Ochre or Purple Sea Star Facts and a Deadly Wasting Disease

Updated on April 5, 2020
AliciaC profile image

Linda Crampton is a writer and teacher with an honors degree in biology. She loves to study nature and write about living things.

The ochre sea star on Salt Spring Island in British Columbia
The ochre sea star on Salt Spring Island in British Columbia | Source

A Strange and Deadly Disease

The ochre sea star or starfish is a colourful sight in the intertidal zone. Most individuals are orange or purple. The animal is found on the northeastern coast of the Pacific Ocean from Alaska to Baja California. For the past few years, the species has been subject to a strange disease that causes its body to change into a goo and white debris. Many animals have died. Recent discoveries suggest that the population may finally be recovering, but the situation is still worrying.

The scientific name of the ochre sea star is Pisaster ochraceus. The wasting disease may be caused by a virus, but this is far from certain. The disease has affected other species, but P. ochraceus has been especially hard hit. The condition is technically known as sea star wasting disease or as sea star wasting syndrome. In the popular press, it's sometimes referred to as a melting disease. The animal doesn't really melt, but the disintegration of its body into slime can give that impression.

Different colours of ochre sea stars on an Oregon beach
Different colours of ochre sea stars on an Oregon beach | Source

The Ochre Sea Star

Like other sea stars, Pisaster ochraceus belongs to the phylum Echinodermata and the class Asteroidea. Although the word starfish is still a popular term, scientists generally use the word sea star for the animals because they aren't fish or even vertebrates.

The ochre sea star has a variety of colours. In addition to purple and orange animals, reddish, yellow, and brown ones exist. A group of animals having different colours is an attractive sight.

The species is said to be a keystone species in its ecosystem, or one that has a major influence. In particular, it keeps the mussel population under control. Mussels are the main component of the sea star's diet. If they become too numerous, they can crowd out other organisms in the area.

A discovery on a beach in Vancouver's Stanley Park
A discovery on a beach in Vancouver's Stanley Park | Source

I use to find ochre sea stars quite easily at low tide in Stanley Park in Vancouver. On a recent visit, I found only the specimen shown above. Unfortunately, it was dead. (The crab in the corner of the photo was dead, too.)

Diet and Feeding

The sea star's mouth is located in the centre of its lower surface. The animal everts its stomach through its mouth and engulfs its prey. It's able to insert the stomach into even tiny openings between the two halves of a mussel shell. It creates an opening by pulling on each half of the shell with the numerous tube feet located under its arms. Digestion begins outside the mouth. The stomach then pulls the partially digested prey into its body. The sea star also eats snails, limpets, chitons, barnacles, and smaller echinoderms and crustaceans. It swallows small prey whole.

Anatomy of a sea star
Anatomy of a sea star | Source

The Water Vascular System and Locomotion

The sea star's body consists of a central area surrounded by five arms. The upper surface (the aboral surface) is covered with a mesh-like network of small spines, or ossicles, which are made of calcium carbonate. The central area on the aboral surface contains an opening called the madreporite, or the sieve plate. This is usually visible as a light spot a little to one side of the body's centre. Seawater enters the animal through the madreporite and travels through the canals of the water vascular system. This system enables a sea star to move.

Tube feet are small structures that are visible in the grooves on the underside (oral surface) of the arms. They are attached to the radial canal of the water vascular system. Each tube foot consists of a bulb called an ampulla and a elongated structure called a podium, as shown in the illustration above. The podium has a wide tip. The ampulla contracts and expands, causing water to move into and out of the podium.

The exact mechanism of sea star movement is still being studied. Both adhesion and suction involving the tube feet may be responsible for the movement. The video below shows the tube feet of an ochre sea star.

Other Features of the Animal

Surface Features

The upper surface of the sea star has small, pincer-like structures called pedicellariae. The pedicellariae grasp and crush harmful items that land on the animal. The animals's surface also has skin gills, or papillae. The skin gills function in respiration and excretion. They absorb oxygen and excrete harmful gases. The tube feet perform these activities as well.

Circulatory and Nervous Systems

Instead of a true circulatory system, sea stars have a haemal system consisting of fluid-filled channels. The system transports nutrients, oxygen, and carbon dioxide around the body cavity. A small sac in the haemal system contracts at intervals, acting as a heart.

The animals have a nerve net but no brain. They have sensory cells on their surface that detect chemical and mechanical stimuli. The tip of each arm contains an eyespot that can distinguish light areas from dark ones but is unable to form an image.

Reproductive System

The male and female sea stars contain two gonads (reproductive organs) in each of their arms. Eggs and sperm are released from the gonads into the ocean. Some of the eggs are fertilized by sperm. A fertilized egg develops into a bilaterally symmetrical larva, which is free swimming. The larva later changes into a form with a typical sea star shape.

An animal with wasting disease
An animal with wasting disease | Source

Sea stars along much of the North American Pacific coast are dying in great numbers from a mysterious wasting syndrome. Similar die-offs have occurred before in the 1970s, 80s, and the 90s, but never before at this magnitude and over such a wide geographic area.

— UC Santa Cruz

Sea Star Wasting Disease or Syndrome

The wasting disease was first noticed in ochre sea stars along the coast of Washington in June, 2013. Researchers say that previous outbreaks of a similar disease have occurred, but the latest one was far more serious and widespread than the other events. Sick sea stars were found all the way from Baja California to southern Alaska, although not in a continuous band.

The first noticeable symptom of the disease is the appearance of white areas, or lesions, on the body of the sea star. The lesions are areas where tissue is being destroyed. The animal soon becomes limp. The destruction often spreads along the arms of the animal, which drop off. The sea star gradually "melts". At least some of the affected animals die only a few days after the first symptoms appear.

The disease typically appears in some species of echinoderms in an area first and then in other ones later. Researchers don't know whether the infection passes from one species to another or whether some species are more resistant to the disease than others. They are currently studying the genetics of healthy and sick animals in the hope of understanding resistance and susceptibility better.

In the south, adult sea stars declined by 99 percent or more at over half of the 39 sites surveyed.

— UC Santa Cruz News Center (in reference to southern California)

Cause of the Disease

While some researchers talk of a viral cause for the wasting disease, others are not sure that this is the correct explanation or at least not sure that it's always correct. Vibrio bacteria have been found in some affected echinoderms, for example. In addition, the water in several affected areas was unusually warm water before the outbreak of the disease. An increased temperature may play a role in the condition.

One point emphasized by researchers is that there is no evidence that the latest outbreak was caused by the accident at the Fukushima nuclear facility. A rumour about this potential cause was popular in the early stages of the outbreak.

Evidence for a Viral Cause

The virus that has been linked to the disease is known as the sea star-associated densovirus. A study performed in 2014 discovered the following facts.

  • Material containing virus-sized particles extracted from affected tissue and inoculated into healthy sea stars consistently caused the recipients to develop a wasting disease
  • When the donated material was treated with heat, it didn't make the recipient animals sick.
  • Genetic tests showed that the densovirus was the "most likely candidate" for the particles of interest in the infected tissue.
  • As animals became sicker, the amount of densovirus in their body increased.
  • A survey of wild sea stars showed that the sick ones contained a higher level of the virus than the healthy ones.

There is a possible problem associated with the last two observations described above. As sea stars become sicker they may become more susceptible to infection by the virus, causing its level to increase. The actual cause of the disease may be another infectious agent instead of the densovirus, however. More recent research has found that some animals develop the wasting disease without a high level of densovirus in their body.

Researchers at Cornell University suspect that multiple factors can cause the wasting disease. They propose that its name be changed to Asteroid Idiopathic Wasting Syndrome. The word "idiopathic" means that the cause of the condition is unknown. A syndrome is a collection of symptoms that occur together and characterize a condition.

Another Possible Cause

The latest outbreak of the wasting disease has lasted for a long time. A recovery is underway in some areas, but not everywhere. Scientists find this situation puzzling. Some think that the viral infection was just a "side effect" in animals weakened by another cause. They've noticed that in at least some areas the die-off was correlated with events that added a large amount of organic material to the ocean. These events included storms that washed material from the land into the ocean and massive growths of algae known as blooms.

A microbiologist named Ian Hewson suspects that organic matter may have covered the sea stars, preventing them from getting enough oxygen and causing them to behave abnormally, as the quote below describes. The events that transported the organic material may become more common or more severe due to climate change. It will be interesting to see how Hewson's theory progresses as more evidence is obtained.

Sea stars have a natural defense mechanism known as “programmed cell death,” which they can use to shed one of their arms, called rays, if it’s injured or developed an infection. But Hewson thinks sea stars are sending that signal to the entire body as they suffocate.

— KTOO Public Media

A Partial Recovery

In British Columbia, where I live, the sea star recovery has been described as a "mixed bag" by some scientists. In mid 2019, they said that they had discovered "unusually high numbers of juvenile ochre sea stars", which was a hopeful sign. On the other hand, only one population of sunflower sea stars was recovering. Other observers on the west coast of North America have also reported partial recoveries. Unfortunately, in some places the decreased numbers of sea stars has allowed the populations of other animals to increase, which has created problems.

A puzzling aspect of the situation is that certain species of sea stars weren't affected by the event at all or experienced only a slight reduction in population. It's good that the situation is getting better in some areas, but it would be helpful if scientists understood the cause or causes of the disease. The wasting disease has appeared before and could flare up again. It has the power to affect the ecosystem as well as the ochre sea star.


Questions & Answers

    © 2018 Linda Crampton


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      • AliciaC profile imageAUTHOR

        Linda Crampton 

        11 months ago from British Columbia, Canada

        Thank you for the comment, Anita. It's a shame that the animals have experienced the problem, though it's interesting biologically.

      • profile image

        Anita Hasch 

        11 months ago

        Thank you for sharing. So interesting. Sounds as if the sickness is similar to the HiV virus. Also a wasting disease.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        22 months ago from British Columbia, Canada

        I agree, Devika. The situation does need our attention.

      • profile image


        22 months ago

        The species is greatly affected and needs attention!

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        23 months ago from British Columbia, Canada

        Thanks, Larry. It's a great shame that the disease exists. I hope its cause is discovered soon.

      • Larry Rankin profile image

        Larry Rankin 

        23 months ago from Oklahoma

        Beautiful animal. It's a shame there's a disease like this.

        Great read and always educational.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        23 months ago from British Columbia, Canada

        Hi, Cynthia. Thank you for the comment. I hope the situation improves for all the species that have been affected. Sea stars are lovely creatures, as you say.

      • techygran profile image

        Cynthia Zirkwitz 

        23 months ago from Vancouver Island, Canada

        HI Linda, a thorough treatment of a sad, frightening predicament with these lovely creatures, the sea stars. I was interested to hear about how the Salish Sea has such a wide variety of sea stars. I learn quite a lot from your multi-modal presentation. Thank you!

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        23 months ago from British Columbia, Canada

        Hi, Heidi. It is a sad situation, though things are looking more hopeful at the moment for some species. Thanks for the visit.

      • heidithorne profile image

        Heidi Thorne 

        23 months ago from Chicago Area

        So sad to hear about the plight of these beautiful creatures! I hope they figure out a way to make their environment better.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        23 months ago from British Columbia, Canada

        Thank you for the comment, Dora. (Your mind is definitely not "little"!) It's amazing that there is so much to learn about the living world. Even the simplest life forms are impressive.

      • MsDora profile image

        Dora Weithers 

        23 months ago from The Caribbean

        Linda, your articles certainly motivate expansion of my little mind to embrace these strange facts like five arms and "everts its stomach through its mouth and engulfs its prey." Humbling to humans also that there they are some things about smaller lesser creatures that remain mysterious. Thanks for sharing your interesting research.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        23 months ago from British Columbia, Canada

        Hi, Peggy. I find it sad that scientists are uncertain about the cause, too. I hope they are soon able to clarify the situation.

      • Peggy W profile image

        Peggy Woods 

        23 months ago from Houston, Texas

        It is sad that this is happening to so many of these beautiful creatures and that we do not yet definitively know the reasons why this wasting disease is killing so many sea stars. Hopefully they will survive in numbers enough to keep the ecosystem intact and functioning as it should.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        23 months ago from British Columbia, Canada

        Thank you for the visit, Bill. I hope scientists can solve the mystery, too.

      • bdegiulio profile image

        Bill De Giulio 

        23 months ago from Massachusetts

        Hi Linda. How interesting and sad. Such a beautiful and colorful creature. I was not aware that they were being affected by this terrible disease. Hopefully scientists can solve the mystery to prevent these outbreaks.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        23 months ago from British Columbia, Canada

        Thank you for the interesting comment, Bede. Calcium carbonate is certainly a useful substance!

      • Bede le Venerable profile image


        23 months ago from Minnesota

        Linda, I was unaware of such colorful sea stars. Their plight is sad but it is good to know that scientists are working to find a solution. I’m probably the only reader intrigued that their body has a network of calcium carbonate. It seems to be a very common material found in nature- in our bones, in rocks (limestone and marble), in shellfish and coral; I work with it frequently in art materials (gesso); it’s in antacids, cleansers, and the Sistine chapel ceiling!

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        23 months ago from British Columbia, Canada

        Thanks for the comment, Manatita. Warm water may play a role in the problem, but it's probably not the only factor at work. As in past outbreaks, the animals do seem to be recovering, although this outbreak was more severe and the recovery is taking longer. In addition, some sea star species weren't affected by the disease. It's an interesting and confusing situation.

      • manatita44 profile image


        23 months ago from london

        A fascinating subject and a truly interesting colour for a starfish. Sorry to hear that they are dying off. Must be the higher temperature and yes, perhaps it is wise to think of man-made problems. Your video man reminds me of Sir Richard Attenborough. Educational Hub as usual.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        23 months ago from British Columbia, Canada

        Hi, Flourish. Increased water temperature due to climate change has been proposed as a contributor to the problem. As far as I know, though, there is no proof for this at the moment. The disease is puzzling.

      • FlourishAnyway profile image


        23 months ago from USA

        I wonder if global warming is a culprit, making the waters uncharacteristically warm and allowing the suspected pathogens to thrive. I wonder why the Pacific but not Atlantic?

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        23 months ago from British Columbia, Canada

        Hi, Bill. It's sad when such a common animal disappears. It seems like a recovery is underway for at least some species, but I think the disease outbreak should be taken seriously. It would be horrible if it happens again and no recovery occurs.

      • billybuc profile image

        Bill Holland 

        23 months ago from Olympia, WA

        Fascinating read, Linda! This is one of those things we take for granted, but don't really notice when it is gone, if that makes any sense. I have seen star fish since I was a toddler, in plentiful numbers. It just never dawned on me that the day would come when they disappeared.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        23 months ago from British Columbia, Canada

        I grew up calling them starfish too, Patricia. It's hard to drop the name. I hope they recover well and that the disease becomes a thing of the past. Thank you so much for the angels.

      • pstraubie48 profile image

        Patricia Scott 

        23 months ago from North Central Florida

        O I hope whatever the cause is of the disease that they can find a way to stem the tide of its spread so that our sea stars (I grew up calling them starfish) do not become a thing of the past. Very detailed article ...so much I did not know. Thanks for sharing. Angels are on the way to you this morning. ps

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        23 months ago from British Columbia, Canada

        I agree, Mary. It is a joy to find them alive. I hope all of the populations involved in the latest outbreak recover and that no new outbreaks occur.

      • aesta1 profile image

        Mary Norton 

        23 months ago from Ontario, Canada

        This is worrying and I hope ey find out soon the cause of this wasting disease. The Starfish population are so colourful, it is a joy to find them alive not dead.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        23 months ago from British Columbia, Canada

        Hi, Jackie. The ways in which we treat the ocean are very sad. I hope researchers soon discover the cause or the causes of the disease. If they are able to do this, I hope they can help the sea stars in some way.

      • Jackie Lynnley profile image

        Jackie Lynnley 

        23 months ago from the beautiful south

        Pollution would be my best guess.I expect it will only increase with many more fish. It is as though you cannot trust fish to eat from any of our waters anymore so this really is not surprising, ut such a shame. I hope they can discover the cause quickly enough to not have them wiped out completely. They are very beautiful.


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