Lying on or crawling ever so slowly across the bottom, the sea star looks deceptively simple and slightly out of place — like it might have fallen from a child’s crude drawing of the night sky and landed on the sea floor. But don’t let the sea star’s appearance fool you. It’s a complex, fascinating creature, indeed. 

For example, the sea star doesn’t have to ingest its food; it covers it with its oral disc (body) and then extrudes its stomach (which resembles a tiny, opaque balloon) through its mouth and envelops the meal, digesting it in situ. Lose an arm? The sea star regenerates it. Some species are even able to regenerate a whole new animal from a severed arm.
Sea stars are members of the Phylum Echinodermata (spiny skinned), which includes sea urchins, brittle stars, sea lilies and sea cucumbers. All of them have pentamerous radial symmetry; parts arranged in groups of five around a common center point, which is easily seen in sea stars. Echinoderms also have a unique water-vascular system, i.e., tubes throughout the body that connect to the tube feet. Water is taken in via the perforated sieve plate on the top of the central disk. Hydraulically operated tube feet are found in a groove on the underside of each arm, usually have flat tips, and can be suckered or nonsuckered. They provide motion and are also sensory organs. 
No doubt you’ve heard of a person who “wears their heart on their sleeve”? Well, the sea star wears its eyes on its arms. Kind of. You see, sea stars have a light sensitive eyespot on the tip of each arm and they curl those tips up when moving, for maximum exposure.
Sea stars are members of the Class Asteroidea. Those off SoCal waters can have as few as four arms and as many as 24. That being said, however, arms are frequently missing — partially or altogether — and many are in the process of being regenerated. 
On the tops of their bodies sea urchins and most sea stars have minuscule stalked pedicellariae, which are equipped with a specialized organ on the end. Sea urchins can have several different types. Most have three calcareous blades or valves, which open and close. One type of pedicellariae (not found on any of our West Coast species) is poisonous. Another is used for grasping and one is used for pinching/grasping/biting. Sea stars have a version of this latter pedicellariae but theirs have two valves and in some species the valves have teeth. Sea star pedicellariae occur in thick bunches and are only found on the tops of the animals. They are much too tiny to cause harm to humans. It is thought they are used primarily to remove debris or carry food to the mouth.
The sea stars found off the SoCal coast have few predators, though sea gulls are known to eat both ochre and giant stars, which are often found in tide pools. Sea otters also prey on sea stars.
Sea star sexes are separate and reproduction is via broadcast spawning or brooding as well as regeneration. In spawning, the eggs and sperm are released into the water simultaneously. Larvae (some species are free-swimming), hatch from the fertilized eggs, eventually settling on the bottom and morphing into juvenile sea stars. 
Of the more than 105 sea stars found off the SoCal coast, I have chosen four to feature here: the giant, the six arm, the sunflower and the bat star.
Giant Sea Stars
Giant sea stars (Pisaster giganteus) can reach a diameter of 22 inches. They have long, uniformly spaced, blunt-tipped spines surrounded by a blue circle. The spaces outside the circles are densely packed with tiny pedicellariae with pliers-shaped valves. The giant sea star lives on rocky and sandy bottoms, from intertidal waters to depths of 300 feet. It eats mollusks; most especially mussels, and is found from British Columbia to Baja CA. 
Six Arm Sea Stars
Six arm sea stars (Leptasterias sp.), which remind me of snowflakes, only reach one to three inches in diameter. I haven’t included a species name because these stars are difficult to identify without lab work. The best guess is that it is aequalis. Common in shallow water, six arms are carnivores, as are most sea stars, munching on small snails, mussels, barnacles and sea cucumbers. Six arms gather in mating aggregations and the females brood the fertilized eggs under their oral discs. They can’t eat for the six to eight weeks it takes for the eggs hatch and the larvae to morph into juvenile sea stars that crawl away. Six arm stars are found from Puget Sound to Southern California and from low intertidal waters to 100-foot depths.
Sunflower Stars
Sunflower stars (Pycnopodia helianthoides) come in jewel-like colors. These asteroids, however, are not gems but juggernauts, the largest and fastest sea stars found off our coast. They can have as many as 24 arms and measure as much as 39 inches from arm tip to arm tip. They have been clocked moving across the sea floor at a top speed of 40 inches per minute, propelled by an estimated 15,000 suckered tube feet with flat tips. That’s some serious “star power.” Sunflower stars are said to be voracious predators, with a diet consisting of crabs, sea cucumbers, snails, chitons, sea urchins, squid and even other sea stars. They have no known predators in SoCal waters and are found from intertidal waters down to 145 feet, from Alaska (where king crabs eat them) to San Diego. 
Bat Stars
Bat stars (Patiria miniata, formerly Asterina miniata) are ubiquitous off the West Coast. They only grow to be about eight inches in diameter and usually have five arms though there can be as few as four or as many as nine. Their color varies from bright red to tan and they can even be multicolored. The bat star is an anomaly for several reasons: it lacks spines and pedicellariae and is an omnivore. It is the only member of the Asterinidae family. The star’s aboral (top) surface is covered with small, flattened, overlapping crescent shaped ossicles that resemble shingles. While the tube feet are considered nonsuckered by scientists they are capable of adhering to hard substrates, even glass aquaria. Like the six arm, the bat star broods its eggs/larvae. It is found from Alaska to Baja, California, on rocks and sandy bottoms from intertidal waters to 950 feet.