Sea star on colorful rock reef

Sea stars and brittle stars

When I learned how to dive, a whole new world opened up—but it soon felt like this world was dying!

Sargassum seaweed was taking over where kelp forests had been, and bristle stars took over entire reefs. It was at the height of the mysterious sea star wasting syndrome, and many thought one of the coolest sea stars of California had been lost—but then we saw them return.

What to expect in this episode:

  • What kind of animals are sea stars?
  • Sea star anatomy
  • A stomach that digests outside the body
  • Stars moving through the seas
  • The Sunflower Star
  • Spiny brittle stars
  • Crown-of-thorns
  • Ochre sea star (kinda)

What kind of animals are sea stars?

Sea star on colorful rock reef
Sea star on colorful rock reef by Kate Hildenbrand

A while ago, we split the world of animals into the Protostomia and the Deuterostomia depending on where that pre-mouth ends up in the final organism, both during early development and evolution.

The last few, okay many, episodes were about the Protostomia. Mollusks, crustaceans, and a shitton of worm stuff: all Protostomia.

The other huge branch of the Bilateria is the Deuterostomia, the animals whose mouths became anuses. I know, flattering. But it’s what human embryos go through, too. They fold in their blastula to form the gastrula with its one opening, and then they fold stuff around in something called radial cleavage until everything ends up where it’s supposed to be.

We’ll stay in the deuterostome realm for the rest of this series until the very final episodes about plants and fungi, where we leave the animal kingdom altogether.

There are three groups of Deuterostomia: the echinoderms that will occupy us next, the chordates, which will keep us busy for a while, and the hemichordates. The hemichordates, the acorn worms, are worms, so I am over them already. Evolutionary biologists like the hemichordates because they form the link between all the mushy organisms and the animals with a backbone.

Within the echinoderms, there are the sea stars, the brittle stars, the sea urchins, the sea cucumbers, and the Articulata. There is a lot of uncertainty and debate as to how these five groups fit together, so let’s not even try. Some people think the sea stars and brittle stars belong together because of the position of their mouth, others think they aren’t closely related. It’s a mess. Let’s not.

My trusted NCBI database has the echinoderms divided as follows:

Taxonomic tree of Echinodermata
Taxonomic tree of the echinodermata. Based on the NCBI database. Drawn by Kate Hildenbrand.

Instead of getting into that mess, let’s talk about sea stars and brittle stars. Sea stars, like all echinoderms, are strictly marine. You won’t find them in limnic habitats, let alone on land. While they are limited to salty waters, there is a large variety of them in the benthic communities anywhere from the tropics to cold polar waters.

Just the fact that you can find them in tide pools already tells you a lot: If something can withstand the extreme temperature variation of the shallow tide pools, they can withstand a lot.

Sea star anatomy

Sea stars and, to a degree, brittle stars look like a child’s drawing of a star—well, except for the googly eyes, of course. Those should be put on the end of the arms to make them more scientifically accurate, but they’d probably look a lot less cute. Sea stars have eye spots made up of ocelli—the same simple pigment eyes used by insect species—at the end of each arm.

Childlike drawing of a sea star with googly eyes on each arm.
Childlike drawing of a sea star with googly eyes on each arm.

Like the other echinoderms, they don’t have a centralized brain, but they do have a complex nervous system. A nerve ring is situated around their mouth and nerves move outward from there. It’s the same for all echinoderms.

One characteristic of the echinoderms, including the sea stars, is their radial symmetry, usually pentamerism, so repeating one part five times. If you just think of a sea star, this will probably be easy to understand. Take a leg, rotate it around, and all the other legs line up in a beautiful pentameral symmetry.

But those of you who’ve paid attention throughout this series remember that all the protostome and deuterostome animals belong to the Bilateria, so animals with a bilateral symmetry like humans. So, sea stars are both divisible by five and two? Something doesn’t add up here.

Again, the answer is in early development. When the larvae first hatch, they are bilaterally symmetrical, with a left half and a right half. This symmetry gets ducked up in their later metamorphosis to an adult.

To make this even more ducked up, there are sea stars with six arms or more that completely destroy all of these assumptions about pentameral symmetry—and that’s before they lose some of their arms and have to regrow them, of course. Oh, yes, they can regrow arms. As long as they still have one arm left, the rest can be regenerated.

Their skin is made up of calcareous plates and a spongy structure in between that keeps the armor from getting too heavy. The plates are called ossicles; the spongy stuff is the stereom. These plates can be spines or flat, fused or individually mobile.

The brittle stars and true sea stars have a lot in common, but they are still easy to distinguish if you take a closer look. The brittle stars, the Ophiuroidea, have a central disk with thin arms branching out from it, while the true sea stars, the Asteroidea, look like nothing but arms.

Sea star drawing by Kate Hildenbrand
Brittle star drawing by Kate Hildenbrand

In true sea stars, there are two features on the top side of the stars: the anus in the middle, and the madreporite or sieve plate a little offset to one side. In brittle stars, this sieve plate thing is on the bottom instead, and they just don’t have an anus at all. Eww. And because biologists like to name things, the three arms away from this pore are called the trivium, the other two the bivium. Yeah, don’t ask me why that’s important.

Let’s have a look at the underside: both brittle and sea stars have the mouth at the very center of their body. Around that are the arms with tiny tube feet. Though they are way more than feet, as they also aid in some other processes such as gas exchange. Can your feet get rid of nitrogen? No, well, sea stars have a leg—or should I say arm?—up on you then.

The area where the tube feet are is called the ambulacral groove. I told you biologists like naming things. There are some canals that move water through the vascular system, and the ampullae that control the tube feet.

Before we talk about the feet on their arms—seriously, who decided on those terms?—let’s talk about the coolest feature of sea star anatomy:

A stomach that digests outside the body

Sea stars have an exceptional digestive system: they can drop their stomach. The stomach is directly attached to the mouth, and when there’s food, the stomach drops out, wraps around the prey, and then gets pulled in after digestion. Yes, you heard that right: food is digested outside the body. And if they don’t like what they ate, they can discard the entire thing and grow a new one. That does sound neat, indeed. Definitely would solve quite a few of my digestive issues.

And while sea stars are typically carnivorous, they are also opportunistic feeders who will scavenge on anything from dead things to the organic film that builds up on surfaces.

Brittle stars are likely just as jealous about this ability to digest outside the body as I am. They can’t do it either.

Stars moving through the seas

Brittle stars and sea stars move quite differently. If they are nice enough to move for you, it’s another way to tell them apart—though I really think looking at the form is the best and easiest way.

Both kinds of stars have tube feet, small extensions that kind of work like suction cups. The ampullae move water out of the tube feet to make them stick to surfaces and into the tube feet to release them. Sea stars use these feet to actually move. It’s why they look like they are gliding or rolling across the surface.
Brittle stars, in contrast, use their actual arms for movement. Their plates aren’t fused, so they can much better move the entire foot to drag and push themselves around. They still have the tube feet for sticking to things, but it’s not how they like to get around. They also use their tube feet to filter-feed.

Well, and then there’s the part where they also use the feet to get rid of excess nitrogen and take up other gases.

The Sunflower Star

The Sunflower sea star isn’t just my favorite, it’s also one of the two stars people keep claiming holds the record for heaviest echinoderm, along with something very eloquently named the “Fat Starfish.” Common names are always fun, aren’t they?

By Sam Wilson – originally posted to Flickr as Sunflower Sea Star (Pycnopodia helianthoides), CC BY-SA 2.0,

The Sunflower Star, Pycnopodia helianthoides, is among the largest sea stars with a diameter of up to a meter (3 feet). They are opportunistic hunters, but they really like munching on sea urchins—an echinoderm we’ll get into very soon. But before you complain that these echinoderms eat echinoderms, I’d like to remind everyone that humans eat mammals.

When we started diving, we were excited to see a few of these stars during our trips a little further North than our home turf around Catalina island. According to the IUCN red list profile, the sea star wasting syndrome killed off more than 90% of these beautiful ginormous stars—more than five billion of them. It also made them vanish from the southern half of their range. The southern half of their range is where we usually went diving, so this was pretty obvious to us.

But what is this sea star wasting syndrome I keep talking about? In 2013 and 2014, a shitton of sea stars from more than 20 species died off in masses—and no one knew why. We’ve seen similar die-offs before, but those weren’t as extreme as the 2013/14 one. They also didn’t have a range this broad. And, it’s not gone. According to the UCSC, “SSWS has persisted at low levels in most areas, and continues to kill sea stars.”

The reason for the sea star wasting syndrome are still pretty unclear, and it’s been a while. In essence, the sea star just looks unwell for a few days, and then dies. It’s a quick and ugly death with lesions on the body before the body just falls apart. Oh, and it’s contagious apparently, as this usually affects quite a few stars of the population.

There were—or rather are—more than twenty species affected, but the sunflower star and the ochre star have been pretty badly hit. Unfortunately, those two are also considered keystone species. And we saw why first hand: when the sunflower stars don’t feed on the urchins, the urchins take over entire reefs. And while you might think that it doesn’t make much difference if one echinoderm dominates a reef or another, that’s like saying it doesn’t make a difference if humans or deer inhabit a forest.

Urchins like munching on kelp. Kelp forests are my favorite ecosystem, so seeing the Giant Kelp of California being eaten up by urchins—well, and pushed out by Sargassum—was no fun at all.

But, they made it. The sunflower star is still out there, fighting for survival. And it’s looking ducking cool while doing so. I mean, remember all that talk about symmetry? The sunflower star has the obligatory five legs during early development—and even in the larval stages—but grows more arms as it ages. Adults have 16 to 24 arms. That’s a lot of arms. I can barely coordinate two of those things…

They reproduce through broadcast spawning, so when it’s time for sex, they use some of their arms to heave their bodies away from the sea floor and squirt out their gametes. The sperm and eggs then need to find each other for external fertilization. Yes, sea stars have separate sexes: there are males and females.

The tiny stars then float around with the plankton crew for a while until they are ready to settle down—and grow more arms.

But no matter how cool they look, the coolest thing for me will always be that dive when we saw our first Sunflower Star—and the excitement on the boat afterward.

Spiny brittle stars

Brittle stars, along with basket stars, belong to one of those taxonomic groups I’ll never get the pronunciation right of: Ophiuroidea. They are ophiuroids.

By Ed Bierman from CA, usa – Spiny Brittle Star, Ophiothrix spiculata, CC BY 2.0,

These ophiuroids like to carpet. I assume they benefited when other stars vanished and the kelp cleared. Thousands upon thousands of them crawl over each other with their fuzzy-looking arms searching, crawling, sticking into the air.

Brittles aren’t picky, so if the conditions are even remotely right, they populate the area, no matter if it’s near the shore or at depths of up to 2,000 meters (more than 6,500 feet).

And because these brittles along with a few other members of the ophiuroids—you think it’ll get better if I keep saying that word? Ophiuroidea. Ophiuroidea. Ophiuroidea. Well, they’ve got two methods of reproduction, so they are pretty ducking good at, well, ducking. They can either broadcast spawn like their asteroid friends, or they can brood.

Brooding brittles develop young in their central cavity until they are ready to be released. This gives them a bit of an arm-up compared to floating around as plankton for a while. Pretty cool!

They don’t usually grow more arms as they age, by the way. The ophiuroid stars have a pretty high tendency to have near-perfect fivefold symmetry. Though, because why would anything ever be easy, there are six-armed species of brittles.

They, by the way, have an extra trick up their furry little sleeves: Ducking fission. They just split their central disk in half with three arms for each new “offspring” and live on as two individuals.

No wonder, brittles can reproduce quickly enough to form ducking carpets. The carpets look like furry wastelands, but if you look closely, they are fascinating to watch—though I’d much prefer the kelp forests to be dense enough that I have to search for the fuzzy arms sticking out of rocks.


When we talked about corals dying left right and center, we talked about the crown of thorn that is so devastating to reefs. When we talked about shrimp, I mentioned the sneaky little bastards that are the Harlequin shrimp, which exclusively feed on the crown of thorns. Today, we’re finally actually talking about the crown of thorns.

By Kris Mikael Krister – Imported from 500px (archived version) by the Archive Team. (detail page), CC BY 3.0,

Acanthaster planci, the crown-of-thorns star, can be found mostly in the Indo-Pacific region. They are corallivores. Yes, that’s apparently a word. It means they eat corals. Like other true sea stars, feeding includes everting the stomach. And due to their general size (60-70 cm in diameter) and an extra-large stomach, they can munch a lot.

They can be pretty devastating to coral reefs, especially during “outbreaks” or blooms. These things are ducking fecund, so they create a duckton or offspring. A female can release tens of millions of eggs. The larvae then float around as plankton for about two weeks before they settle and start feeding on algae. They join the coral-devastation at about six months old. And they start reproducing at just about two years, or even earlier. An outbreak can just gobble up a reef and leave nothing but the skeletons behind.

To make them more fun, they have sharp spines on the upper side—the spines on the bottom are dull. These spines can reach 5-6 cm, so a good two inches. And those lovely spines are toxic, too. Yep, all-round fun balls.

Probably not unrelated, few things actually feed on these things. In the Great Barrier Reef, researchers observed twelve species that feed on A. planci. We know one of them from our shrimp episode: harlequin shrimp.

I have one piece of good news, though: the same researchers also figured out that coral cover returns to affected reefs after about a decade. Now humanity just needs to stop ducking with the climate, so the corals aren’t fighting on all fronts.

Ochre sea star (kinda)

By NOAA’s National Ocean Service –, CC BY 2.0,

I wanted to end this one with a fun example of a tide pool sea star, as seeing those is something a lot more people have access to. A walk along a rocky shore is all that’s needed to spot them in many parts of the world. Unfortunately, this brought me full-circle to the ochre sea star, a very common tide pool inhabitant along the West coast of the US. And, hey, they are pretty, too. A variety of colors with what looks like strings of pearls across the surface. Stunning!

But they are also one of the keystone species affected pretty badly by the sea star wasting syndrome outbreak in 2013.

So, instead, I decided to end things here. I know, this was even more doom-and-gloom than the Climbing the Tree of Life series usually gets. It’s also why this episode took so long to write and produce. I wanted to keep things light and educational, but just couldn’t manage.

So, duck the gloom-and-doom rule. This crab is important. To not leave you in a crabby mood:

USCS has an ongoing update section on the sunflower star, and the first specimens have been reported in the intertidal zones of California. They say this hasn’t happened since before the SSWS outbreak of 2013. Yay! They are also getting more reports of sunflower stars in general. The first juveniles are surviving in the Southern ranges. So, while SSWS is persisting at most sites, most populations are leaning toward recovery. Let’s keep our fingers crossed for my favorite sea star.



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