Sea Stars

Sea Stars
Luidia clathrata, the gray sea star. Photo by Joel Wooster,
While wandering a deserted beach at dawn, stagnant in my work, I saw a man in the distance bending and throwing as he walked the endless stretch toward me. As he came near, I could see that he was throwing starfish, abandoned on the sand by the tide, back into the sea. When he was close enough I asked him why he was working so hard at this strange task. He said that the sun would dry the starfish and they would die. I said to him that I thought he was foolish; there were thousands of starfish on miles and miles of beach. One man alone could never make a difference. He smiled as he picked up the next starfish and hurled it into the sea, "It makes a difference to that one."

~ adapted from The Star Thrower, by Loren Eiseley

Some marine scientists have undertaken the ambitious task of replacing the popular starfish's common name with sea star because, well, the starfish is not a fish. It's an echinoderm (a spiny-skinned invertebrate), closely related to sea urchins and sand dollars. There are some 1,500-2,000 species of sea stars living in all the world's oceans, from shallow, tropical waters to the deep, cold seafloor. The five-arm varieties are the most common, hence their name, but species exist with ten, twenty, and even forty arms. Sea stars belong to the subphylum Asterozoa and the class Asteroidea. Yep, that means they're asteroids! Brittle and basket stars also belong to Asterozoa, but they branch off into another class, Ophuroidea. Both classes have a central disc and five or more radiating arms. However, compared to sea stars, brittle stars have a much smaller central disc and no anus, meaning the entrance is also the exit (which would no doubt lead to a complex, psychological relationship with food if brittle stars had a brain). Part of the reason for this digestive system is possibly a simple lack of space. Starfish utilize their entire body to house their viscera. Brittle and basket stars cram it all into that tiny central disc. The other main difference between asteroids and ophuroids is locomotion. Both have a water vascular system (meaning seawater takes on the duties of blood), and sea stars use hydraulically powered tube feet on the bottom side of their arms to move around. The tube feet in brittle stars are a little different in that they don't end in suckers, and they aren't used for getting around at all. Brittle and basket stars move by a sinuous flexing of their arms; this snaking movement gave them their class name, Ophuroidea, "serpent star."

But enough about serpent stars this is a story about sea stars! As we already know, sea stars have no brains, no blood, and no backbone. They do have hard plates under their skin and special cells on their skin to gather information about their surroundings. These cells then report to the nervous system, which is spread throughout the body. Sea stars have no eyes, not like we have, but they do have eyespots, groups of cells that can detect light, on the tips of their arms. These eyespots have only two settings: light (safe) & dark (mortal peril).

The mouth is located on the underside of the sea star's central disc; the anus is located directly opposite on top. Interestingly, sea stars have two stomachs. The cardiac stomach is the one they thrust out of their mouth to begin predigestion, allowing them to consume a larger organism than what they could fit into their mouth. Once the cardiac stomach and half-digested meal are retracted back into the body, the food transfers to the pyloric stomach to fully digest. Of course, this method only works on prey that is willing to sit quietly with a sea star's stomach draped over it. Although they're fairly opportunistic carnivores, sea stars tend to feed mainly on such foodstuffs as microalgae, sponges, snails, sea anemones, other sea stars, coral, shellfish, and detritus. They find their food by smell. Specialized skin cells can detect chemicals in the water, and the tube feet also aid in sensory detection. Once a sea star smells food, it can move towards its next meal. Normally, these critters move very slowly, a typical speed being just six inches a minute. When chasing prey (or perhaps being chased), it can crawl up to two and a half feet a minute. Some burrowing species have points instead of suckers on their tube feet and are capable of much more rapid motion, shooting across the ocean floor at speeds up to nine feet a minute. Prey clearly doesn't stand a chance against this blinding speed.

Sea stars, in turn, are food for triton snails, crabs, fish, gulls, sea otters, and other sea stars. Their first line of defense is their echinoderm status: bony plates and SPINES. If sharp points don't deter the predator, next up is chemical armory. Many species have saponins in their body walls, which taste bitterly unpleasant. Some sea stars, such as the crown of thorns star, are laced with powerful toxins, and the slime star can ooze out large quantities of mucus. Unfortunately, none of these defenses protect against the Vibrio bacteria, known to cause a wasting disease in several species, or Orchitophrya stellarum, a protozoan that infects the gonads of sea stars and damages tissue. They're also quite ill-equipped for high temperatures, though at least one species can absorb seawater to keep cool when it is trapped in the sunlight by a receding tide. A new disease has also appeared on the western coast, affecting millions of sea stars, both wild and captive. First observed in June 2013, this disease causes the star's legs to fall off (more quickly than they can regenerate) and its internal organs to fall into the water. It is tentatively being called "Starfish Wasting Syndrome." The cause is, as yet, undetermined.

In most sea star species, individuals are gonochorous (either male or female). Individuals are not sexually dimorphic as their gonads are internal, but sex is apparent when they spawn. In the tropics, a plentiful supply of phytoplankton is continuously available for sea star larvae to feed on, so spawning can take place year round, each species having its own personalized timing. In temperate regions, spring and summer bring an increase in food supplies and are therefore the best time to spawn. In some species, an individual may trigger a mass spawn by releasing a pheromone that entices other sea stars to aggregate and release their eggs or sperm synchronously. Other species prefer a more one-on-one approach: the male climbs on top and places his arms between the female's. When she releases eggs into the water, he is induced to spawn pseudocopulation. In a few species, fertilization is internal and female's brood the eggs. Brooding may be accomplished in pockets on the sea star's aboral (top) surface, inside the pyloric stomach, or even in the interior of the ovaries themselves. Those species that brood their eggs by "sitting" on them generally assume a humped posture with the central disc raised off the seafloor. In brooding species, eggs are comparatively large, are supplied with yolk, and usually develop directly into miniature sea stars, bypassing the larval stage. In the Tasmanian live-bearing sea star, the young sea stars obtain nutrients by eating other eggs and embryos in the brood pouch. Natural selection starts early in this species Brooding is especially common in polar and deep-sea species, where environments are unfavorable for larval development, and in smaller species that produce fewer eggs.

A few species are simultaneous hermaphrodites, producing both egg and sperm, er, simultaneously. A few other species are sequential hermaphrodites, beginning life as one sex and changing into the other. Starlet cushion stars start life as males and change into females as they grow older. Large females of the scaly sea star (Nepanthia belcheri) can split in half, each half growing into a whole sea star, and also turning back into a male. When these grow large enough, they can change back to females. In this way, asexual reproduction is accomplished. Regeneration is one of the sea star's claims to fame. Most species can regrow a severed limb, provided the central disc is in tact, but a few species, such as the multicolored sea star (Linckia multifora) and the Luzon sea star, can regrow the entire star from just one arm. Regrowth can take several months or years. A separated limb survives on stored nutrients until it regrows a disc and can feed again. Other than dividing for reproductive purposes, body parts may be detached by a predator, or actively shed to escape the predator.

When conditions are appropriate, the larvae of several species of sea stars may take the path of asexual reproduction rather than normal development. Though this costs time and energy and delays maturity, it allows a single larva to give rise to multiple adults. Most sea stars hatch at the blastula stage and are not much larger than pinheads. The blastula is a hollow sphere of cells surrounding an inner fluid-filled chamber. After developing a mouth, an anus, a gut, bands of cilia on the exterior, and some arm-like outgrowths, the larva is classified as a bipinnaria, a free-swimming, zooplanktonic form. When enough arms develop, it becomes a brachiolaria. Both bipinnaria and brachiolaria larvae are bilaterally symmetrical. The brachiolaria eventually settles on the seabed, attached by a short stalk, and metamorphosis begins: the left side becomes the underside, and the right side becomes the top. Radial symmetry is achieved. The stalk is cast off and a juvenile sea star makes its way in the wide ocean. Sea star lifespans vary considerably between species with the larger species usually living longer (up to 35 years!).

Sea stars are keystone species, species that have a disproportionately large effect on their environment relative to their abundance. The role is analogous to the role of a keystone in an arch. Though the keystone is under the least amount of pressure of any of the stones in an arch, the arch still collapses without it. Similarly, an ecosystem may experience a dramatic shift if a keystone species is removed, even though that species may have been a small part of the ecosystem by measures of biomass or productivity. Some sea star species are also considered bioindicators, species that can be used to monitor the health of an environment or ecosystem by monitoring the organisms for biochemical, physiological, or behavioral changes that may indicate a problem in their ecosystem. Bioindicators can show the cumulative effects of some pollutants in an ecosystem where physical and chemical testing might not.

However, not all sea star species are so saintly. The Northern Pacific sea star is on the Invasive Species Specialist Group's list of the world's 100 worst invasive species, and the crown of thorns sea star is making its way to the podium. The crown of thorns is covered entirely on its upper body with long sharp spikes, lending to its biblical name. The spines are so sharp that they require absolutely no force whatsoever to pierce skin (or wetsuits, for that matter). I think you bleed just from looking at them. Plus, they are poisonous, filled with a neurotoxin that causes pain, nausea, and swelling. Needless to say, they're rather solitary creatures, with few enemies and fewer friends. They specialize in eating coral; each individual can ingest up to sixty-five square feet of coral beauty every year. Ordinarily, they promote coral diversity by stopping any one species of coral from completely dominating. However, massive population increases since the 1960s have had a devastating effect on Pacific coral reefs, leaving coral ghost towns that can take decades to recover. The cause of these population explosions isn't fully known. Could be the nutrient-filled runoffs; could be overfishing of the triton snail, one of the crown's main predators. People used to just slice and dice the buggers, but that, of course, just increased the numbers. Nowadays, people inject them with carefully selected poisons that break down in seawater. Luckily, the vast majority of sea stars have, so far, zero inclination to be so troublesome. In fact, some are downright nifty. The sunflower sea star, though starting life with just the basic five-limb package, can grow up to twenty-four arms and can reach three feet in diameter. The leather sea star is one of the few smooth-skinned stars, owing to the fact that its spines don't reach above the skin. It can feel leathery, though it's usually slippery, and smelly (just a hint of garlic). None of the aforementioned are locals, though.

The Texas crew consists mostly of the gray sea star (Luidia clathrata), the banded sea star (Luidia alternata), and the two-spined sea star (Astropecten duplicatus). The gray and banded sea stars are common along the beach and are occasionally caught in seines or while crabbing. The gray star is completely cream or flesh colored. The banded star has dark brown bands on a cream-colored background. Both can reach about ten inches in diameter, have fairly slender arms, and lack suckers on their tube feet (meaning they have to swallow food whole and regurgitate the undigestible parts). The two-spined sea star lives a bit deeper than the gray or banded stars. It's a smaller species, often caught in shrimp trawls, and is easily distinguished from the gray and banded stars by rows of large plates along the sides of its arms. The two-spined star also lacks suckers and must swallow its prey whole. I've heard the beach of choice for sea star spotting is San Jose ("St. Joe"), a privately owned, undeveloped isle, accessible only by boat (you can get a ferry across to it in Port Aransas). However, South Padre Island was the place to see them in January. Rough seas, high winds, and strong currents forced thousands of gray sea stars, close to shore for feeding, onto the beach. There were many opportunities for star throwers. Mass mortalities such as this aren't common (only three occurrences with this species in the Northern Gulf); usually you should let the living sea stars you find stay in their tide pools, leaving them for the next people to enjoy.

I found a tiny starfish
In a tide pool by the sand.
I found a tiny starfish
And I put him in my hand.

An itty-bitty starfish
No bigger than my thumb,
A wet and golden starfish
Belonging to no one.

I thought that I would take him
From the tide pool by the sea,
And bring him home to give you
A loving gift from me.

But as I held my starfish,
His skin began to dry.
Without his special seaside home,
My gift for you would die.

I found a tiny starfish
In a tide pool by the sea.
I hope whoever finds him next
Will leave him there, like me!

And the gift I've saved for you?
The best that I can give:
I found a tiny starfish,
And for you, I let him live.

~Dayle Ann Dodds

Where I learned about sea stars, and you can, too!

Texas Parks & Wildlife Magazine

National Geographic

Animal Planet

Beachcomber's Guide to Gulf Coast Marine Life: Texas, Louisiana, Mississippi, Alabama, and Florida

By Susan B. Rothschild

Real Monstrosities


"Thousands of Starfish Wash Up on South Padre Island"

By George Cox, Coastal Current

"Mysterious Diseas has been Devastating Starfish"

By Lisa Winter