal shallow habitats and the deep ocean floor of the abyssal plains. Echinoderms have a long history in the fossil record. Fossils of primitive echinoderm have been found in rocks over 500 million years old. These species are classified into five main extant taxa, including the Asteroidea, Echinoidea, ophiuroidea, Holothuroidea and the Crinoidea.
Asteroidea
Echinoderms, such as sea stars, within this group have five or more arms radiating from a central body disc. The arms can range in size form a few centimeters to 96 centimeters, which is the largest arm recorded in a sea star. Sea stars show symmetrical multiples of the basic five arms, with some species, such as the labidiaster annulatus having more than fifty arms and the sea-lily Comaster schelgelii possessing two hundred arms. Most sea stars are carnivorous, although some are detritivores, filter feeders and suspension feeders. They have a mouth, esophagus, intestine and anus. Sea stars have the ability of everting their stomachs and feed on organisms outside of their body.
Sea stars have the ability to regrow their arms. They sometimes intentionally detach their arms to engage in asexual reproduction. Sea stars have a set of tube feet spread along grooves under each of their five arms. They use these tube feet to move along a surface by adhering to a substrate by suction. To defend themselves from predators, sea stars may detach themselves from an arm. This may distract the predator, allowing the sea star to escape away. Sea star plays an important role in benthic habitats. The crown of thorns sea star feeds on coral polyps and can be detrimental to the coral reef ecosystem when its populations increase.
Echinoidea
The echinoderms in this group include sea urchins, heart urchins and sand dollars, among others. Echinoderms range in size from about 6 centimeters to up to 35 centimeters long. Sea urchins are principally herbivorous, while sand dollars are deposit feeders. Echinoids have adapted to thrive on both hard and soft substrates by utilizing either tube feet or spines for locomotion. Sea urchins play an important role as grazers in their habitat, often becoming detrimental when their populations grow. Echinoids have a rigid skeleton of interlocking ossicles. The test is covered with either long spines in urchins or tiny spines in sand dollars. The spines are sharp, and in some species, they can be toxic.
The majority of echinoderms have five pairs of teeth positioned in a circular ring form around their mouth in a configuration known as the Aristotle’s lantern. Echinoids are dioecious, and fertilization of eggs occurs externally. The eggs are protected by the spines until a larva forms after which it may feed for several months before turning into an immature urchin. Urchins possess the ability to regenerate lost or damaged spines. To avoid being catch by predators, they get camouflaged by carrying sea grass, corals or anemones on their backs. Some marine animals live in a symbiotic relationship with sea urchins. Small fish and shrimp may find shelter and protection against predators among the poisonous spines of the fire urchin.
Ophiuroidea
Ophiuroidea, such as brittle stars or basket stars resemble sea stars. They have very flexible arms that radiate from a central disc, which is smaller than that of the sea star. Brittle stars lack a mouth or anus and they have no intestines; all wastes are expelled out through the mouth. They are scavengers, although, some are also suspension feeders. They usually use their spines and rarely their tube feet to propel themselves through the water. Brittle stars, like sea stars, possess the ability of detaching their arms or segments of arms when threatened. Ophiruoids reproduce sexually, though some reproduce by fission and some are hermaphroditic.
Holothuroidea
Holothurians (sea cucumbers) are bilateral, soft-bodied creatures, ranging in size from one centimeter to about 3 meters. Sea cucumbers possess an elongated, flexible body with a mouth on one end and the anus at the opposite end. They possess modified tube feet tentacle-like structures used for collecting food. Most species feed on organic film covering the sandy ocean floor. They also ingest sand, extracting nutrients and expelling the sand through the anus. When threatened by predators, they discharge part of their guts, which they quickly regenerate. Sea cucumbers are abundant in the ocean floor, and their largest diversity is found within the coral reef habitat.
Holothuroids utilize their tube feet-like to structures to move in a number of ways for burrowing, swimming and crawling through the water. The majority of sea cucumbers are dioecious. Most species spawn and fertilization occurs externally. Others are viviparous, with embryos developing inside the mother’s body. Sea cucumbers host a number of symbiotic organisms within their body, including shrimps, worms and crabs. The pearl fish lives in the gut cavity of the sea cucumber, feeding on the tissues of the host. Some species of sea cucumbers are economically important in many Asian countries; furthermore, sea cucumbers have been found to contain certain chemicals important in the pharmaceutical industry.
Crinoidea
Crinoids, including sea lilies and feather stars abound in the deep ocean, although feather stars may be found at shallower waters less than 200 meters. Feather stars possess a radial symmetrical body, which is cup-shaped. Some have five arms, although others may have a few hundred. Crinoids are suspension feeders. Both the mouth and anus are located in the oral surface. They use their large tube feet to take food particles to their mouths. Most sea lilies’ locomotion is limited to bending. Feather stars may crawl along the sea floor substrate and some are able to displace by swimming. Crinoids possess the ability to regenerate detached arms and fragments of their body.
Echinoderms are highly valuable economically and scientifically. Tons of echinoderms are consumed each year, mainly in Asian countries where they’re included in the diet. The shells of echinoderms are used by farmers as a source of lime. Sea urchins are utilized in scientific research, especially in embryological research. The regeneration of the arms in brittle stars is being researched in an attempt to understand neurodegenerative diseases in human beings. According to www.starfish.ch, echinoderms are found exclusively in the marine environment with no freshwater representative species. They abound in almost all sea floor habitats from sand, to rubble and in the cold and tropical waters of the world.
