Are Starfish Keystone Species? (Explained)

Keystone species define an entire ecosystem and without them, the ecosystem would be completely different. Many other species wouldn’t be able to survive creating an unbalanced environment. Have you ever wondered if starfish are keystone species? In this blog post, we’ll talk all about that but let’s begin with a quick answer:

Starfish are keynote species because their feeding habits affect the entire ecosystem. Without starfish, the mussel population would expand rapidly, driving out other species.

However, this certainly doesn’t tell the whole story. Below I’ll explain more about why starfish are keystone species and about the experiment that was conducted on them. Furthermore, I’ll explain what starfish’s biggest threats are and if they’re endangered species.

Why starfish are keystone species?

Starfish are keystone species because their feeding habits control other organisms’ populations. Starfish mostly prey on clams, mussels, oysters, and other small animals. Depending on the environment, some animals such as mussels, have no other predators.

Removing starfish from this environment would increase the number of mussels and drive out other species. The food web would be completely unbalanced with a lack of other organisms. Interestingly, starfish were the first animals to be identified as keystone species, thanks to Paine’s experiment.

Starfish keystone species experiment

In 1966, Robert Treat Paine, a groundbreaking ecologist, wanted to prove how different organisms co-exist in their environment and what controls their population. He claimed that predators play an essential role in maintaining the number of other organisms in the food web.

To test that idea, he conducted an experiment in Makah Bay, Washington, USA. He chose the rocky intertidal pool because the subwebs and their predators appeared to be particularly distinct and easy to observe in their natural environment.

The community there was dominated by mussels, barnacles, snails, and their main predator: Purple Sea Star (Pisaster ochraceus). Dr. Paine removed purple starfish from the bay and observed the changes.

The results

At the beginning of the experiment, the studied coastline had sixteen species, and one year later, the number went down to eight. Interestingly, five years later, only one species remained: California mussels (Mytilus californianus). 

California mussels didn’t have other predators except the starfish because of their very thick shells. No animals could open the strong shells, which protected them from being eaten. Thanks to the starfish’s unique way of eating, the purple sea star would extend its stomach out of its body and squirt digestive enzymes through a tiny hole in the shell. Next, the starfish would digest the mussels until they couldn’t hold their shells closed anymore.

Since the starfish are the only animals in Makah bay that can eat mussels, they are truly essential to the ecosystem of this area. The absence of this sea star species allowed mussels to take over, driving out other species. Paine’s experiment led to the keystone species concept, with the Pisaster ochraceus sea star as the first defined keystone species. 

purple sea star on the rock
Pisaster ochraceus sea star

His conclusions had a significant effect on conservation and defining other keystone species. For instance, we understood that sharks are essential in controlling many species’ distribution, abundance, and diversity in ocean ecosystems.

Starfish threats

Knowing that starfish are keystone species, do you wonder if they have any threats that could possibly cause their extinction? The biggest threats to starfish are the sea star wasting disease and climate change.

The sea star wasting disease

The sea star wasting syndrome was a disease that caused a massive die-off in 2013 and 2014 along the west coast of North America. Symptoms of this disease include the appearance of white lesions followed by tissue decay, body fragmentation, and death, usually within only a few days.

The condition is transmissible, and the disease-cause agent seems to be a virus or microorganisms at the animal-water interface. So far, twenty different starfish species have suffered from this syndrome, and the most affected areas are the West Coast of America, from Mexico to Alaska.

Interestingly, some scientists think that the other cause of the disease may be rising water temperatures.

Climate change

Climate change and what comes with it, rising water temperatures affect all organisms on our planet. One of the reasons this affects starfish is the lower oxygen level in warmer water, and lower oxygen level makes it harder for starfish to breathe.

Starfish breathe through their skin gills and tube feet, absorbing the oxygen directly from the seawater and exchanging gases through diffusion. Thus, with oxygen levels being too low, starfish cannot get enough and will suffocate.

In addition to that, higher water temperatures cause coral reefs to die, which are home to many marine animals, including many species of starfish. What’s more, if the coral reefs die, 25% of marine life would lose their habitat.

Are starfish endangered?

Unfortunately, there are starfish species that are currently endangered. In August 2021, the International Union for the Conservation of Nature (IUCN) added the sunflower sea star to the Red List, which means that the species is critically endangered.

The sunflower sea star is the largest sea star in the world, with a maximum arm span of 3.2 ft (1 m). The starfish mostly inhabit the northeast Pacific. Interestingly, adult species usually have 16 to even 24 arms!

The series of marine heatwaves that raised the temperatures of the eastern Pacific Ocean caused the reduced number of sunflower sea stars. The divers noticed an 80 to 100% decline in sunflower sea star abundance across 2,000 miles (3220 km).

Many different species died during the combination of rising water temperatures and the outbreak of the sea star wasting syndrome, but the sunflower sea star was affected the most.


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