Warming Oceans May Be Causing Sea Stars to 'Drown'

A mysterious wasting disease has been devastating sea star populations around the world for several years. Now scientists believe that it may be respiratory distress. Increased organic matter and bacteria due to warming oceans are using up the oxygen, causing the sea stars to “drown.”

In a new study published in Frontiers in Microbiology, researchers explain sea star wasting disease. Signs include changes in color, puffiness, arm twisting, and eventually death. Disease outbreaks have been noted for the past seven years to the extent that several species have been threatened with extinction.

“Sea stars breathe by passing oxygen through their outer tissues. This occurs primarily through two structures: small gill-like structures called papulae and through their tube feet,” study co-author Ian Hewson, professor of microbiology in the College of Agriculture and Life Sciences at Cornell University, tells Treehugger.

“Sea stars do not ventilate (i.e. they don't pump water across these structures) but rather rely on waving their tube feet and water movement over these papulae to breathe.”

When there isn’t enough oxygen surrounding their papulae and tube feet, the sea stars can’t breathe.

When the Oceans Warm

The oceans face substantial threats due to changing environmental conditions. As waters warm, bacteria are thriving, limiting the oxygen available to sea stars.

“​The total amount of oxygen in seawater relates to its temperature by physics, so the warmer the water, the less oxygen it can bear. The ocean is slowly being ‘deoxygenated’ as a consequence of climate change,” Hewson says.

“More immediately, however, more frequent storm events and large algal blooms deliver larger amounts of organic matter to coastal habitats; this organic matter is consumed by marine bacteria which subsequently draws down oxygen concentrations.”

When there isn’t enough oxygen in the surrounding water, the sea stars drown in their own environment.

“The animals have a certain respiratory demand – a minimum amount of oxygen they need to survive – which is normally met by the oxygen in water that surrounds them,” Hewson says. “When organic matter is unusually high in concentration (and bacterial respiration causes depleted oxygen), their respiratory demands are not met. This is a bit like drowning or suffocating.”

Jumping Between Sea Stars

Researchers have seen sea star wasting disease in more than 20 species of starfish, but in different concentrations, Hewson says. 

“​Based on some experiments and field observations, it looks like the disease can jump between individuals​. However, this is not because a germ or infectious agent moves between the diseased and healthy specimen,” Hewson says.

“Rather, when one starfish starts to die because it ‘drowns,’ the organic matter released from this individual (during decomposition) then enriches bacteria living near other starfish nearby, and they too subsequently ‘drown.’”

Researchers say the findings are significant for several reasons.

“We now have a much clearer picture of what's causing sea star wasting disease, which is the largest marine disease event ever seen. Second, these results suggest that ocean change and unusual conditions may be responsible for the disease, which may provide clues for remediation,” Hewson says.

“Our work reframes marine disease in context of environmental conditions; in other words, disease may generate from microorganisms that are not directly associated with the animals. Rather, microorganisms living in proximity may generate environmental conditions that may subsequently cause disease.”