Sea stars (also often called " starfish ") are echinoderms belonging to the class Asteroidea . The names "sea star" and "starfish" essentially refer to members of the Class Asteroidea. However, common usage frequently finds "sea star" and "starfish" also applied to ophiuroids which are correctly referred to as "brittle stars or "basket stars".
There are over 1800 species of living species of sea stars that occur in all the world's oceans, including the Atlantic, Pacific, Indian as well as in the Arctic and the Southern Ocean (i.e., Antarctic) regions. Sea stars occur across a broad depth range from the intertidal to abyssal depths (>6000 m).
Sea stars are among the most familiar of marine animals and possess a number of widely known traits,such as regeneration and feeding on mussels that are largely based on the frequently encountered Atlantic Asterias . Sea stars possess a wide diversity of body forms and feeding methods. The extent that asteroids can regenerate varies with individual species. Broadly speaking, sea stars are opportunistic feeders, with several species having specialized feeding behavior, including suspension feeding and specialized predation on specific prey.
The Asteroidea occupy several important roles throughout ecology and biology. Sea stars, such as the Ochre star ( Pisaster ochraceus ) have become widely known as the example of the keystone species concept in ecology. The tropical Crown of Thorns starfish ( Acanthaster planci ) are voracious predators of coral throughout the Indo-Pacific region. Other sea stars, such as members of the Asterinidae are frequently used in developmental biology.
General anatomy
Sea stars express pentameral or pentardadial symmetry as adults. However, the evolutionary ancestors of echinoderms are believed to have had bilateral symmetry. Sea stars, as well as other echinoderms, do exhibit bilateral symmetry, but only as larval forms.
Most sea stars typically have five rays or arms, which radiate from a central disk. However, several species frequently have six or more arms. Several asteroid groups, such as the Solasteridae, have 10-15 arms whereas some species, such as the Antarctic Labidiaster annulatus can have up to 50. It is not unusual for species that typically have five-rays to exceptionally possess five or more rays due to developmental abnormalities.
The bodies of sea stars are composed calcium carbonate components, known as ossicles. These form the endoskeleton, which takes on a variety of forms that are externally expressed as a variety of structures, such as spines and granules. The architecture and individual shape/form of these plates which often occur in specific patterns or series, as well as their location are the source of morphological data used to classify the different groups within the Asteroidea.
Terminology referring to body location in sea stars is usually based in reference to the mouth to avoid incorrect assumptions of homology with the dorsal and ventral surfaces in other bilateral animals. The bottom surface is often referred to as the oral or actinal surface whereas the top surface is referred to as the aboral or abactinal side.
The body surface of sea stars often has several structures that comprise the basic anatomy of the animal and can sometimes assist in its identification.
The madreporite can be easily identified as the light-colored circle, located slightly off center on the central disk. This is a porous plate which is connected via a calcified channel to the animal's water vascular system in the disk. Its function is, at least in part, to provide additional water for the animal's needs, including replenishing water to the water vascular system.
Several groups of asteroids, including the Valvatacea but especially the Forcipulatacea posses small bear-trap or valve-like structures known as pedicellariae. These can occur widely over the body surface. In forcipulate asteroids, such as Asterias or Pisaster , pedicellariae occur in pom-pom like tufts at the base of each spine, whereas in goniasterids, such as Hippasteria, pedicellariae are scattered over the body surface. Although the full range of function for these structures is unknown, some are thought to act to act as defense where others have been observed to aid in feeding. The Antarctic Labidiaster annulatus uses its large, pedicellariae to capture active krill prey. The North Pacific Stylasterias has been observed to capture small fish with its pedicellariae.
Other types of structures vary by taxon. For example, Porcellanasteridae employ additional cribriform organs which occur among their lateral plate series, whcih are thought to generate current in the burrows made by these infaunal sea star.
Internal anatomy
As echinoderms,sea stars possess a hydraulic water vascular system that aids in locomotion. 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 aid with feeding. Tube feet emerge through openings in the endoskeleton and are externally expressed through the open grooves present along the bottom of each arm.
The body cavity not only contains the water vascular system that operates the tube feet, but also the circulatory system, called the hemal system . Hemal channels form rings around the mouth (the oral hemal ring), closer to the top of the sea star and around the digestive system (the gastric hemal ring). A portion of the body cavity called the axial sinus connects the three rings. Each ray also has hemal channels running next to the gonads.
On the end of each arm or ray there is a microscopic eye (ocellus)which allows the sea star to see, although it only allows it to see light and dark, which is useful to see movement. Only part of the cells are pigmented (thus a red or black color) and there is no cornea or iris. This eye is known as a pigment spot ocellus.
Several types of toxins and secondary metabolites have been extracted from several species of sea star. Research into the efficacy of these compounds for possible pharmacological or industrial use occurs worldwide.
Digestion and excretion
Sea star digestion is carried out in two stomachs in the mouth: the cardiac stomach and the pyloric stomach. One that remains inside the body and another than can be protruded through the starfish's mouth (membranous or Cardiac stomach), and one normal pyloric stomach. The Cardiac stomach fetches the prey then passes it to the Pyloric stomach. The cardiac stomach is a sack-like stomach located at the center of the body and may be everted out of the organism's body to engulf and digest food. Some species are able to use their water vascular systems to force open the shells of bivalve mollusks such as clams and mussels by injecting their stomachs into the shells. With the stomach inserted inside the shell, the sea star is able to digest the mollusk in place. The cardiac stomach is then brought back inside the body, and the partially digested food is moved to the pyloric stomach. Further digestion occurs in the intestine. Waste is excreted through the anus on the aboral side of the body.
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, such as clams and oysters, arthropods, small fish, and mollusks.
Some echinoderms live several weeks without food under artificial conditions. It is believed that they may receive some nutrients from organic material dissolved in seawater.
Sea stars and other echinoderms have endoskeletons, suggesting that echinoderms are very closely related to chordates, animals with a hollow nerve chord that usually have vertebrae.
Diversity
As mentioned above there are over 1800 species; with many species awaiting discovery. Some of the better known sea star include:
- Bat star
- Blue sea star
- Carpet sea star
- Comb sea star
- Common starfish
- Crown-of-thorns sea star
- Eleven-armed sea star
- Japanese sea star
- Ochre sea star
- Pincushion sea star
- Pink sea star
The Northern Pacific sea star ( Asterias amurensis ) known as gohongaze is considered an edible delicacy.
Physiology
Nervous system
Echinoderms have rather complex nervous systems, but lack a true centralized brain. All echinoderms have a network of interlacing nerves called a nerve plexus 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. Th
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