Sea stars are star-shaped invertebrates living on the rocky sea floor. There are about 2000 known living sea star species that are distributed across the globe. The greatest variety of these are found in the tropical Indo-Pacific. Some other regions where they are widely found are the temperate regions of Australia, the cold-temperate waters of the North Pacific (California to Alaska), and the tropical East Pacific regions. These marine creatures can be sized from ½ an inch to over 3 feet in diameter; up to 11 pounds in weight and can live up to 35 years. Leather Star, Vermilion sea stars and Cushion stars are some of the common types of sea stars along the Pacific coast. Let's have a look at some more details about these sea stars.
Sea star is not a fish
Sea stars are often referred to as starfish, however, the point to be noted is that the sea star is not a fish, which is why marine scientists have preferred to change its name from starfish to sea star. Sea stars and fish come under Kingdom Animalia. However, since fish are vertebrates, they belong to phylum Chordata, and sea stars along with sea urchins, brittle stars, feather stars and sea cucumbers come under phylum Echinodermata. Echinoderms exhibit radial symmetry, which means that their arms radiate from a central disc outwards. Echinoderms are also characterized by tube feet, an endoskeleton made up of ossicles and plates in their body wall (may be large or microscopic).
Sea stars are often brightly colored
Live sea stars are brightly colored, unlike the dull, colorless skeletons we find on the beach. When alive, they feature wonderful colors such as red, orange, yellow, pink, blue, green or even purple. They even exhibit patterns on their body such as mosaic-like tiles, spots or even mottles. These colors and patterns assist the sea star to camouflage itself from predators. The bright hues also keep predators away by scaring them off.
Arms of sea stars are not limited to the number '5'
We come across pictures of sea stars with five arms and assume that they are five-armed marine creatures. On the contrary, sea stars' arms are not limited to 5. In fact, there exist sea stars with 10, 25 and even 40 arms. Cool isn't it! Moreover, each arm contains an extension of the body organs and body cavity.
Sea stars use tube feet for locomotion
Sea stars do not possess a jointed and movable skeleton for locomotion, instead their locomotion is carried out with the help of the water vascular system. This water vascular system has hundreds of projections called tube feet (suction cups) adorning its ventral side. When the creature contracts, water enters these tube feet and makes them protrude from the body. They enable these creatures to move along a surface by latching themselves to the surface. The tube feet function in the form of a wave, and when one portion of the body's tube feet attach themselves to the surface, the other portion releases its grip, thereby enabling the sea star to move from one place to another. The adult sunflower sea star has the amazing ability of moving at a speed of 1m/min with the help of its 15,000 tube feet! However, in general sea stars are slow-moving creatures.
Sea stars are carnivorous and have two stomachs
Sea stars are carnivorous and feed on shellfish, mussels, clams, sea anemones, gastropods, snails and small fish. Some even feed on sea urchins and sea cucumbers. They will feed on any animal they can get their stomachs on! Sea stars do not use their mouth to draw in food, instead they use their stomach. They possess two stomachs: cardiac stomach and the pyloric stomach, and use both for digestion. The sac like cardiac stomach is ejected out of the body through the mouth opening and is used to engulf and digest prey from the surroundings. The pyloric stomach is used solely for digestion of the partially digested food in the cardiac stomach. Since they can eject their stomach into the surroundings, they are able to catch animals larger than their mouth would naturally allow, such as mollusks, arthropods and so on.
Sea stars can pry open a shell
Sea stars are capable of ejecting their pouch like cardiac stomachs from their body; to devour the surrounding food. For example, if they find a mollusk, they can pry open the shell using the powerful suction cups on their arms. Once the sea star forces open the shell, it ejects its stomach through the mouth opening and devours the prey. When the meal is over, the stomach is retracted into the body. However, the sea star needs to be careful about what it eats, because a sick mollusk can make it sick as well.
Sea stars have no brains or even blood
This is quite an interesting fact about sea stars: they lack blood. These marine creatures pump water from the sea through their body. Water enters the water vascular system through a sieve plate called madreporite, which it pumps into the tube feet for carrying out locomotion. Sea stars lack the presence of the brain, instead possess a very complex nervous system.
Sea stars are not blind
Although sea stars do not have eyes, they are not blind. This doesn't mean they can see! They possess a small eye spot at the tip of each arm, which is sensitive to light and enables the sea star to detect shapes and differentiate between darkness and light.
Sea stars can regenerate their lost arms
Sea stars have the ability to regenerate a lost arm, and in some cases even the entire body. The vital organs in these creatures are located on the arms itself and so in cases when their arms are eaten by predators or get damaged, these creatures can regenerate fresh arms. From the central disc and one arm, an entirely new sea star can be formed (this can take up to a year).
Sea stars are dioecious
This means sea stars comprise male and female individuals, who possess 10 gonads each, situated at the end of each arm in pairs. The female sea star sends signals to the male and releases her eggs (about 2 million) into the water. The male sea star senses the signal and releases his sperms into the water. The water current causes the egg and sperm to drift along. When the egg and sperm meet, fertilization takes place and bilaterally symmetrical larvae are released into the water. This is called mass spawning. Sea stars can also reproduce asexually, wherein, part of the sea star's body breaks off and grows into a new individual. This process of reproduction is termed as fragmentation.
Some sea stars help keep sensitive coral reefs clean by scavenging for food, thereby being quite useful in reef setups. Most of the sea star species are harmless, however, there is also the poisonous variety (Crown of Thorns). These fascinating creatures are not listed as endangered species. However, the rate at which water is being polluted, we cannot be sure what the future holds!