They can have up to 40,000 feet, produce millions of offspring and eat with their stomachs outside their bodies. Sounds like something from a sci-fi film? You probably see them on every dive you make! Christian Skauge and Peter Symes provide 12 intriguing insights into sea stars.
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Sea stars (or starfish—but they are not fish) are a common sight for divers, as they occur in all the world’s oceans, from tropical reefs to the polar seabed. But how much do you know about these slow-moving predators?
They can have between five and 30 arms, arranged in radial symmetry, and they lack blood, a brain, a heart and other vital organs. Yet they have conquered the world’s oceans. Here are some interesting facts about starfish:
1. Around 2,000 species
Starfish belong to a group of animals called echinoderms, which also includes sea urchins, feather stars, brittle stars and sea cucumbers. The name literally means “spiny skin”, and the common starfish has around 2,000 cousins around the world. Sea stars have between five and 30 arms and a wealth of interesting features and characteristics, as well as colours, structures and shapes.
2. Feeding from the inside out
Sea stars move quite slowly, but many species are nevertheless voracious predators that consume large quantities of mussels. The sea star will grab a mussel with its suction feet and continues to pull on its shell until the mussel’s closing muscle is exhausted and starts to open. It then turns its stomach inside out, inserts it into the mussel and releases enzymes that dissolve the innards. After a while, the mussel meat becomes liquid, and the sea star can slurp up the delicious, nutritious soup that remains.
3. They can see you
Sea stars are found almost everywhere in the sea. They thrive in all kinds of habitats, no matter how silty, current-ridden, cold or deep they are. Sea stars generally live on the bottom, but can climb rock walls, piers or even ropes to get to where the food is. At the tip of each arm, many sea stars have a primitive eye that can distinguish between light and dark. Even though they do not move fast, they are surprisingly active animals.
4. Lack of organs
Sea stars have no heart, brain or even blood. In fact, they do not even have gills. They obtain oxygen from papillae on their tube feet and on their backs. Despite these “deficiencies”, sea stars are a very successful group of animals. Because they breathe the way they do, sea stars can be threatened by water pollution.
5. Shellfish on the menu
Most sea stars are predators, and some, like the red sea star, even feed on other sea stars. In general, sea stars prefer mussels, but their varied menu also includes sea snails, bristle worms, jellyfish, coral polyps and sometimes crustaceans. Even animals larger than themselves are at risk of being attacked and eaten; size is not so important when you are able to digest your food before putting it in your mouth. Sea stars also quickly gather in large numbers to scavenge if something edible falls to the bottom.
6. Suction feet
A sea star can have up to 40,000 suction feet, all controlled by a hydraulic system of water-filled channels inside the animal. This is called a water vascular system, and the feet are arranged in two or four rows along an open groove on the underside of the arms. The feet are used for locomotion, clinging to objects and searching for food. The suction power of the feet is enormous, and sea stars never get tired because they do not depend on muscles in the same way as we do.
7. Radial symmetry
Starfish are different from us in many ways. In addition to lacking several organs that are important to us, one of the most striking differences is that they are not bilateral (composed of two identical halves) like us. Instead, they consist of five identical segments. This five-point radial symmetry is clearly visible in most species. It can also be seen in species with more arms—there will simply be two or more arms per segment.
8. Millions of babies
Sea stars mostly reproduce sexually and require a mate. When the female releases up to 2.5 million eggs into the water, the male responds to her call, and the fertilised eggs develop into tiny larvae that live mid-water for a few weeks before settling on the bottom.
Other species, such as those in the Solaster and Henricia families, lack this larval stage and instead guard their eggs until they hatch as miniature sea stars. Some species are even capable of asexual reproduction and produce offspring entirely on their own.
9. Shooting stars
Sea stars have great self-healing powers and can easily regenerate an arm that has been lost to a predator or otherwise damaged. They are able to survive even if only the central disc remains, and sometimes you may encounter sea stars that resemble small shooting stars, where one arm is intact and all the others are just small buds waiting to grow out.
10. Water-powered
The starfish has a sieve-like opening on its back called a madreporite plate. This round, hard and porous opening allows water to be sucked into the water-filled channels of the vascular system to propel the hydraulic tube feet. In a way, you could say that sea stars are water-powered! Scientists also believe that the madreporite plays a role in how sea stars navigate and orient themselves.
11. Eating and being eaten
Although starfish are predators, they are also prey. They are hunted by other sea stars, crabs, birds, sea otters, harlequin shrimp and a few species of sharks. Even seagulls can be seen pecking at sea stars at low tide. Their most dangerous enemy is, of course, humans. In some Asian countries, sea stars are considered a delicacy. Would you like a bite? Despite its name, you will not find it at Starbucks.
12. Found in all oceans
Sea stars are found in all the world’s oceans. They thrive in all kinds of habitats, no matter how silty, current-ridden, cold or deep they are. However, they cannot live in fresh water because it lacks the calcium they need to build their bodies. That is why they are not found in the Baltic Sea and similar waters with low salinity. The largest numbers are found in the Indo-Pacific region, but they also thrive in the North Atlantic. ■
