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Biology

Sea-ing fewer stars: Virus linked to sea star mass die offs.

Paper

Hewson, I., Button, J. B., Gudenkauf, B. M., Miner, B., Newton, A. L., Gaydos, J. K., … Harvell, C. D. (2014). Densovirus associated with sea-star wasting disease and mass mortality. Proceedings of the National Academy of Sciences of the United States of America, 111(48), 2–7. doi:10.1073/pnas.1416625111

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Sea stars on the move. Credit: Royce Bair (The Stock Solution)

Background

Fig 1: SSWD-affected stars (A) asymptomatic P. helianthoides, (B) symptomatic P. helianthoides, and (C) symptomatic P. ochraceus. Disease symptoms are consistent with loss of turgor, loss of rays, formation of lesions, and animal decomposition. (D) Map showing occurrence of SSWD based on first reported observation. (E) Transmission electron micrograph of negatively stained (uranyl acetate) viruses extracted from an affected wild E. troschelii from Vancouver

Fig 1: SSWD-affected stars (A) asymptomatic P. helianthoides, (B) symptomatic P. helianthoides, and (C) symptomatic P. ochraceus. Disease symptoms are consistent with loss of turgor, loss of rays, formation of lesions, and animal decomposition. (D) Map showing occurrence of SSWD based on first reported observation. (E) Transmission electron micrograph of negatively stained (uranyl acetate) viruses extracted from an affected wild E. troschelii from Vancouver

Call them starfish or sea stars, these iconic ocean animals are in danger. Recent widespread declines have affected at least 20 sea star species in the U.S. Pacific Northwest. Since June 2013, millions of sea stars have succumbed to the die off. The problem exists from Mexico to Alaska and is known collectively as sea star wasting disease (SSWD). The sea stars suffer behavioral changes, lesions, limb curling, loss of rigidity, and often face a death called melting characterized by rapid degradation. The cause of the disease has remained a mystery.

Not only have wild sea stars have been affected, but also their captive counterparts in aquariums throughout the Pacific coast. Symptomatic sea stars appeared only when seawater was not treated with a UV light. This led scientists to believe a microscopic entity was to blame. The researchers in this paper began to look for the culprit.

Methods

What is causing the problem?

The scientists scoured hundreds of infected tissue slides but found no evidence of microbial infection leading to a virus hypothesis. To search for the virus causing so many problems, the investigators found unaffected stars in the wild and brought them to the lab. Tissue from diseased stars was ground up in seawater to create a liquid that was injected into the healthy stars. Disease and destruction ensued! This confirmed diseased individuals could pass along the sickness through small, virus-sized particles.

(A) Proportion of stars remaining asymptomatic after inoculation with control (heat-killed) or virus-sized fraction (VSF) of asteroid homogenates in first (Expt 1) and second (Expt 2) challenge. Survival analysis (20) indicates that the time to lesion development differs among treatment and control groups (log-rank test: χ2 = 18.6, df = 1, P =  0.05). Error bars = SE.

(A) Proportion of stars remaining asymptomatic after inoculation.  (B) Change in SSaDV load between initiation of viral challenge and termination of experiment.  Viral load was determined by quantitative PCR (qPCR).  Different letters above bars indicate that the mean viral abundance change is significantly different.

The next step was to identify the virus. Tissue samples were collected and genetic tests were run to identify all existing and relevant viruses. Only one virus fit all criteria- a densovirus similar to viruses found in insects and other animals related to sea stars such as urchins. The researchers named the virus sea star- associated densovirus.

What is happening in the wild?

Fig 2: Viral load in asymptomatic and symptomatic asteroids in San Diegan (i.e., south of Point Conception) and Oregonian (i.e., north of Point Conception) biogeographical provinces.

Fig 3: Viral load in asymptomatic and symptomatic asteroids in San Diegan (i.e., south of Point Conception) and Oregonian (i.e., north of Point Conception) biogeographical provinces.

After successfully identifying the virus in the lab, the scientists hit the coast and conducted field studies. The field studies confirmed a higher amount of the virus in diseased individuals.

The researchers now know what virus causes sea star deaths, but want to better understand how the virus is transmitted. Through sampling museum specimens, the investigators showed the virus has been present in sea stars for 75 years. The reason for the recent increase in virulence remains unexplained.

 

 

 

 

Significance

The large geographic range and the number of species facing sea star wasting disease could make this the largest localized, marine wildlife disease outbreak yet. Marine pathogens affecting non-commercial species may often be overlooked and are not well understood. This study helps to advance our understanding of one such pathogen as well as help to solve the mystery of mass sea star deaths.

Discussion

Trackbacks/Pingbacks

  1. […] by many scientists to be caused by a virus, as previously covered by Oceanbites writer Sarah Giltz here. However, further research has discovered that the virus was present in individuals that were not […]

  2. […] Exmples of sea stars affected by the virus. (source: oceanbites.org) […]

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