Sea star wasting disease

The leg of this Pisaster ochraceus sea star in Oregon is disintegrating as a result of sea star wasting syndrome

Sea star wasting disease or starfish wasting syndrome is a disease of starfish and several other echinoderms that appears sporadically, causing mass mortality of affected starfish.^[1] There are around 40 different species of sea stars that have been affected by this disease. The disease seems to be associated with raised water temperatures. It starts with the emergence of lesions, followed by body fragmentation and death.^[2] In 2014 it was shown that the disease is associated with a densovirus now known as the sea star-associated densovirus (SSaDV).^[3]

Symptoms

Typically the first symptom of starfish wasting disease is white lesions that appear on the surface of the starfish and spread rapidly, followed by decay of tissue surrounding the lesions. Next the animal becomes limp as the water vascular system fails and it is no longer able to maintain its internal hydrostatic balance. The body structure begins to break down, signs of stretching appear between the arms which may twist and fall off, and the animal dies. The arms may continue to crawl around for a while after being shed. Progression of these events can be rapid, leading to death within a few days.^[4][5]

A deflated appearance can precede other morphological signs of the disease. All of these symptoms are also associated with ordinary attributes of unhealthy stars and can arise when an individual is stranded too high in the intertidal zone (for example) and simply desiccates. "True" wasting disease will be present in individuals that are found in suitable habitat, often in the midst of other individuals that might also be affected.^[5]

The final result is a white, mushy blob, which no longer seems to be a sea star.^[6]

1972 plague

The 1972 plague was the first notable case of Sea Star Wasting Disease.^[7] Scientists noticed a rapidly declining population of common starfish (Asterias rubens) occurring off the east coast of the United States. The symptoms were that the starfish became limp and lost limbs until finally melting into a white mucus like paste.^[7][8]

1978 plague

In 1978 large numbers of the predatory starfish Heliaster kubiniji succumbed to a wasting disease in the Gulf of California. At the time it was suspected that high water temperatures were a causal factor. This starfish became locally extinct in some parts of the gulf and some populations had still not recovered by the year 2000. Because this starfish is a top-level predator, its disappearance had profound effects on the ecosystem.^[9] In the Channel Islands off the coast of California, ten species of starfish were recorded as being affected as well as three species of sea urchins, two brittle stars and a sea cucumber, all of which experienced large population declines.^[10]

2013 plagues

In July 2013, populations of starfish declined rapidly on the east coast of the United States between New Jersey and Maine. There had been a great increase in starfish numbers three years earlier, though in 2013 they were dying off. No cause for the mysterious deaths was apparent.^[11] The meltdown of the sea stars were first found in ochre stars and sunflower stars in Howe Sound (BC). In late August, the disease had also been found stretching from Alaska to the border of Mexico.^[12]

At the beginning of September 2013, a mass die-off of starfish was reported off the coast of British Columbia. The sea bed was littered with disintegrating sunflower stars (Pycnopodia helianthoides), their detached arms and discs. Another species also suffering mortalities was the morning sun star (Solaster dawsoni) but no cause for the deaths was apparent. If they were caused by infection or toxins, the two species might have affected each other because the diet of each includes starfish.^[13]

In October 2013, in a marine laboratory seawater tank in California holding various species of starfish, other species started displaying similar symptoms. The ochre star (Pisaster ochraceus) was the first affected. Most of these developed symptoms, lost arms and died over the course of a week or so. Later the rainbow star (Orthasterias koehleri) developed the disease and died, but the bat star (Patiria miniata) and leather star (Dermasterias imbricata), which were living in the same tank and had been scavenging on the corpses, showed no ill effects.^[4] At Natural Bridges State Marine Reserve in California, the ochre star is normally a very common resident on the mussel beds, but in November 2013 it was reported to have completely disappeared.^[4]

Locations

Currently, most cases are located on the west coast of North America, affecting sea stars from Baja California to the gulf of Alaska.^[14] Map of Sea Star Wasting Syndrome Locations

Causes

As of November 2013, no identifiable cause for the disease had been found. Pathogenic bacteria did not seem to be present, and though the plague might be caused by a viral or fungal pathogen, no causal agent had been found. Each episode of plague might have a different cause.^[8]

Other possible causes of the condition that have been suggested include high sea temperatures, oxygen depletion and low salinity due to freshwater runoff. Research suggests that high water temperatures are indeed linked to the disease, increasing its incidence and virulence. The disease also seems more prevalent in sheltered waters than in open seas with much wave movement. One result of global warming is higher sea temperatures. There is a wave of unusually warm water along the west coast of the United States, which is where all of the sea stars are dying off.^[15] These may impact both on starfish and on echinoderm populations in general, and a ciliate protozoan parasite (Orchitophrya stellarum) of starfish, which eats sperm and effectively emasculates male starfish, thrives at higher temperatures.^[16]

Research in 2014 showed that the cause of the disease is transmissible from one starfish to another and that the disease-causing agent is a microorganism in the virus-size range. The most likely candidate causal agent was found to be the sea star-associated densovirus (SSaDV), which was found to be in greater abundance in diseased starfish than in healthy ones.^[3][17]

Treatment

In 2014, Point Defiance Zoo and Aquarium lost more than half of its 369 sea stars, and by September 2015 they numbered fewer than 100. The aquarium treated its affected sea stars with antibiotics in 2014, which proved effective.^[18] Although a mechanism is still unknown, evidence suggests that a single mutation in the elongation factor 1-alpha locus in Pisaster ochraceus may be associated with reduced mortality.^[19]

Species affected

Most affected (high mortality rates):

• Solaster dawsoni (morning sun star),
• Pisaster brevispinus (giant pink star),
• Pisaster ochraceus (ochre/purple star),
• Pycnopodia helianthoides (sunflower star),
• Evasterias troschelii (mottled star),

Affected (some mortality):

• Dermasterias imbricata (leather star),
• Solaster stimpsoni (striped sun star),
• Orthasterias koehleri (rainbow star),
• Henricia spp. (blood star),
• Leptasterias spp (six-armed star),
• Patiria (Asterina) miniata (bat star), and
• Pisaster giganteus (giant star)

^[20]

See also

• Effects of global warming on oceans

References

^[21]