Brittle Star

Ophiura ophiura is referred to as the brittle star scientific name which is otherwise commonly referred to as serpent stars or ophiuroids. These are primarily the members of the phylum Echinodermata that are closely related to the starfish. They belong to the class of Ophiuroidea and the genus Ophiothrix. The genus Ophiotherix of the brittle star is the widest genus that is found worldwide in almost all oceans stretching from the Arctic to the Antarctic region. Recently the genus Ophiothrix has been subdivided into three important subgenera that include a few hundred of the species of the brittle star. The subgenera are Theophiothrix, Acanthophiothrix and Ophiothrix. 

The brittle star is made up of a circular disc with five to six arms attached to the centre disc that they use for locomotion. They crawl up on the sea bed with the help of their flexible arms. The five or six arms of the brittle star are usually long, slender and whip-like structures that can grow up to a length of about 60 cm (24inches) in the largest specimen.

The class Ophiuroidea are divided into two large class, namely, Ophiurida which is of brittle stars and Euryalida that is meant for basket fishes. There are about 2000 species of brittle stars that are known today out of which about 1200 prefer to dwell in deep of the ocean, that is, about 200meters or even more in-depth.

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Description of Brittle Fish 

The brittle star is made up of a prominent central disc with which there are five or six long, elongated, slender flexible arms attached. The centre disc is completely offset from the arms of the bitter star. Like a sea star, they also have a tube-like foot underneath their body but they do not have any succession cups at the end of the feet that can be used for locomotion. They are thus only used while feeding and to sense the environment and threats around it. Ophiura ophiura, that is the brittle star scientific name, just like any other starfish use their vascular system that uses the water to control various physical activities like controlling locomotion, respiration, digestion and waste transportation. Their feet are also filled with water. A trap door that is biologically termed as Madreporite that is situated at the undersurface of the bitter fish also known as ventral surface, controls the in and out movement of the water from the brittle’s body. The central disc of the brittle star consists of its organs that include a big stomach, its genitals, muscles and mouth surrounded by five arms. The brittle star does not possess brains and eyes. 

Vertebral ossicles that are primarily made up of calcium carbonates support the arms of the brittle star. The plats together work as balls and joints just like a human shoulder that provides flexibility to the arms. There is a special type of connective tissue that helps in the movement of the plates. The two such tissues are biologically identified as mutable collagenous tissues (MCT) which are completely controlled by the vascular system. Therefore the brittle star has a special characteristic with its arms, unlike other starfishes that have stiff arm muscles. All the arms of the brittle stars are very agile and graceful and have snake-like properties that enable the species to move much faster and quickly in the water and can squeeze into very narrow openings within the corals.

The measurement of the total body of the brittle star considered the diameter of the central disc as well as the length of its arms. Thus the diameter of its disc ranges from 0.1 to 3 inches on average and the length of the arms are two to three times more in length than the disc size in general. But sometimes the arms may grow as big as 20 times the size of the disc in a few species. Ophiopsammus maculata is the largest brittle fish known so far with a disc diameter 2-3 inches and the length of the arm is around 7-8 inches. The average weight of brittle starfish is around 0.01 to 0.2 ounces and all the species of the starfish have a variety of colour. Few of the species also generate bio-luminance that is they create their light. 

Diet and Range of Brittle Star

Diet 

The brittle starfish mostly feeds on detritus and small living organisms like plankton, clads, small fishes and mollusks. Some of the species, to trap prey, tends to raise their arms and when the prey such as small fishes approaches nearby the starfish, it wraps them in the arms and eats them. The brittle starfish often raise their arms for the tiny living organisms and the sea algae that the mucous strands present inside the tubes of their feet and then the food is swept into its mouth that is situated beneath the disc through the feet. The food then travels down the oesophagus to the stomach. The brittle star has almost 10 pouches in its stomach to digest the food that eventually takes up a lot of space on the central disc. Since the brittle fish do not have any anus opening thus the waste of their body comes out of their mouth.

Range of Brittle star

The ophiuroids first diverged about 500 million years ago, and are in the second geographical period of the six periods of the palaeozoic era. This geographical period is known as the Early Ordovician era. Today the ophiuroids can be found worldwide in all kinds of oceans stretching from the poles to the tropical regions. The basket stars are known to be dwellers of the deep regions of the ocean whereas the ophiuroids are known to dwell on the sea beds at a depth of more than 6000 meters in sea level. Whereas the brittle fish are known to habitat mostly in the coral reefs where they hide inside a rock hollow or within other living organisms growing in reefs. Though the tolerance and survival ability of other echinoderms are still unknown. Though, some of the ophiuroids can tolerate and survive the brackish water that has high salinity. The skeleton study of a brittle star stated that it consists of some calcareous elements that are built in the dermis of the body wall of many ophiuroids. 

Anatomy of Brittle Star 

The Ophiuroidea class where the echinoderms are considered as a part has the strongest affinity for the symmetry towards the five-segment radius. The body structure of the echinoderms is very similar to that of a starfish except in the ophiuroids the central disc is well marked off from its arms. All the viscera are contained inside the disc. The disc consists of the digestive system and the reproduction system that never enters any of the arms as is seen in Asteroidea. The disc contains the mouth that comprises five jaws as per their skeleton study. A light colour calcareous opening that is used for the transportation of the water into the vascular system known as Madriporite is situated in the upper jaw which is uncommon among animals like starfish.

Water Vascular System

All the vessels of the water vascular system end at the tip of the feet that contain one Madriporite. The other species consists of the Madriporite on the arboreal surface of each of the arms. The other forms of the Ophiuroidea do not have any Madriporite at all. The sucker and the ampualle are absent in the tubes of the feet

Nervous System

The central nervous system of the species consists of a nervous ring that surrounds the central disc. At the base of each arm, the ring is attached to a radial nerve that runs to the end of each of its flexible arms. At the base of each of the Vertebral Occicles, the nerves run through a vessel. Though the species do not have eyes or any other special sensory organs. But they have several numbers of sensory nerves that end in the epidermis that allows them to sense the water, touch any surface even the presence or absence of light. Their tube feet are capable of identifying light as well as odour

Digestion

The ophiuroids have five jaws in their mouth that is situated in the central disc and act both as anus and mouth. They are the scavengers as they eat their prey as a whole. Behind the mouth, there is a small oesophagus and the stomach cavity that occupies maximum space of the dorsal disc. They usually have 10 pouches in their stomach for digestion but they do not extend to their arms as in the case of a starfish. Their stomach also comprises of granular liver cells. The ophiuroids mostly feed on small worms crustaceans. A species of ophiuroids called basket starfish use their arms that are filled with mucus to catch their prey and sweep it into the mouth while the other arms act as anchors to hold it in the place.

Respiration

The exchange of gas, as well as waste products, takes place through bursae that is cilia-lined sacs. These are mainly present at the base of each arm underneath the central disc. Each sac is positioned exactly between the two digestive pouches and there is a total of 10 bursae present in a brittle sea star. By muscular contraction or through cilia the flow of water takes place. They also possess a series of vessels and sinuses together called the Helem system that is separated from the vascular system that supports oxygen transportation.  

Reproduction of Brittle Fish

There are both male and female brittle stars but it is very difficult to discriminate between male and female brittle stars without looking at their genital structures that are located inside the central discs. There are two different methods by which they reproduce majorly. One, the female releases its eggs and the male releases its sperm into the water which is then fused to produce free-floating larva. The larva then settles down on the sea bed and eventually forms a brittle star shape.

In the second method, the female brittle star, for example, Amphipholis Squamata also known as small brittle stars, brood their own eggs. As they release their eggs they store them near the base of their arms that are in cilia-lined sacs that are known as bursae. Finally, they are fertilized by the sperms that are released by their male counterparts into the water. The embryos also grow inside those sacs until they are fully mature and hatches out crawling from the bursae.

Some of the other brittle species are known to reproduce asexually by splitting their central discs in half. Then the split discs grow into two brittle stars. This process of reproduction is called fission. The lifespan of the brittle stars are 5 years and they reach sexual maturity at the age of 2 years and are fully grown by the age of 3 or 4 years. 

Behaviour

Brittle stars when under attack possess a unique defence mechanism that is scientifically termed as self-amputation or autotomy. This is the process by which the brittle star can lose or detach one of its arms from the central disc when feeling threatened or attacked by its predators. Whenever the brittle star is nervous, the system indicates the mutable collagenous tissues that are situated at the base of the arms to disintegrate itself from the central disc and the arm falls off. Though they are regrown and the wound is healed, this procedure can take weeks or even months depending on the nature of the species.

The brittle stars are known to have a radially symmetrical body but they do not use their tube feet to move forward or for locomotion. They use their symmetrical arms to wriggle through the water for movement. They are the first species that was seen that is radially symmetric and is also capable of bilateral symmetrical movement as that of humans or other mammals. When one of their arms points forward for the movement the other two on the right and the left side of the pointer arm co-ordinates the brittle star movement to move forward. The rowing movement is very similar to the flipping movement of the sea turtle swimming using its flippers.

Fun Facts About the Brittle Star

1. In the increased acidic condition of the water, the larva of the brittle starfish face difficulty surviving. By the middle of the century not all, but most of the larvae of the specimen died due to the increased acidic level of the seawater throughout the world.

2. There is a special species of brittle star that have legs that are about just 1 to 2 cm long and can survive their entire lifespan in sponges. The feet are so tiny that they don’t leave a hole while moving over the sponges. 

3. Brittle stars are the fastest moving echinoderms that are ever known. They are even faster than a starfish due to their symmetrical body structure with the presence of long, slender, spinny arms that are so flexible that it makes a wriggling motion to move forward more like a water snake.