Scientists link massive starfish die-off to warming ocean

Desdemona Despair

By Lynda V. Mapes
21 February 2016

(Seattle Times) – It was the starfish arms walking off on their own that alerted biologist Steven Fradkin that something was terribly wrong at Starfish Point at Olympic National Park.

Next he noticed white lesions pitting the skin of the usually colorful orange, purple and brick-red starfish that are the signature of Olympic tide pools. Worse, the starfish, usually so thick and clinging robustly to their rock, were melting into goo.

“They were just falling apart,” said Fradkin, Olympic National Park coastal ecologist. “It was a horror show.”

The observations he made and shared June 7, 2013, would turn out to be the first reported sighting of a mysterious starfish wasting disease that in 2013 and 2014 would devastate more than 20 species of starfish from Alaska to Mexico.

In its geographic scope, the number of species of starfish affected, and duration of the outbreak — still not over — the sea star wasting syndrome Fradkin first documented is now understood to be the largest observed die-off of a wild animal in the ocean. Nearly three years later, the epidemic of sea star wasting disease has left many coves, tide pools, pilings and beaches still largely bereft of starfish. Some locations saw complete mortality of sea stars.

Scientists working ever since to understand the outbreak have published the first evidence of a link between warmer ocean temperatures and the devastation of the wasting disease. Unusually warm ocean temperatures coincided with the 2014 die-off analyzed in the paper published last week in the Philosophical Transactions of the Royal Society.

“We were able to show warmer temperatures were related with the higher risk of disease,” said Drew Harvell, professor of ecology and evolutionary biology at Cornell University and a co-author of the study, along with Fradkin and others. “We suspected there was a temperature link, but we really needed to look at the field data to pull that out, and we were able to back that up with lab experiments that found that in warmer temperatures, they died faster.”

The scientists also figured out why so few sick juvenile sea stars were being seen that summer: While younger sea stars took longer to show symptoms, once they did, they died right away.

Scientists focused on the most abundant species on the West Coast, Pisaster, the ochre star. The wasting syndrome was linked to a virus and resulted in rapid disease spread, high death rates, and wasting that continued in survivors in 2015. Prevalence of the disease was as high as 100 percent in some locations studied, with an overall mean of 61 percent prevalence at the 16 total study sites in the San Juan Islands, South Puget Sound and Washington’s outer coast.

Adult starfish were hit first — leading to a restructuring of communities with adult populations falling to 25 percent of pre-outbreak levels. The role of temperature was unmistakable. In lab experiments, the death rate of adult starfish was 18 percent higher in warmer water than infected stars in cooler tanks.

Timing of the outbreak varied in different parts of the coast, noted Morgan Eisenlord, lead author on the paper. At first there was no sign of the disease in the San Juans — but then it spread to the point that 100 percent of the starfish in some coves died. “You would see more and more animals be affected to the point that you could see just the tissue where the animals used to be,” Einsenlord said. [more]

Scientists now link massive starfish die-off, warming ocean

ABSTRACT: Over 20 species of asteroids were devastated by a sea star wasting disease (SSWD) epizootic, linked to a densovirus, from Mexico to Alaska in 2013 and 2014. For Pisaster ochraceus from the San Juan Islands, South Puget Sound and Washington outer coast, time-series monitoring showed rapid disease spread, high mortality rates in 2014, and continuing levels of wasting in the survivors in 2015. Peak prevalence of disease at 16 sites ranged to 100%, with an overall mean of 61%. Analysis of longitudinal data showed disease risk was correlated with both size and temperature and resulted in shifts in population size structure; adult populations fell to one quarter of pre-outbreak abundances. In laboratory experiments, time between development of disease signs and death was influenced by temperature in adults but not juveniles and adult mortality was 18% higher in the 19°C treatment compared to the lower temperature treatments. While larger ochre stars developed disease signs sooner than juveniles, diseased juveniles died more quickly than diseased adults. Unusual 2–3°C warm temperature anomalies were coincident with the summer 2014 mortalities. We suggest these warm waters could have increased the disease progression and mortality rates of SSWD in Washington State.

Ochre star mortality during the 2014 wasting disease epizootic: role of population size structure and temperature