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Sea star
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Sea stars, also known as starfish, are echinoderms belonging to the class Asteroidea. The names "sea star" and "starfish" are sometimes differentiated, with "starfish" used in a broader sense to include the closely related brittle stars, which make up the class Ophiuroidea, as well as excluding sea stars which do not have five arms, such as the sun stars and cushion stars.
Sea stars exhibit a superficially radial symmetry.
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Sea stars, also known as starfish, are echinoderms belonging to the class Asteroidea. The names "sea star" and "starfish" are sometimes differentiated, with "starfish" used in a broader sense to include the closely related brittle stars, which make up the class Ophiuroidea, as well as excluding sea stars which do not have five arms, such as the sun stars and cushion stars.
Sea stars exhibit a superficially radial symmetry. They typically have five "arms" which radiate from a central disk (pentaradial symmetry). However, the evolutionary ancestors of echinoderms are believed to have had bilateral symmetry. Sea stars do exhibit some superficial remnant of this body structure, evident in their larval pluteus forms.
Sea stars do not rely on a jointed, movable skeleton for support and locomotion (although they are protected by their skeleton), but instead 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. Sea stars usually hunt for shelled animals such as oysters and clams. They have two stomachs. One stomach is used for digestion, and the other stomach can be extended outward to engulf and digest prey. This feature allows the sea star to hunt prey that is much larger than its mouth would otherwise allow. Sea stars are able to regenerate lost arms. A new sea star may be regenerated from a single arm attached to a portion of the central disk.
External anatomy
Sea stars are composed of a central disc from which arms sprout in pentaradial symmetry. Most sea star have 5 arms, but some have more or fewer. Some sea stars have shown differing numbers of limbs within a single species.
The mouth is located underneath the sea star, on its ventral surface. The spiny upper surface is called the aboral or dorsal surface. On the aboral surface there is a structure called the madreporite, a small white spot located slightly off-center on the central disc which acts as a water filter and supplies the sea star's water vascular system with water to move. Porcellanasteridae employ additional cribriform organs used to generate current in the burrows made by these infaunal sea star.
While having their own basic body plan, sea star radiate diversely in shapes and colors, the morphology differing between each species. A sea star may have dense rows of spines as a means of protection, or it may have no spines at all. Ranging from nearly pentagonal (example: Indo-pacific cushion star, Culcita novaeguineae) to gracile stars like those of the Zoroaster genus.
Surrounding the spines on the surface of the sea star 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 colonizing the sea star. The radial canal which is across each arm of the sea star has tooth-like structures called ampullae, which surround the radial canal.
On the end of each arm or ray there is a microscopic eye which allows the sea star to see, although it only allows it to see light and dark, which is useful to see movement.
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 which is displayed by many marine animals as a means of protection against predation. 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.
Internal anatomy
The body cavity also contains the water vascular system that operates the tube feet, and the circulatory system, also 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.
Digestion and excretion
Sea star digestion is carried out in two stomachs: the cardiac stomach and 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.
Variety
As mentioned above there are over 1800 species and many are undiscovered. Some of the better known sea star include:
A Japanese variety known locally 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. The ring nerves and radial nerves coordinate the sea star'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 sea stars are sensitive to touch, while eyespots on the ends of the rays are light-sensitive.
Diet
Most species are generalist predators, eating clams, oysters or any animal too slow to evade the attack (e.g. dying fish). Some species are detritivores, eating decomposed animal and plant material or organic films attached to substrate. Others may consume coral polyps (the best-known example for this is the infamous Acanthaster planci), sponges or even suspended particles and plankton (sea star from the Order Brisingida).
The processes of feeding and capture may be aided by special parts; Pisaster brevispinus or short-spined pisaster from the West Coast of America may use a set of specialized tube feet to extend itself deep into the soft substrata to extract prey (usually clams). Grasping the shellfish, the sea star slowly pries open the shell by wearing out the adductor muscle and then inserts (also called evisceration) its stomach into an opening to devour the organism.
Sea stars have also been known to attack animals such as marine snails. Protoreaster Nodotus, or horned sea stars, eat snails in an aquarium setting if the detritus in the tank is not sufficient to satisfy the sea star's hunger.
Reproduction
Sea stars are capable of both sexual and asexual reproduction. Individual sea stars are male or female. Fertilization takes place externally, both male and female releasing their gametes into the environment. Resulting fertilized embryos form part of the zooplankton.
Sea stars are developmentally (embryologically) known as deuterostomes. Their embryo initially develops bilateral symmetry, indicating that sea stars probably share a common ancestor with the chordates, which includes the fish. Later development takes a very different path however as the developing star fish settles out of the zooplankton and develops the characteristic radial symmetry. Some species reproduce cooperatively, using environmental signals to coordinate the timing of gamete release; in other species, one to one pairing is the norm.
Sea stars commonly reproduce by free-spawning: releasing their gametes into the water where they hopefully are fertilized by gametes from the opposite sex. To increase their chances of fertilization, sea stars probably gather in groups when they are ready to spawn, use environmental signals to coordinate timing (day length to indicate the correct time of the year, dawn or dusk to indicate the correct time of day), and may use chemical signals to indicate their readiness to each other.
Fertilized eggs grow into bipinnaria and later into brachiolaria larvae, which either grow using a yolk or by catching and eating other plankton. In either case, they live as plankton, suspended in the water and swimming by using beating cilia. The larvae are bilaterally symmetric — unlike adults, they have a distinct left and right side. Eventually, they undergo a complete metamorphosis, settle to the bottom, and grow into adults.Some species of sea star brood their young: the males spawn gametes which fertilize eggs held by the females. The females may hold the eggs on their surface, in the pyloric stomach (as in Leptasterias tenera), or even attach them to the ground (as in Asterina gibbosa). Brooding is especially common in polar and deep-sea species, environments less favourable for larvae.
Male and female sea stars are not distinguishable from the outside; one needs to see the gonads or be lucky enough to catch them spawning. The gonads are located in each arm, and release gametes through gonoducts located on the central body between the arms.
Some species of sea star also reproduce asexually by fragmentation, often with part of an arm becoming detached and eventually developing into an independent individual sea stars. This has led to some notoriety. Sea stars can be pests to fishermen who make their living on the capture of clams and other mollusks at sea as sea stars prey on these. The fishermen would presumably kill the sea stars by chopping them up and disposing of them at sea, ultimately leading to their increased numbers until the issue was better understood. A sea-star arm can only regenerate into a whole new organism if some of the central ring of the sea star is part of the chopped off arm.
Locomotion
Sea stars move using a water vascular system. Water comes into the system via the madreporite. It is then circulated from the stone canal to the ring canal and into the radial canals. The radial canals carry water to the ampullae and provide suction to the tube feet. The tube feet latch on to surfaces and move in a wave, with one body section attaching to the surfaces as another releases. Most sea stars cannot move quickly. However, some burrowing species from the genera Astropecten and Luidia are capable of rapid, creeping motion: "gliding" across the ocean floor. This motion results from their pointed tubefeet adapted specially for excavating patches of sand.
Regeneration
Some species of sea star have the ability to regenerate lost arms and can regrow an entire new arm in time. Most species must have the central part of the body intact to be able to regenerate, but a few can grow an entire sea star from a single ray. Included in this group are the red and blue Linckia star. The regeneration of these stars is possible due to the vital organs kept in their rays.
Larval development
The larvae of echinoderms are ciliated, free-swimming organisms. They tend to look like embryonic chordates because they organize themselves bilaterally. As the organism grows, one side of the body grows more than the other, and eventually absorbs the smaller side. After that, the body is formed into five parts around a central axis. Then the echinoderm has radial symmetry.
Distribution
There are about 1,800 known living species of sea star, and they occur in all of the Earth's oceans. The greatest variety of sea stars is found in the tropical Indo-Pacific. Areas known for their great diversity include the tropical-temperate regions around Australia, the tropical East Pacific, and the cold-temperate water of the North Pacific (California to Alaska). Asterias is a common genus found in European waters and on the eastern coast of the United States; Pisaster, along with Dermasterias ("leather star"), are usually found on the western coast. Habitats range from tropical coral reefs, kelp forests to deep-sea floor, although none of them live within the water column; all species of sea star found are living as benthos. Echinoderms need a delicate internal balance in their body; no sea stars are found in freshwater environments.
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