Researchers might have discovered why sea stars have been dying worldwide for the past seven years, with some species pushed to the brink of extinction.
A team of scientists led by Cornell University members suggests that starfish, the main victim of a phenomenon known as sea star wasting disease (SSWD), might actually be suffering from a form of a respiratory disorder - the starfishes are basically "drowning" underwater. Additionally, researchers also hinted at increased microbial activity from surrounding organic matter, and increasing ocean temperatures make it harder for these species to respirate in water.
"As humans, we breathe, we ventilate, we bring air into our lungs, and we exhale," explains Ian Hewson, microbiology professor from Cornell, in a press release from the university. He explains that these sea stars diffuse oxygen through little structures on their skin - called papulae, or skin gills - and that the lack of oxygen around these breathing structures makes breathing difficult.
The results of their study, titled "Evidence That Microorganisms at the Animal-Water Interface Drive Sea Star Wasting Disease," appears in the journal Frontiers in Microbiology, January 6.
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Uncovering Sea Star Wasting Disease
In their report, researchers describe the sea star wasting disease as a condition that affects asteroids - the class of echinoderms that contains sea stars - and has resulted in significant population decline in the Northeastern Pacific region after a 2013 mass mortality event. However, the etiology or causes behind this phenomenon remains "unresolved."
Arguing that many signs of the sea star wasting disease are "subject to observer bias," researchers standardized reported occurrences of the disease through a combination of experiments and surveys regarding the appearance of body wall lesions among the sea stars - one of the main symptoms of the disease. In the study, asteroids were considered to be "dead" when their tube feet do not move for a 30-second observation period or when all limbs are found to have "automized" or separated from the central disc.
Researchers then conducted longitudinal studies to check whether the disease was either preceded or accompanied by shifts in bacteria and inhabiting tissues. They also tested if there was a connection between oxygen gas (O2) depletion and the tendencies of developing the wasting disease.
A Combination of Different Factors
Hewson explains that ocean conditions can encourage "unusual amounts of organic material," prompting bacteria's growth and survival. As these organisms consume organic material in the water, oxygen levels decrease - creating a "low-oxygen micro-environment" surrounding the asteroids and leading to various symptoms such as deflation, discoloration, puffiness, and curling of limbs.
"It's a cascade of problems that starts with changes in the environment," Hewson said. He added that most of these organic matters are from microscopic algae discharges, zooplanktons, and decaying carcasses from other animals. These materials stimulate bacteria called copiotrophs found in carbon-rich environments and thrive in locations with a lot of organic matter.
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"If you have a dead and rotting starfish next to starfish that are healthy, all of that dead one's organic matter drifts and fuels the bacteria, creating a hypoxic environment," Hewson added. This creates the impression that the wasting disease is being transmitted across sea star populations.
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