Ochre Sea Star or Starfish Facts and a Major Wasting Disease

A Strange and Deadly Disease

The ochre sea star or ochre 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 overall situation has improved, but the animals are still dying at an abnormal rate.

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.

In an article that was recently updated, the University of California Santa Cruz says that the sea star wasting disease is still occurring at a low level. Readers interested in the latest news can explore the second reference at the end of this article.

Introduction to 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 the 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 one in its ecosystem, or a species 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.

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

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. It then travels through the canals of the water vascular system and into the tube feet. 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.

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 the sea star’s body. The animal also eats snails, limpets, chitons, barnacles, and smaller echinoderms and crustaceans. It swallows small prey whole.

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Other Features and Body Functions

Pedicellariae and Papillae

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.

Elimination and Excretion

Elimination is the removal of the portion of a meal that wasn’t digested and absorbed, which is often known as feces. In starfish, the undigested food is released through the anus and the mouth.

Excretion is the removal of nitrogenous waste (mostly in the form of ammonia) created by chemical reactions in the animal’s body. The ammonia diffuses through the papillae and the tube feet of a starfish and into the ocean. We release nitrogenous waste in our urine.

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 somewhat like 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.

Sea stars along much of the North American Pacific coast experienced a massive die-off in 2013/14 due to a mysterious wasting syndrome. The disease, called “sea star wasting syndrome” (SSWS) has persisted at low levels in most areas, and continues to kill sea stars.

— UC Santa Cruz (in mid 2021)

Sea Star Wasting Disease or Syndrome

The latest 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. During 2013 and 2014, sick sea stars were found all the way from Baja California to southern Alaska, although not in a continuous band. According to a report from the University of California in Santa Cruz that was updated in the middle of 2021, although the dramatic and widespread effects of the disease have abated, the problem is still present. In addition, the disease is still described as “mysterious.”

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. An affected 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 an 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. 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 outbreak was caused by the accident at the Fukushima nuclear facility in 2011. 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.

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