Starfish or sea stars are marine invertebrates belonging to the kingdom animalia, phylum Echinodermata, class Asteroidea. The names sea star and starfish are also (incorrectly) used for the closely related brittle stars, which make up the class Ophiuroidea. They exhibit a superficially radial symmetry. Starfish typically have five or more "arms" which radiate from an indistinct disk (pentaradial symmetry). In fact, the evolutionary ancestors of echinoderms are believed to have had bilateral symmetry, and starfish do exhibit some superficial remnant of this body structure, which is evident in their larval pluteus forms. Starfish do not rely on a jointed, movable skeleton for support and locomotion (though 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, located on the ventral face of the starfish's arms, which function in locomotion and aid with feeding.
There are about 1,800 known living species of starfish, and
they occur in all of the Earth's oceans. The greatest variety of
starfish 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 could range from tropical coral reefs, kelp
forests to deep-sea floor, although none of them live within the
water column; all species of starfish found are living as
benthos. Echinoderms need a delicate internal balance in their
body; no starfish are found in freshwater environments.
Starfish are composed of a central disc from which arms sprout in pentaradial symmetry. Most starfish have five arms, however some have more or fewer; in fact some starfish can have different numbers of arms even within one species. The mouth is located underneath the starfish on the oral or ventral surface, 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, a small white spot located slightly off-center on the central disc which acts as a water filter and supplies the starfish'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 starfish.
Starfish, while having their own basic body plan, radiate diversely in shapes and colors and the morphology differs between each species; for example, a species of starfish may have rows of spines on their arms as means of protection. Ranging from nearly pentagonal (example: Indo-pacific cushion star, Culcita novaeguineae) to gracile stars like those on Zoroaster genus.
Starfish have a simple photoreceptor eyespot at the end of each arm. The eye is able to "see" only differences of light and dark, which is useful in detecting movement.
On the surface of the starfish, 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 colonizing the starfish. The radial canal which is across each arm of the starfish has tooth-like structures called ampullae, which surround the radial canal. 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 means of protection against predation. Several types of toxins and secondary metabolites have been extracted from several species of starfish and now being subjected into research worldwide for pharmacy or other uses such as pesticides.
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 starfish and around the digestive system (the gastric hemal ring). The axial sinus, a portion of the body cavity, connects the three rings. Each ray also has hemal channels running next to the gonads.
Starfish digestion is carried out in two 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 endurance of 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, it digests 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 and waste is either excreted through the anus on the aboral side of the body, or if the anus is absent (as in brittle stars), waste is excreted through the mouth.
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, 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 (internal skeletons), suggesting that echinoderms are very closely related to chordates: animals with a hollow nerve chord that usually have vertebrae.
Echinoderms have rather complex nervous systems, but lack a true
centralized brain. 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
Most species are generalist predators, some eating bivalves like mussels, clams, and 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. The others may consume coral polyps (the best-known example for this is the infamous Acanthaster planci), sponges or even suspended particles and planktons (starfish 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 Starfish slowly pries open the shell by wearing out the Adductor muscle and then inserts an arm into an opening to devour the organism.
Starfish are capable of both sexual and asexual reproduction. Individual starfish 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.
Starfish are developmentally (embryologically) known as deuterostomes. Their embryo initially develops bilateral symmetry, indicating that starfish probably share a common ancestor with the chordates, which includes the fish. Later development takes a very different path however as the developing starfish 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.
Some species of starfish also reproduce asexually by fragmentation, often with part of an arm becoming detached and eventually developing into an independent individual starfish. This has led to some notoriety. Starfish can be pests to fishermen who make their living on the capture of clams and other mollusks at sea as starfish prey on these. The fishermen would presumably kill the starfish by chopping them up and disposing of them at sea, ultimately leading to their increased numbers until the issue was better understood. A starfish arm can only regenerate into a whole new organism if some of the central ring of the starfish is part of the chopped off arm.
Starfish 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 starfish cannot move quickly. However, some burrowing species like starfish from genus 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.
Some species of starfish 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 starfish 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 arms.
Fossil starfish and brittle stars are first known from rocks of Ordovician age indicating that two groups probably diverged in the Cambrian. However, Ordovician examples of the two groups show many similarities and can be difficult to distinguish. Complete fossil starfish are very rare, but where they do occur they may be abundant. Most fossil starfish consist of scattered individual plates or segments of arms. This is because the skeleton is not rigid, as in the case of echinoids (sea urchins), but is composed of many small plates (or ossicles) which quickly fall apart and are scattered after death and the decay of the soft parts of the creature. Scattered starfish ossicles are reasonably common in the Cretaceous Chalk Formation of England.
This Starfish Page is Copyright The Animal Web Guide © 2004 - 2009 Chuck Ayoub