Echinoderm, belonging to the superphylum Deuterostomia, is a constitution of marine animals. It includes 7000 species, which makes it the second largest phylum in the Deuterostomia. The physical property of the marine animals from this phylum includes the radial symmetry (5 parts), which makes it easy to recognize. This phylum includes many commonly seen and well-known animals such as Starfish, Sea-cucumbers, Sea lilies, Sand dollars and Sea urchins. It can be seen at the deep bottom of Oceans and seas ranging from abyssal zone (bottom of the sea) to the intertidal zone.
History of Fossil
Fossils of Echinoderms have been acquired from the ancient Paleozoic era, approximately 540 million years ago. Abundant fossils have been left by this phylum. It is believed that many species were extinct by the end of that particular era. Some of those species are Blastoids, Cystoids, Carpoids and the strange looking Edrioesteroids. Many siltstones and sandstones of the Paleozoic period, acquired from the Victoria consist of Echinoderms fossils like Sea star, Cystoids, Brittle star, Blastoids and others. Some studies also reveal that the oldest fossil ever found from the Echinoderm phylum was an organism known as 'Arkarua' that belonged to the pre-Cambrian period, which means more than 600 million years ago, and was discovered from the sandstone from the Flinders ranges in South Australia.
In the Echinoderm phylum, both sexual and asexual reproduction takes place. In sexual reproduction, the animals become sexually mature only after 2 or 3 years of growth. Most of the species in this phylum are gonochoric (unisexualism). The sperm and the eggs are usually released into the water, where the fertilization takes place. In some species, during the reproductive period, individual organisms aggregate, which leads to the successful fertilization. In some species like the Sea Stars and the Brittle stars, internal fertilization is carried out. Some species are seen to be brooding their eggs in the special chambers inside the ovary or coelom and also sometimes on the oral surface of the body. Some develop directly into a new juvenile, whereas some transform into a free flowing larvae, depending on the species.
In the asexual reproduction, some species undergo asexual reproduction till they reach sexual maturity. The Sea star species undergo asexual reproduction by parthenogenesis. Mostly the asexual reproduction takes place by the transverse fission process in which an individual organism splits into dual new organisms that have the capability to create the missing body parts of their own.
Distribution and Habitat
The Echinoderms can be found throughout the globe, equally distributed in all the latitudes and climates. It can be found in deep oceans as well as in shallow seas. Some species like Sea urchins, Sand dollars and Asteroids are mostly seen near the seashore areas. The species growing near the seashore does not settle down beyond 100 meters of depth, whereas the species growing in the deep oceans are found at a depth of nearly 5000 meters, and the species like the Sea cucumbers can even reach a depth of more than 10,000 meters.
The feeding mode is different in each of the species. Some are filter-feeders whereas some are active hunters. The filter-feeders like the Brittle star and Crinoids sieve the food particles floating in the water and consume them. Some suspension feeders acquire their food from the water by raising their arms, mucus strands, or by the help of spines and tube feet. Some species are carnivores, which suddenly attack and capture the waterborne prey by their limbs, and then the limbs bend towards the mouth or jaw to propel the food. Other species like Sea urchins consume the algae, found over the rocks that cover the surface. The thin layer on the surface of the algae is scratched by the Sea urchins with a special organ, referred as 'Aristote's lantern'.
Echinoderm species like Sea urchins and Sea cucumbers are considered as a special delicacy in certain cuisines. In China, Sea Urchin is used to prepare gelatinous stews and soups. The shells of some species are used by farmers for the lime deposits present in it.