Nav: Home

Sloppy sea urchins

July 10, 2019

Sea urchins have gotten a bad rap on the Pacific coast. The spiky sea creatures can mow down entire swaths of kelp forest, leaving behind rocky urchin barrens. An article in the New York Times went so far as to call them "cockroaches of the ocean." But new research suggests that urchins play a more complex role in their ecosystems than previously believed.

A team led by Christie Yorke, a postdoctoral scholar at UC Santa Barbara's Marine Science Institute, studied how urchins might function to break up tough kelp into more manageable pieces that can feed other scavengers, also known as detritivores, living on the kelp forest floor. The paper, published in the Proceedings of the Royal Society B, is the first to look at sea urchins' role as shredders in the kelp forest ecosystem.

Urchins can have an outsized effect on kelp forests, especially when their predators aren't around to keep their population in check, Yorke explained. Overhunting of the sea otter, one of urchins' most significant predators, has allowed some urchin populations to clear cut vast tracts of kelp forest, drastically reducing the productivity and biodiversity of sites they've munched through. Some groups have even taken to indiscriminately smashing urchins to stem this scourge.

Nevertheless, urchins may be crucial to the health of the kelp forest ecosystem. Giant kelp is highly productive, growing up to 18 inches per day under ideal conditions. But a significant amount of this material gets transported away from the ecosystem, washing up on beaches, getting swept out to the open ocean or drifting into the deep sea. Kelp is also rather unpalatable compared to single-celled phytoplankton.

Despite its incredible growth rate and availability, giant kelp is not the first choice for many animals, as it is tough to digest.

Yorke and her colleagues were curious whether anything might be able to retain this food source within the kelp forest. "We know that kelp affects animals by providing habitat for fish and other species, but does it actually feed any of these animals?" said Bob Miller, a research biologist at the Marine Science Institute and one of the paper's coauthors.

Scientists have hypothesized that kelp sheds small particles that could be a food source. But the team's previous work found that kelp didn't appear to be nourishing the filter feeders in this way. The activity of sloppy sea urchins offered a promising alternative pathway for funneling nutrients from kelp to the ecosystem's detritivore.

Urchins as Marine Shredders

To test their hypothesis, Yorke set up several tanks with a number of detritivore from several species, along with some labeled kelp. Half of the tanks also got sea urchins.

To label the kelp, the team spiked it with rare forms of carbon and nitrogen by letting the algae photosynthesize in seawater enriched with these isotopes for three days, allowing them to trace the extent to which the tank residents ate the kelp. After 28 days, the researchers compared the isotope measurements for the specimens after the experiment to baselines they had established beforehand.

"We found that a whole host of detritivores can take advantage of kelp as long as urchins are there to process it for them, whereas otherwise they can't," said Miller. Indeed, only one species, a type of brittle star, ate a significant amount of kelp in the absence of sea urchins.

"Even then, the brittle stars used much more kelp when the urchins were present," added Yorke.

Urchins excel in their role of processing kelp for other detritivores. They are remarkably messy eaters, scattering all sorts of bits and pieces as they chow down on giant kelp. What's more, sea urchins digest remarkably little of what they actually eat. Meanwhile, their guts contain a rich assortment of microbes, some of which can pull nitrogen from the seawater itself, enriching the urchin's waste. Some studies have shown that urchin feces can be more nutritious than fresh algae, said Yorke.

"Essentially, they create a kelp smoothie for the reef," Miller said.

Yorke agreed and stated that "without the urchins there, it's possible that this kelp would just get washed out of the kelp forest by the current and be unavailable altogether."

A Broader Perspective

Extrapolating from small experiments in the lab to the processes out in the environment can be difficult, so the team used historical data collected by the Santa Barbara Coastal Long Term Ecological Research project (SBC LTER) to place their results in context. SBC is one of several long-term research sites throughout the country funded by the National Science foundation. It is one of two overseen by UC Santa Barbara.

The scientists looked at 11 years of relevant data, including the amount of kelp litter over time, as well as sea urchin abundance and biomass. Their analysis suggested that the amount of kelp that urchins shred and process could be a significant portion of the resources available to the creatures that live on the seafloor.

"A lot of times urchins are portrayed as grazers," said Yorke, "but that's actually an uncommon condition. Most of the time the urchins are just sedentary detritivores that wait for leaf litter from the kelp to fall and drift past them. They capture this detritus and consume it.

"Urchins switch from this sedentary behavior to active grazing if drift kelp becomes limited," she explained. This can happen for a number of reasons. If urchins become super-abundant there may not be enough drift kelp to sustain them. Alternatively, oceanographic conditions like El Niño can impact kelp productivity.

In this way, urchins are more like grasshoppers. Under normal conditions, grasshoppers are a healthy part of their ecosystem. But in certain circumstances, some species will swarm, becoming a plague of locusts.

"Urchins are generally cast as the villain in the kelp forest," said Miller, "but this study shows that they can play an important role as an intermediary in the food web."

"We should not go around and vilify or smash sea urchins before we understand their role in the ecosystem better," he added. "They're not necessarily always the bad guy they're made out to be."
-end-


University of California - Santa Barbara

Related Ecosystem Articles:

Breast cancer 'ecosystem' reveals possible new targets for treatment
Garvan researchers have used cellular genomics to uncover promising therapy targets for triple negative breast cancer.
Unparalleled inventory of the human gut ecosystem
Scientists gathered and published over 200 000 genomes from the human gut microbiome.
Cycad plants provide an important 'ecosystem service'
A study published in the June 2020 edition of the peer-reviewed journal Horticulturae shows that cycads, which are in decline and among the world's most threatened group of plants, provide an important service to their neighboring organisms.
More ecosystem engineers create stability, preventing extinctions
Biological builders like beavers, elephants, and shipworms re-engineer their environments.
Ecosystem degradation could raise risk of pandemics
Environmental destruction may make pandemics more likely and less manageable, new research suggests.
Improving the operation and performance of Wi-Fi networks for the 5G/6G ecosystem
An article published in the advanced online edition of the journal Computer Communications shows that the use of machine learning can improve the operation and performance of the Wi-Fi networks of the future, those of the 5G/6G ecosystem.
A lost world and extinct ecosystem
The field study site of Pinnacle Point, South Africa, sits at the center of the earliest evidence for symbolic behavior, complex pyrotechnology, projectile weapons, and the first use of foods from the sea, both geographically and scientifically, having contributed much on the evolutionary road to being a modern human.
Ecosystem services are not constrained by borders
What do chocolate, migratory birds, flood control and pandas have in common?
Late cretaceous dinosaur-dominated ecosystem
A topic of considerable interest to paleontologists is how dinosaur-dominated ecosystems were structured, how dinosaurs and co-occurring animals were distributed across the landscape, how they interacted with one another, and how these systems compared to ecosystems today.
How transient invaders can transform an ecosystem
Study finds microbes can alter an environment dramatically before dying out.
More Ecosystem News and Ecosystem Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Debbie Millman: Designing Our Lives
From prehistoric cave art to today's social media feeds, to design is to be human. This hour, designer Debbie Millman guides us through a world made and remade–and helps us design our own paths.
Now Playing: Science for the People

#574 State of the Heart
This week we focus on heart disease, heart failure, what blood pressure is and why it's bad when it's high. Host Rachelle Saunders talks with physician, clinical researcher, and writer Haider Warraich about his book "State of the Heart: Exploring the History, Science, and Future of Cardiac Disease" and the ails of our hearts.
Now Playing: Radiolab

Insomnia Line
Coronasomnia is a not-so-surprising side-effect of the global pandemic. More and more of us are having trouble falling asleep. We wanted to find a way to get inside that nighttime world, to see why people are awake and what they are thinking about. So what'd Radiolab decide to do?  Open up the phone lines and talk to you. We created an insomnia hotline and on this week's experimental episode, we stayed up all night, taking hundreds of calls, spilling secrets, and at long last, watching the sunrise peek through.   This episode was produced by Lulu Miller with Rachael Cusick, Tracie Hunte, Tobin Low, Sarah Qari, Molly Webster, Pat Walters, Shima Oliaee, and Jonny Moens. Want more Radiolab in your life? Sign up for our newsletter! We share our latest favorites: articles, tv shows, funny Youtube videos, chocolate chip cookie recipes, and more. Support Radiolab by becoming a member today at Radiolab.org/donate.