Sea star

? Sea stars
Ochre Sea Star on beach at Olympic National Park
Scientific classification
Kingdom: Animalia Phylum: Echinodermata Class: Asteroidea
Orders
Forcipulatida Paxillosida Platyasterida Spinulosida Valvatida

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.

Distribution

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.

External Anatomy

Red-knobbed Starfish Protoreaster linckii, a sea star from the Indian Ocean

The tube feet can be seen on this sea star.

Deep Blue starfish found off of Fiji.

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.

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.

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.

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. This mainly serves 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 curing diseases or other uses such as pesticides.

Internal Anatomy

The body cavity also contains the water vascular system that operates the tube feet, and 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.

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 molluscs.

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.

Nervous System

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.

Behaviour

Reproduction

Most starfish reproduce in a method similar to the sponge. The starfish gather in a group (using environmental signals to coordinate the timing), metamorphose into juvenile sea stars and can begin living on the ocean floor.

Sea stars are developmentally ( embryologically) known as deuterostomes. Since echinoderms and chordates share this same embryological pattern, they are thought to be closely related. Nevertheless, as these creatures are invertebrates and not actually fish, most marine biologists are pushing to completely replace the term starfish with sea star.

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.

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 starfish from a single ray. These species will regenerate several starfish from a single one which is torn apart.

Geological history

Fossil sea stars 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 similarites and can be difficult to distinguish Complete fossil sea stars are very rare, but where they do occur they may be abundant. Most fossil sea stars 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 sea star ossicles are reasonably common in the Cretaceous Chalk Formation of England.