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Echinoderms are a Phylum of Invertebrate Animals which live in the oceans. Echinoderms include Animals like Starfish, Brittle Star, Sea Urchin, Feather Star, Sea Cucumber etc. There are about 7,000 living Species of Echinoderms. Echinoderms are simple organisms lacking a brain and complex sensing organs. The first definitive members of the Phylum appeared near the start of the Cambrian period (544-500 million years ago). The oldest-known fossil Echinoderm is Arkarua, which lived in pre-Cambrian times, during the Vendian period or the Ediacaran Period (650 to 540 million years ago).
Echinoderms are important geologically as they are major contributors to many limestone formations, and provide valuable clues as to the geological environment. They are also important biologically as they are abundant in both the biotic environment of the deep sea, as well as the shallower oceans. Echinoderms like Sea cucumbers are considered a delicacy in some countries of south east Asia; particularly popular are the (Pineapple) roller 'Thelenota ananas' (susuhan) and the red Halodeima edulis. Japan, Peru and France are leading consumers of Echinoderms.
Kingdom - Animalia.
Sub-kingdom - Eumetazoa.
Superphylum - Deuterostomia.
Phylum - Echinodermata.
1. Crinoidea - Example Sea Lilies. There are about 600 Species of Crinoid.
2. Asteroidea - Example Starfishes. There are about 1800 Species.
3. Ophiuroidea - Example Brittle Stars. There are about 1500 Species of Brittle Star.
4. Echinoidea - Example Sea urchins and Sand Dollars. There are about 1000 Species.
5. Holothuroidea - Example Sea Cucumbers. There are about 1000 Species of Sea Cucumber.
Size - There are two contenders for the title of 'Largest Echinoderm'. First is Catala's Starfish (Thromidia catalai), also known as the 'Fat Star', weighing in at 6 kilograms (13 pounds). However, its fat arms are only just over 1 feet (30 cm) long. Second is the Midgardia Xandaros, which has arms that can reach up to twice that length. Midgardia has very long arms but a tiny body. It's each arm is 67cm (over 2 feet) long but its body measures only 1 inch (2.6cm).
Diet - Various Echinoderms eat variety of food. Sea Urchins graze on the surfaces of rocks, scraping off the thin layer of algae covering the surfaces. Sea Cucumbers suck vast quantities of sea water through their guts and absorb any useful matter. Some Sea Stars are Detritivores (scavengers), extracting the organic material from mud, and others mimic the Crinoids' filter feeding, most are Active Hunters, attacking other Sea Stars or Shellfish.
Predators - They are preyed upon by Triton Shell, Fish like the Trigger Fish, Crabs and Shrimps and by other Echinoderms like Starfish etc. Sea Otter is another Animal that consumes Echinoderms like Sea Urchins regularly.
Echinoderms are found in almost all depths, latitudes and environments in the ocean. They reach highest diversity in reef environments but are also widespread on shallow shores and around the poles. They occur in various habitats from the intertidal zone down to the bottom of the deep sea trenches and from sand to rubble to coral reefs and in cold and tropical seas.
The first universally accepted Echinoderms appear in the Lower Cambrian Period. In spite of extensive fossil records, there is isn't a definite answer to the evolution of Echinoderms. There are various theories which aren't uniform with each other. According to the traditional view the first Echinoderms were Sessile and became Radial (capable of movement) as an adaptation to that existence, and then gave rise to free-moving groups. This theory perceives the evolution of endoskeletal plates with stereom structure and of external ciliary grooves for feeding as early Echinoderm developments. Attachment to a substratum would have selected for Radial Symmetry and may have marked the origin of the Class Crinoidea. Members of Crinoidea, along with the extinct members of Class Cystoidea, were primitively attached to a substratum by an aboral stalk. An ancestor that became free-moving might have given rise to Asteroidea, Ophiuroidea, Holothuroidea, and Echinoidea.
Although reproduction is usually sexual, involving fertilization of eggs by spermatozoa, several species of Echinoderms, such as Sea Stars and Sea Cucumbers, can also reproduce asexually. They can break a ray or arm or by deliberately splitting the body in half. Each half then becomes a whole new animal. This process in known as 'Fission'.
Sexual reproduction involves the external fertilization of eggs by spermatozoa. The fertilized eggs develop into planktonic larvae. The larvae typically go through two stages, called 'Bipinnaria' and 'Brachiolaria'. They are Bilaterally Symmetrical and have bands of cilia used in swimming and feeding. As the larvae gradually metamorphose into adults, a complex reorganization and degeneration of internal organs occurs. The left side of the larva becomes the oral surface of the adult, which faces down, and the right side becomes the aboral surface, which faces up. The larvae settle to the sea floor and adopt their distinctive adult Radial Symmetry.
