Over the last 70 years, invasive seaweeds have inundated the northeast coast of the Hawaiian island of Oʻahu, spreading hundreds of feet or more per year and smothering the coral reefs in the Kāne‘ohe Bay. The island’s largest embayment, Kāne‘ohe Bay has been called the “Coral Kingdom,” and researchers travel from far and wide just to see it. Today, though, many of its majestic reefs are engulfed by seaweed. At least one of these interloping species hitched a ride to the bay in the 1950s via the hull of a barge as it made its way from Guam. But much of the seaweed, broadly called macroalgae, was intentionally introduced in the 1970s by researchers from the University of Hawaiʻi who were experimenting with commercial cultivation of marine vegetation. These same species still litter the reefs, forming mats that block the sun, absorb oxygen and other nutrients, crowd the reefs and inflict damage.
To rescue the corals and the multitudes of marine life they harbor, the State of Hawaii Division of Aquatic Resources (DAR) and The Nature Conservancy began deploying “supersucker” barges to vacuum up the seaweed. Divers would pluck the seaweed from the reefs and siphon it through a hose and back to the barge, where it could be sorted and later distributed to local farmers for fertilizer. However, divers found it nearly impossible to pull in every microscopic fragment, and the seaweed would often return in a manner of months. The DAR realized it needed reinforcements: hundreds of thousands of hungry collector urchins (Tripneustes gratilla) that could consume the tiny seaweed fragments and access the remote crevices sheltering the root-like structures that anchor the seaweed to the reef. After several years, the DAR retired its supersucker barge and began to rely solely on collector urchins to remove the remainder of the seaweed.
Collector urchins get their name because they have a propensity for amassing coral rubble, rocks and algae for camouflage. Often no larger than a baseball, these dark, rotund critters are endowed with sharp spines and tiny tube feet. They’re ideal for reef cleanup because they are ravenous for many types of algae and seagrasses, and—unlike fish—do not migrate far after they’ve settled. Although some scientific reports suggest that collector urchins were once the most abundant urchin in Kāne‘ohe Bay, the natural population is relatively sparse today. In 2010, the DAR began gathering these spiky critters from the ocean and bringing them back to the state-run hatchery to spawn. The number of fertile urchins it gathers at any given time depends on the weather and wave surges, but the hatchery technicians are often able to raise enough juvenile urchins to release batches of 4,000 to 7,000 back into the bay a few times a month. There, the urchins spend their time slowly migrating across the reef, grazing as they go. By November, the DAR will have released nearly a million urchins, but more are still needed.
To augment the urchin population and simultaneously curb the invasive seaweed, local researchers are investigating ways to complement the DAR’s efforts. In July, scientists at the Hawaiʻi Institute of Marine Biology became the first to cryopreserve collector urchin embryos and rear them to competency. This is the culmination of a multiyear project spearheaded by University of Hawaiʻi graduate student Charley Westbrook. He devised a method to store these embryos for extended periods of time by cooling them to ultralow temperatures below 300 degrees F and then slowly thawing them so they can develop into full-blown urchins. These efforts mean that hungry young urchins could easily be raised in the lab year-round. Frozen embryos could also provide a “Noah’s Ark” from which to restock urchin populations in the event of a collapse—or even fill the plates of hungry diners hankering for sea urchin sex organs, also known as the uni one might find in a sushi restaurant.
After several years of experimentation, Westbrook finally pinpointed the freezing and warming rates that would cause the least damage to the embryos, as well as the best cryoprotectant chemicals that would help keep them alive during these temperature changes. But even once he had honed his protocol, the hardest was yet to come.
Westbrook says cultivating the proper food for each developmental stage was particularly challenging. The tiny larvae that develop from the thawed embryos swim freely around their containers and require a specific diet of phytoplankton for many weeks. Once they’re about the size of the tip of a thumbtack, they grow spines and settle at the bottom of the container. There, they munch on biofilms of bacteria for several months until their mouths become big enough to manage large seaweeds.
Westbrook was intrigued to find that urchins reared from cryopreserved embryos took even longer to develop than their non-cryopreserved counterparts. At first, he feared his larvae would never stop swimming, remaining in their final larval stage without settling into urchins. But one day this past June, he saw a tiny, spiky speck resting on the bottom of the beaker. “No one else was in the lab, but I was just hooting and hollering,” he recalls. “It worked!”
Mary Hagedorn, a senior research scientist at the Smithsonian Conservation Biology Institute and Hawaiʻi Institute of Marine Biology and one of Westbrook’s mentors, says collector urchins aren’t the only marine organisms that have fussy diets during development; marine fish embryos are quite finicky as well. Scientists have a hard time finding and culturing the right sequence and size of food for them as they develop because they have such tiny mouths, she explains, which is one of the factors holding back aquaculture for marine fish species.
Hagedorn herself is a pioneer in the field of cryopreservation. To date, she and her colleagues have cryobanked sperm from over 50 different coral species to help conserve the diversity of the world’s reefs. She says it will still take some time to transition Westbrook’s cryopreservation protocols from a “boutique” process to a high-throughput tool that can aid conservation, restoration and aquaculture efforts, but he’s getting there.
David Cohen, who manages the DAR’s sea urchin hatchery and was not involved in the cryopreservation research, is excited to see more attention paid to the collector urchin. While his hatchery doesn’t have plans to incorporate cryopreservation into its urchin-rearing protocol, he explains that “every bit of research that goes into this urchin and its life cycle and its habits is helpful.”
Seaweed smothering is, unfortunately, not unique to the Kāne‘ohe Bay. Cohen hopes the methods Westbrook is developing to rear urchins could help foster other urchin species in different parts of the world—such as the Caribbean’s Diadema antillarum. Researchers have only recently learned how to raise these urchins, which nearly went extinct after a virus swept through local populations and allowed invasive seaweed to overtake the corals. Additional grazers are still needed to prune the seaweed, and the more urchins that scientists can foster in the lab, the faster balance will be restored to the world’s reefs.
Serean Adams, a former aquaculture group manager at Cawthron Institute who was not involved in the research, notes that sea urchins have utility even beyond reef conservation. Researchers around the world use them for a variety of purposes, from studying fertilization and developmental biology to conducting ecotoxicological tests that assess the effects of marine pollution. Besides providing access to urchin reproductive materials year-round, cryopreservation also allows scientists to store samples with specific genetics for future selective breeding experiments.
Although Adams’ focus for the past several years has been on cryopreservation, hatchery production and breeding shellfish, in graduate school she conducted sea urchin research and developed protocols to freeze their sperm. She considers Westbrook’s cryo-reared embryos to be a coup and refers to them as an “insurance plan for the planet” that could help rebuild threatened habitats including Kāne‘ohe Bay.
Back in the lab, Westbrook is working hard to optimize and scale up his procedures so his urchins can eventually help to restore the reefs. To date, he has raised just a couple of juveniles from frozen embryos, and it will still take a few more months before they’re ocean-ready. To release them he will need permission from the state’s management agencies, but he’s looking forward to the day when these voracious critters can roam the reefs and snack on smothering seaweed invaders.