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The ocean is a vast and mysterious world, home to countless weird and wonderful creatures. Yet, among them all, the sea star stands out as truly one-of-a-kind. With more than 2,000 species of these stunning marine animals, they are a true wonder of the underwater world.
From their unusual mode of movement to their complex feeding behaviors, there is so much to uncover and admire about these captivating creatures. Join us on a deep dive into the enchanting world of sea stars as we explore their biology, behavior, and the diverse array of species that call our oceans home.
What are Starfish/Sea Stars?
Sea stars, also known as starfish owing to their star-like shape, are a group of invertebrates called echinoderms, of which there are around 2000 known species.
What’s special about sea stars is that they are what is known as a keystone species. This means that they prey on animals that have no other natural predators. If there were no sea stars, their prey would easily multiply and take over the environment, making it difficult for other animals to survive.
While they may have long been called starfish, these creatures are not fish at all since they lack gills, fins and other body parts that would class them as fish.
Sea Star Anatomy
Sea stars are amazing creatures in that their anatomy is very different from what we might view as normal. With eyes yet no brain and several arms but no blood, it might seem as though they’d have a hard time surviving. But that certainly isn’t the case.
Generally speaking, sea stars have five arms. However, this can vary across species, with some having ten or twenty. There are even some starfish that have up to 40 arms, such as the sun star.
Sea stars may lose their arms which are sensitive to light because of the eye at the end. However, when their arms are lost, these amazing creatures have the ability to regenerate them – more on that later!
At the end of each of their arms they have hundreds of tiny tube feet which act like tiny suction cups which are driven by a hydraulic-like system as they fill with water. They can attach and detach to surfaces very rapidly, allowing the starfish to move around.
Sea stars do not have brains. However, they do have a nervous system that comprises a main nerve that runs around the mouth and others that run the length of each of the arms.
Sea stars have a single eye on the end of each of their arms. However, it was thought for a long time that they might not even be able to see. Scientists have now discovered that while sea star vision isn’t the clearest, they likely use their eyes for navigation and can form images to stop them from straying too far from their home.
Sea stars don’t have blood so how are nutrients and oxygen carried around their bodies? Well, they do have a vascular system; it’s just that the liquid pumped through it is different from other animals. Instead of blood, they pump seawater through their veins which is taken in through the madreporite which looks like a light spot on the top of the animal.
Sea stars come in a variety of different sizes. Some sea stars, such as the sunflower sea star can measure up to 16 inches (40 cm) across, while others may be less than half an inch (1.3 cm), like the paddle-spined sea star.
Due to their delicate electrolyte balance, sea stars are only able to survive in a marine environment and therefore cannot be found in bodies of freshwater. That said, the common sea star has been seen to survive in estuaries owing to its ability to adapt to various environments.
They are benthic animals meaning that they are bottom-dwellers and can be found in seas all over the globe, including warm tropical waters as well as polar regions. While they are mostly found in coastal areas around reefs, mudflats, and tide pools, there are some species that can live at depths of up to 20,000 feet (6,096 meters).
Starfish are mainly carnivorous creatures feeding on creatures that also live on the ocean floor. This can include snails, barnacles, and even small fish.
But what’s really interesting is not what they eat but how they eat. Some of the animals that sea stars feed on, like mussels, for example, can be very difficult to get into. But starfish are perfectly equipped to handle such a meal.
When hunting, they will grab their prey with their arms, allowing them to open the shell slightly. Once they do this, they eject their stomachs out of their mouths, which are located on the underside of the body. The stomach can squeeze into the shell, digesting the animal within before retracting back into the sea star.
The main benefit of eating this way is that sea stars are able to feed on larger animals that they otherwise wouldn’t be able to fit into their small mouths.
Reproduction & Lifecycle
Sea stars may reproduce in one of two ways. In most species, there are separate males and females and while the reproductive organs cannot be seen from the outside, making it difficult to sex any given individual, it becomes apparent who is who when they spawn. Males will release sperm into the water, while females release eggs. They are fertilized and turn into larvae that float down to the ocean floor where they develop into adults.
However, there are some species that are able to produce asexually and have both male and female reproductive parts. To reproduce this way, the starfish must lose an arm where it will then start growing its young. The second starfish will only detach from its parent once it is fully developed.
Sea Star Autotomy – Regeneration of Lost Arms
Sea stars are able to drop their arms when they are threatened which allows them to escape danger. This is known as autotomy or self-amputation, and while it may seem extreme, it’s not that big of a deal for a starfish who can easily regenerate lost limbs.
However, it can take a long time for a new arm to grow; in some cases months while other times, it can take years. This is why we must be very careful when handling sea stars, as they will self-amputate if they’re held by one arm. When they perform this act, they must immediately start sea star first aid to seal off the wound to prevent the entry of pathogens.
As I discussed earlier, there are some sea stars that will reproduce this way. Scientists have noted that, when the new limb begins forming, it comes complete with a mouth, digestive system, and new tube feet.
Types of Sea Stars
The oceans are teeming with more than 2000 species of sea stars, each just as unique and interesting as the next. Below, we’ll discuss some of the traits of the world’s most amazing sea stars.
1. Common Starfish (Asterias rubens)
The common starfish is found in the northeastern parts of the Atlantic and usually grows to between 3.9 inches (10 cm) and 12 inches (30 cm). However, there have been some specimens that have reached up to 20 inches (50 cm).
These sea stars live up to depths of 656 feet (200 meters) and tend to prefer rocky seabeds. While they are largely marine species, they have been known to be able to survive in brackish waters.
With five arms and being orange to brown in color, the common sea star can live for as long as ten years!
2. Morning Sun Star (Solaster dawsoni)
Out of all the starfish, the morning sun star is certainly one of the most unique looking. It can have anywhere between eight and thirteen arms and will grow up to 16 inches (40 cm).
They’ll live at depths of up to 1,377 feet (420 meters) and are feared by other species of starfish upon which they feed. Morning sun stars typically go for leather starfish as they’re much slower moving and can latch on and feed for up to eight days!
They’re bright orange in color and are found in rocky intertidal zones where they are also common predators of the nudibranch and other sea cucumbers.
3. Crown-of-Thorns (Acanthaster planci)
The crown-of-thorns starfish takes its name from the long spines that cover its body that resemble the crown of thorn described in the Bible. This is a species of venomous starfish that can cause pain, fever, headaches, and other unpleasant symptoms in humans. While scientists have discovered that there is saponin in their venom, this isn’t thought to be what causes these symptoms.
These sea stars are among the largest, growing up to 14 inches (35 cm). While they have a beautiful and unique appearance, they are something of a pest, particularly around the Great Barrier Reef where they are thought to be contributing to coral bleaching as this is one of their primary food sources and COT populations are thriving here.
4. Sunflower Star (Pycnopodia helianthoides)
The largest species of sea star, the sunflower star can reach up to 3.3 feet (1 meter) across. They can have up to 24 arms, although 16 are also common. These starfish are found in the north east Pacific but are now considered to be critically endangered as a result of sea star wasting disease. While this is concerning, efforts are being made to reintroduce individuals into the wild to increase numbers.
Sunflower stars prefer shallow waters that are either rocky or muddy. They prey on urchins and can whizz along the seabed at speeds of up to 3.3 feet (1 meter) per minute, making them one of the fastest sea star species.
5. Bat Sea Star (Patiria miniata)
The bat sea star has webbed arms which look similar to the wings of a bat, which is where it gets its name. These sea stars can come in a variety of colors including orange, pink, green, brown, and others.
They’re mid-sized stars growing up to 9.8 inches (25 cm) and are found at depths of up to 984 feet (300 meters) in rocky intertidal zones. Bat sea stars are common in coastal areas of the Pacific from Alaska down to California. They prey on various types of worms, algae, and even other starfish.
6. Cushion Sea Star (Asterina gibbosa)
The cushion sea star is one of the smallest stars that we’ll discuss in this guide, usually growing no bigger than 2 inches (5 cm). These are nocturnal species that spend most of the day hiding under rocks or boulders.
They’re commonly found close to the shore in parts of the North Atlantic as well as in the Mediterranean Sea. While they will go a little further down into the water, they’re not normally found past around 410 feet (125 meters).
Despite being small, cushion sea stars can live as long as seven years and feed on a diet of mainly bacteria.
7. Arctic Cookie Star (Ceramaster arcticus)
No, this isn’t a tasty baked snack, although the strange appearance of this pentagonal shaped sea star might have you believing otherwise. The Arctic cookie star has no arms, is usually pink with red markings, and is one of the rarest starfish in the world.
They are found in the outer reaches of the northwest Pacific and, as their name suggests, in the Arctic Ocean, where they feed on sponges, mollusks, and small shellfish.
These sea stars are capable of reproducing both asexually and sexually and live at depths of up to 610 feet (186 meters).
8. Necklace Starfish (Fromia monilis)
Sometimes called the tiled starfish, the necklace starfish has a very distinct appearance with bright red tips. That said, it commonly comes in other colors, so it’s one of the more difficult to identify using pictures alone.
Necklace starfish are found in the Indian Ocean as well as the western Pacific and prefer clear, warm waters where they will prey on sponges, shrimps, and worms. However, if you go looking for necklace starfish, don’t expect to just stumble upon one as these are among some of the hardest to find in the wild.
9. Pacific Blood Star (Henricia leviuscula)
As you might guess from its name, the Pacific blood star usually has a reddish, or at least orange color. However, there are some individuals that may have a more purplish hue or may be mottled. Found in the north Pacific from Alaska down to California, the blood star likes rocky areas and can live at depths of up to 1,427 feet (435 meters).
They feed primarily on sponges or bacteria and grow no more than 4.7 inches (12 cm), although 3.1 inches (8 cm) is the average size.
10. Blue Sea Star (Linckia laevigata)
The blue sea star has perhaps one of the most striking appearances of all the stars on this list with its solid blue color that can be light through to dark. They’re found living in coral reefs in tropical Indo-Pacific regions, often along the coasts of the Philippines, Tonga, and Bali.
Blue sea stars have five arms and can grow up to 12 inches (30 cm), feeding largely on invertebrates but falling prey to the pufferfish in the wild.
However, they are very popular marine pets and are also common in the sea shell trade, where dead specimens are sold as tourist gifts.
11. Royal Starfish (Astropecten articulatus)
You could be forgiven for thinking that the royal starfish is a mythical creature thanks to its brilliant coloration. They have a deep purple body with orange margins along their five arms.
Royal starfish are found in the western Atlantic along the eastern coasts of North America and all the way down to the Caribbean Sea.
Their primary food source is mollusks, but they eat in a rather unique way. While other starfish may extend their stomachs out of the mouth, the royal starfish swallows its prey whole.
12. Mottled Sea Star (Evasterias troschelii)
Mottled sea stars, unsurprisingly have a mottled appearance but can come in a wide variety of colors, from orange to brown and greenish-gray to purple. They are sometimes called the false ochre sea star.
These sea stars live in rocky areas and are very common in bays, preferring not to live any deeper than around 246 feet (75 meters). They’re common around the North West coasts of the USA and Canada but are also notably found in Kamchatka, Russia.
They grow up to 11 inches (28 cm) and reproduce sexually; their breeding season occurs between April and June.
13. Pink Sea Star (Pisaster brevispinus)
The pink sea star often referred to as the giant pink sea star owing to its size of up to 35 inches (90 cm), can be found in the northeastern parts of the Pacific Ocean.
It has a flexible body and is pale pink in color with five arms. These sea stars prefer shallow waters and won’t head any further than 360 feet (110 meters) into the ocean.
They feed on clams and cockles but are opportunists that will also take snails, crabs, and even dead fish when they have the chance.
14. Leather Star (Dermasterias imbricata)
The leather sea star takes its name because of the leathery, smooth texture of its skin. They are typically yellow, red or gray in color and may have markings in red, brown, or purple.
These sea stars are very similar in their appearance to the bat sea star and are often confused but bat stars are a lot more textured.
Leather sea stars are found no deeper than 295 feet (90 meters) and prefer intertidal regions along the western coasts of North America.
Interestingly, these sea stars emit a smell that is similar to garlic which is why they’re sometimes called the garlic sea star.
15. Giant Spined Star (Pisaster giganteus)
Aptly named, the giant spined sea star can get up to 24 inches (61 cm) across and has five arms that are significantly wider than those of other species. They come in a variety of colors, including purple, brown, and red and have calcium carbonate spines, which are blue with white tips.
Giant spined stars are found along the coast of western America as far north as Vancouver and down to California. They mainly feed on things like limpets and mussels and are common prey for various sea birds.
16. Granulated Sea Star (Choriaster granulatus)
The granulated sea star has to be one of the most interesting in terms of looks with rounded, stubby arms that give it a chubby appearance. They are pale pink in color and typically grow to around 11 inches (27 cm).
These sea stars live on reefs and aren’t usually found any deeper than 131 feet (40 meters). They feed on algae but will also eat dead animals when they have the chance. Sometimes, they’re nicknamed the doughboy starfish.
17. Pincushion Starfish (Culcita novaeguineae)
Pincushion starfish are found in the Indian Ocean and the Pacific in areas around Hawaii and Australia, as well as the Seychelles and Madagascar. While they do come in various colors, they’re almost always mottled.
These sea stars have a pillow-like appearance and seemingly no arms. However, when you look on the underside, you can clearly see five way symmetry.
The pincushion starfish grows to around 12 inches (30 cm) and feeds mainly on a diet of stony corals and various invertebrates.
18. Chocolate Chip Sea Star (Protoreaster nodosus)
Another of the most interesting looking sea stats is the chocolate chip sea star that has brown horns on its back that resemble, you guessed it; chocolate chips. However, these horns also give it the name the horned sea star by which it is commonly referred.
These starfish are found in Indo-Pacific waters and are very common around the Philippines. They are very popular marine pets and are also sadly over-harvested for the sea shell trade, which means numbers are in decline.
They prefer sandy or reef environments and will prey on snails and sea urchins but are opportunists and will take what they can when they can.
19. Red-Knobbed Starfish (Protoreaster lincki)
The red-knobbed starfish has a series of bright red tubercles along each of its arms that are offset by its gray body. These tubercles are what give the animal its name.
These starfish can grow up to 12 inches (30 cm) and have five arms. Their unique appearance makes them another species that is popular among marine tank owners.
Red-knobbed starfish are found only in the Indian Ocean and are common along the coasts of countries like Sri Lanka and Madagascar. They inhabit tidal pools but can be found up to depths of 328 feet (100 meters).
20. Spiny Sand Star (Astropecten armatus)
The spiny sand star is an interesting species since it has a special ability to flip itself over if it ever becomes upturned. What’s more, these sea stars are able to move despite the lack of suckers; instead they have small points on the ends of their five arms which upturned at the ends.
These starfish are commonly found along the east coast of the Americas as far north as California and as far south as Ecuador. They are usually found in shallow water but can go as deep as 377 feet (115 m), where they prefer sandy sediment into which they will partially submerge themselves.
They are dull pink or gray in color and can grow up to 7 inches (17 cm) making them a mid-sized starfish.
Threats to Sea Stars
While there are thousands of species of sea stars, some are now critically endangered. This includes the sunflower sea star which is thought to be at the edge of extinction after a population decline of up to 90%.
There are various threats to sea stars, including things like climate change and a demand for dried specimens in the souvenir trade.
Research has been taking place to find out whether sea stars would be able to survive projected heat waves as far ahead as the year 2100. Sadly, in these tests, with a rise of 14 °F (8 °C), 100% of the sea stars did not survive. According to the same research, these animals, which are sensitive to heat, are struggling to get through +9 °F (+5 °C) heat waves in the present day.
Some species, such as the sunflower sea star, were once incredibly common around the coasts of North America but are now in serious decline and climate change is partly to blame. With rising temperatures in the oceans, sea star wasting disease is being exacerbated and is responsible for the decline of up to 20 species in recent years.
I mentioned sea star wasting disease just a moment ago, and while climate change does seem to make it worse, it’s a problem in its own right.
While it was previously thought that the disease, also known as sea star wasting syndrome, was caused by a virus, this has since been disproved. Scientists have now discovered that the condition is caused by microorganisms that are using up the available oxygen in the water, essentially suffocating the sea stars.
Sea star wasting disease is a condition that seemingly causes sea stars to just fall apart. The condition begins with lesions that soon cause the starfish to fragment and has been responsible for the mass deaths of around 40 species in total.
Sea star wasting disease seems to come in plagues with two in the 1970s and several occurring since 2013. Most cases are found off the coasts of North America.
Starfish do not breathe in the same way as you and I, so water pollution is a massive problem. Since they filter seawater through their skin and around their vascular system to gather nutrients and oxygen, it needs to be clean.
Unfortunately, with things like oil spills and pollution becoming more and more of a problem, sea stars are taking in all kinds of toxins.
Have you ever been to a coastal souvenir store and seen dried starfish and shells for sale? This is sadly one of the biggest threats to sea stars since they are being over-harvested for the souvenir trade. In Mexico, in just one year, it was noted that around 880,000 sea stars were collected to be sold as keepsakes.
But they are just fished from the water. When sea stars wash up on the shore, rather than throwing them back, people will collect them, paint them and go on to sell them.