Invasive Seaweed Harm the Hawaiian Seawaters, but New Research Could Stop the Spread 

Invasive species can really come in many shapes, colors, and size. For example, I’ve just found out that invasive seaweeds have taken over the northeast coast of the Hawaiian island of Oʻahu for 70 years. 

These seaweeds spread themselves over hundreds of feet or more per year and smothering the coral reefs in the Kāne‘ohe Bay.  

It’s unfortunate, because the island’s largest embayment has been called the Coral Kingdom. Tourists and researchers flock to this place just to see the beauty and biodiversity. But nowadays, the seaweeds overwhelm a large part of its majestic reefs.  

One of the intruders must have attached itself a ride to the bay in the 1950s through the hull of a barge from Guam. However, much of the seaweed or macroalgae was actually introduced intentionally in the 70s.  

Around that time, researchers from the University of Hawai’i were experimenting with commercial cultivation of marine vegetation.  

It just so happens that the species broke out of the experiment and still scatter about the reefs. The seaweeds form mats that block sunlight and crowd the reefs, absorbing oxygen and other nutrients, and eventually, inflict damage. 

Previous efforts 

The State of Hawai’i Division of Aquatic Resources (DAR) have made efforts to rescue the corals and the richness of marine life in it. Together with The Nature Conservancy, they began deploying what they call “supersucker barges” to vacuum them all up.  

Divers would pluck the seaweed from the reefs, siphon it through a hose, then back to the barge. It’d then be sorted and distributed to local farmers for fertilizer. 

Although it seems like a good solution, divers find it difficult to pull in every microscopic fragment. With this, the seaweed would frequently return within months. 

So, the DAR knew that there should be something else to help the reefs. There should be hundreds of collector urchins (Tripneustes gratilla) that are hungry and able to consume the tiny seaweed fragments. 

These urchins can place themselves in remote crevices that have sheltered the anchors of the seaweeds to the reef.  

After several years utilizing the supersucker barges, the DAR began to rely only on collector urchins to remove the remainder of the seaweed. 

 

there have been manual efforts to clean the seaweed. Photo by CSIRO Wikimedia Commons

 

Collector urchins 

These urchins get their common name because they could gather coral rubble, rocks, and algae for camouflage. With a size no larger than a baseball, the dark marine creatures are ideal for reef cleanups. They’re known for their big appetites for numerous types of algae and seagrasses. 

And, they don’t migrate like fish do. 

Some scientific reports suggest that these urchins were once plenty, especially in Kāne‘ohe Bay. But their natural population has significantly declined over time. 

In 2010, the DAR started gathering them from the ocean and bring them to the state-run hatchery. Another challenge to this was that the number of fertilize urchins depend on the weather and wave surges.  

Thankfully, hatchery technicians could raise enough juvenile urchins and have been able to release batches of 4,000 to 7,000 back into the bay a few times a month. 

Once they’re in the sea, the urchins spend their time slowly settling themselves across the reef, grazing as they go. Even though the DAR has released almost a million urchins, it’s not sufficient. That’s where the research comes in. 

Bringing up the urchins 

Local researchers are trying to figure out ways to complement the DAR’s efforts. Scientists at the Hawaiʻi Institute of Marine Biology were the first to cryopreserve collector urchin embryos and rear them until they can do their jobs. 

This discovery is a completion of a project led by a graduate student, Charley Westbrook. He created a method to store the embryos for extended periods of time. As the name “cryo” suggests, this method involves cooling urchin embryos to ultralow temperatures.  

Then, the researchers thaw them slowly so they can develop well. This means that they can raise these young urchins in the lab without needing to depend on weather and wave surges. 

Other things that the cryopreservation could do is, firstly, offering a supply whenever there’s a need to restore urchin populations when there’s a collapse.  

Secondly, it could satisfy people who simply want to enjoy urchins like the ones we find in a sushi restaurant. 

 

collector urchin. Photo by Lisa Bennett Wikimedia Commons

 

Challenges during the research 

After experimenting for several years, Westbrook managed to determine the freezing and warming rates that wouldn’t damage the embryos’ development. Westbrook also found the best cryoprotectant chemicals that would help keep them alive during the temperature changes. 

Despite everything that looks good, everything didn’t go as smoothly as planned. 

According to Westbrook, cultivating the right food for each developmental stage wasn’t easy. The tiny larvae that grow from the thawed embryos swim around their containers and need a specific diet of phytoplankton for many weeks. 

Once they’ve grown to the size of the tip of a thumbtack, they’ll grow spines and settle at the bottom of the container. At this stage, the food’s different: biofilms of bacteria. This diet will last for several months until their mouths become big enough to manage large seaweeds.  

In the experiments, Westbrook found that urchins reared from cryopreserved embryos took even longer to develop than their non-cryopreserved counterparts. 

Concerns came when the larvae would never stop swimming, staying in final larval stage without transforming into urchins. But one day, around June this year Westbrook saw tiny, spiky speck on the bottom of the beaker. 

“No one else was in the lab, but I was just hooting and hollering. It worked!” the researcher said. 

Preserving embryos in the cold 

Senior research scientist at the Smithsonian Conservation Biology Institute and one of Westbrook’s mentors Mary Hagedorn said that these urchins aren’t the only marine life with fussy diets during development. Hagedorn stated that marine fish embryos are also the finicky type. 

Due to the tiny mouths of these organisms, scientists have a hard time finding and culturing the right sequence and size of food for them as they were growing up. That’s one of the many factors which have held back aquaculture for marine fish species. 

Hagedorn is actually a pioneer when it comes to cryopreservation. Up to this, she and her colleagues have cryobanked sperm from over 50 different coral species to help conserve the diversity of the world’s reefs. 

According to the senior scientist, it will take some time to turn Westbrook’s cryopreservation methods from a process that aren’t ready for real-time solutions to a working tool that can aid conservation and restoration efforts. But, Westbrook is getting there, his mentor said. 

Not only important to Hawai’i 

The result of this research may be important to all shores which have been invaded by invasive seaweeds. This type of intrusion is, sadly, not unique to the Kāne‘ohe Bay. 

Therefore, many experts hope that Westbrook’s methods could be applicable to other urchin species in different parts of the world, like the Caribbean’s Diadema antillarum. 

Diadema antillarum itself has nearly gone extinct after a virus swept through local populations and allowed invasive seaweed to overtake the corals.  

The more urchins the scientists can grow in the lab, the faster it’ll be for the reefs to restore balance. 

 

Sources

https://www.smithsonianmag.com/science-nature/in-a-first-scientists-raise-a-cryopreserved-urchin-that-may-save-coral-180980792/

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