Thesis Watch – Sea Star Wasting with Mack Hughes

By Rachel Heimke

Mack Hughes defended his thesis on March 5th, titled “Direct and cascading effects of sea star wasting on rocky intertidal communities.”

In 2013 and 2014, sea star wasting caused a mass mortality event on the coasts from Alaska to Mexico. This phenomenon decimated many populations, affecting twenty species of sea stars.

Color palette of Evasterias troschelii. 

Photo by Mack Hughes

This condition rapidly takes hold and kills sea stars soon after onset. They get white lesions, their arms and body can deflate, and their body disintegrates. They can also eject their guts, dismember themselves, and dissolve into goo.

The origin of sea star wasting has yet to be discovered. We don’t know if it’s viral, bacterial, or something else entirely. We know that it has wiped out sea star populations around Alaska. Sea stars are important predators in rocky intertidal populations. Hughes compared them to wolves – if they are removed from the ecosystem, cascading effects could occur throughout the food web.

Hughes came to Alaska in May 2022 and began his thesis work that fall. He studied rocky intertidal populations in Katmai, Kachemak Bay, Kenai Fjords, and Western Prince William Sound. Some areas have since seen their sea star populations rebound, but they haven’t recovered in Kachemak Bay. 

“It’s pretty important because in these ecosystems, the rocky intertidal sea stars are top predators,” said Hughes. “The predation pressure that they exert is important in maintaining the ecosystem and making sure everything is working properly.”

Hughes found that community structure has changed because of the loss of sea stars. Mussels are more common, and there are many more barnacles in Kachemak Bay. However, Hughes didn’t find any evidence of trophic cascades or considerable changes in the structure of the food web.

In Alaska, “they’re not as important as we thought they would be,” said Hughes. He found that some species were absent or more abundant than before sea star wasting occurred.

So why didn’t removing sea stars mess up the food web?

One theory is that other predators eat the same things as sea stars, so their loss wasn’t as significant of a hit. Sea otters and shorebirds like the Black Oystercatcher are voracious predators. Even with the loss of sea stars, the ecosystem still felt the effects of top predators. 

Mack Hughes working in the intertidal in Kenai Fjords National Park.

Photo by Frankie Gerratie

Another theory is that the ecosystem isn’t controlled by top predators at all but instead by the base of the food web. Sometimes, food webs are governed by the primary producers and organisms at the food chain's base, so removing a top predator wouldn’t create massive changes.

There is still much to learn about rocky intertidal systems in Alaska, which the Gulf Watch Alaska program studies.

“I really want to stress how important these monitoring programs are,” said Hughes. In a time of environmental change, we must see how ecosystems respond to disturbance events like sea star wasting. Long-term monitoring programs give us sliding benchmarks, which tell us what “normal” ecological conditions are for the area at the time. 

Since defending his thesis, Hughes will be working on a summer project looking at freeze events during low tide and how they might result in mass mortality events.