Sea stars or starfish are marine invertebrates belonging to phylum Echinodermata, class Asteroidea. The names sea star and starfish are also used for the closely related brittle stars, which make up the class Ophiuroidea. They exhibit a superficially radial symmetry, typically with five or more “arms” which radiate from an indistinct disk (pentaradial symmetry). In fact, their evolutionary ancestors are believed to have had bilateral symmetry, and sea stars do have some remnant of this body structure.
Sea stars do not have movable skeletons, but instead possess a hydraulic water vascular system. The water vascular system has many projections called tube feet, on the ventral face of the sea star’s arms, which function in locomotion and feeding.
As these creatures are echinoderms and not actually fish, most marine biologists prefer to replace the term starfish with the less misleading term sea star.
Sea stars are composed of a central disc from which arms sprout in radial symmetry. Most starfish have five arms, however some have more or less; in fact some starfish can have different numbers of legs within one species. The mouth is located underneath the sea star on the oral or ventral suface, while the anus is located on the top of the animal. The spiny upper surface covering the species is called the aboral or dorsal surface. On the aboral surface there is a structure called the madreporite which acts as a water filter and supplies the sea star’s water vascular system with water to move. Additional parts like cribriform organs present exclusively in Porcellanasteridae are used to generate current in the burrows made by these infaunal sea stars
Sea stars, while having their own basic body plan (pentameral symmetry or anomalies like six, four, seven and nine axis-based), could range diversely in shapes and colors. Ranging from nearly pentagonal (example: Indo-pacific Cushion Star, Culcita novaeguineae) to gracile stars like those on Zoroaster genus.
Sea stars have a simple eye at the end of each arm. The eye is able to “see” only differences of light and dark, which is useful in detecting movement.
The tube feet can be seen on this sea star.On the surface of the sea star, surrounding the spines, are small white objects known as pedicellariae. There are large numbers of these pedicellariae on the external body which serve to prevent encrusting organisms from colonising the sea star. The radial canal which is across each arm of the sea star has what are called ampullae which surround the radial canal. The ampullae are tooth-like structures. The aboral surface is also covered with papulae that are involved with the sea stars respiratory system.
Deep Blue starfish found off of Fiji, Linckia laevigata.Sea stars are often brightly colored, usually from reddish hues to violet, and unusual colors such as green and blue exist in some species, but come in muted colors as well. Patterns including mosaic-like tiles formed by ossicles, stripes, interconnecting net between spines, pustules with bright colors, mottles or spots. These mainly serve as camouflage or warning coloration displayed by many other marine animals as protection to the predator. Several types of toxins have been extracted from several species of sea stars and now being subjected into research worldwide for pharmacy or other uses such as pesticides.
Digestion and excretion
Sea star digestion is carried out in two separate stomachs, the cardiac stomach and the pyloric stomach. The cardiac stomach, which is a sack like stomach located at the center of the body may be everted – pushed out of the organism’s body and used to engulf and digest food. Some species take advantage of the great endurance of their water vascular systems to force open the shells of bivalve molluscs such as clams and mussels, and inject their stomachs into the shells. Once the stomach is inserted inside the shell it digests the mollusk in place.
Because of this ability to digest food outside of its body, the sea star is able to hunt prey that are much larger than its mouth would otherwise allow including arthropods, and even small fish in addition to mollusks.
Some echindoderms have been shown to live for several weeks without food under artificial conditions – it is believed that they may receive some nutrients from organic material dissolved in seawater.
Echinoderms have rather complex nervous systems. All echinoderms have a nerve plexus (a network of interlacing nerves) which lies within as well as below the skin. The esophagus is also surrounded by a number of nerve rings, which send radial nerves that are often parallel with the branches of the water vascular system. The ring nerves and radial nerves coordinate the starfish’s balance and directional systems. Although the echinoderms do not have many well-defined sensory inputs, they are sensitive to touch, light, temperature, orientation, and the status of water around them. The tube feet, spines, and pedicellariae found on starfish are sensitive to touch, while eyespots on the ends of the rays are light-sensitive.
Circulation and respiration
Starfish are almost unique in the fact that, unlike most other animals, they do not have blood but instead use sea water to pump around their bodies.
The water vascular system uses cilia and the constantly contracting ampullae to keep things moving. An ionic imbalance causes water to flow into the madreporite, entering the water vascular system. Some of this water is diverted into the periviscerial coelom (the large cavity in which major organs are suspended), where it is circulated by the beating of cilia. Most oxygen enters the starfish via diffusion into the tube feet (with the water vascular system), or the papulae (small sacs covering the upper body surface.
There are about 1,800 living species of sea star, and they occur in all of the Earth’s oceans. The greatest variety of sea stars are found in the northern Pacific Ocean
Source: Wikipedia, the free encyclopedia