The Ochre or Purple Sea Star and a Deadly Wasting Disease
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.
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.
I use to find ochre sea stars quite easily at low tide in Stanley Park. 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.
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
The surface of the sea star has small, pincer-like structures called pedicellariae as well as skin gills, or papillae. The pedicellariae grasp and crush harmful items that land on the animal's surface. The skin gills function in respiration and excretion. They absorb oxygen and excrete harmful gases. The tube feet also perform these activities.
Circulatory and Nervous Systems
Instead of a true circulatory system, sea stars have a haemal system. The system transports nutrients, oxygen, and carbon dioxide around the body. 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.
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 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 that exists today was first noticed in ochre sea stars in June, 2013 along the coast of Washington. Researchers say that previous outbreaks of a similar disease have occurred, but the latest one is far more serious and widespread than the other events. Sick sea stars have been 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.
While ochre sea star populations shrank in all coastal areas, sites in southern California and the Channel Islands experienced more severe declines than those farther north. 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
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 currently 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.
A Puzzling Event
The latest outbreak of the wasting disease has lasted for a long time. Though it seems to some scientists that a recovery is underway in ochre sea stars and other species, others are cautious about making this claim at the moment.
The population of sunflower sea stars was decimated during the wasting event and is still very low in some areas, which is worrying. On the other hand, some species of sea stars weren't affected by the event at all or experienced only a slight reduction in population. The situation is puzzling at the moment.
It's good that the situation seems to getting better in certain areas, but it would be helpful if researchers understood the cause or causes of the disease. The wasting disease has appeared before and could appear again. It has the power to affect the ecosystem as well as the ochre sea star.
Pisaster ochraceus facts from the Encyclopedia of Life
Information about the ochre sea star from the Slater Museum of Natural History, University of Puget Sound
Sea Star Wasting Syndrome from the University of California, Santa Cruz
Densovirus associated with sea star wasting disease from PNAS (Proceedings of the National Academy of Sciences of the United States of America)
Disease epidemic defies description from UC Santa Cruz
Complex factors of starfish diseases from Cornell University
© 2018 Linda Crampton