starfish

starfish

[stahr-fish]
starfish: see sea star.
or sea star

Principal features of a starfish. Water for the water vascular system enters through the elipsis

Any of 1,800 echinoderm species (class Asteroidea) that have regenerable arms surrounding an indistinct disk and that inhabit all oceans. Species range from 0.4 to 25 in. (1–65 cm) across, but most are 8–12 in. (20–30 cm) across. Their arms, usually five, are hollow and, like the disk, covered with short spines and pincerlike organs; on the lower side are tube feet, sometimes sucker-tipped, used for creeping or clinging to steep surfaces. Some species sweep organic particles into the mouth on the underside of the disk. Others either evert the stomach upon their prey for external digestion or swallow the prey whole.

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Crown-of-thorns starfish

Reddish and heavy-spined starfish (Acanthaster planci) that has 12–19 arms and is often 18 in. (45 cm) across. It feeds on the polyps of coral. Beginning circa 1963, its population on Australia's Great Barrier Reef exploded. Destruction of coral reefs and islands was feared, and intensive efforts were made to kill it off. Since then other outbreaks have been recorded throughout the southern Pacific. The cause of the outbreaks is unknown, but several factors have been proposed, such as the decimation of the starfish's chief predator, the Pacific triton (a marine snail), by shell collectors. Other factors, including the runoff of nutrient-rich soil into reef waters as a result of shorefront development, have also been implicated. Population fluctuations could also be a feature of the starfish's natural ecology, and human influence may alter these cycles.

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Starfish (also called sea stars) are any echinoderms belonging to the class Asteroidea. The names "sea star" and "starfish" are also used in a broader sense to include the closely related brittle stars, which make up the class Ophiuroidea.

Starfish exhibit a superficially radial symmetry. They typically have five or more "arms" which radiate from an indistinct disk (pentaradial symmetry). However, the evolutionary ancestors of echinoderms are believed to have had bilateral symmetry. Starfish 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 starfish's arms which function in locomotion and aid with feeding. The star fish 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 starfish to hunt prey that is much larger than its mouth would otherwise allow. Starfish are able to regenerate lost arms. A new starfish may be regenerated from a single arm attached to a portion of the central disk.

External anatomy

Starfish are composed of a central disc from which arms sprout in pentaradial symmetry. Most starfish have 5 arms, but some have more or fewer. Some starfish have shown differing numbers of limbs within a single species. The mouth is located underneath the starfish, 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 starfish's water vascular system with water to move. Porcellanasteridae employ additional cribriform organs used to generate current in the burrows made by these infaunal starfish. While having their own basic body plan, starfish radiate diversely in shapes and colors, the morphology differing between each species. A starfish 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 starfish 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. On the end of each arm or ray there is a microscopic eye which allows the starfish 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 starfish. 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 starfish 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

Starfish 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 starfish 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 either excreted through the anus on the aboral side of the body, or excreted through the mouth if the anus is absent as in brittle stars.

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, 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 starfish 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 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.

Diet

Most species are generalist predators, some eating 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 its stomach into an opening to devour the organism.

Reproduction

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.

Starfish 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, starfish 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 starfish 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 starfish 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 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.

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 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.

Regeneration

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.

Geological history

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 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 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.

Notes

References

  • Blake DB, Guensburg TE; Implications of a new early Ordovician asteroid (Echinodermata) for the phylogeny of Asterozoans; Journal of Paleontology, 79 (2): 395-399; MAR 2005.
  • Gilbertson, Lance; Zoology Lab Manual; McGraw Hill Companies, New York; ISBN 0-07-237716-X (fourth edition, 1999).
  • Shackleton, Juliette D.; Skeletal homologies, phylogeny and classification of the earliest asterozoan echinoderms; Journal of Systematic Palaeontology; 3 (1): 29-114; March 2005.
  • Solomon, E.P., Berg, L.R., Martin, D.W. 2002. Biology, Sixth Edition.
  • Sutton MD, Briggs DEG, Siveter DJ, Siveter DJ, Gladwell DJ; A starfish with three-dimensionally preserved soft parts from the Silurian of England; Proceedings of the Royal Society B - Biological Sciences; 272 (1567): 1001-1006; MAY 22 2005.
  • Hickman C.P, Roberts L.S, Larson A., l'Anson H., Eisenhour D.J.; Integrated Principles of Zoology; McGraw Hill; New York; ISBN 0-07-111593-5 (Thirteenth edition; 2006).

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