Study identifies key algae species helping soft corals survive warming oceans

Scleractinian corals, or hard corals, have been disappearing globally over the past four decades, a result of climate change, pollution, unsustainable coastal development and overfishing. However, some Caribbean octocorals, or soft corals, are not meeting the same fate.

During a two-year survey of soft corals in the Florida Keys, Mary Alice Coffroth, professor emerita of geology at the University at Buffalo, along with a small team of UB researchers, identified three species of octocorals that have survived heat waves. While the coral animal itself may be heat tolerant, Coffroth said that her team concluded that the symbiotic algae inside the coral serve as a protector of sorts.

"The resistance and resilience of Caribbean octocorals offers clues for the future of coral reefs," Coffroth said.

A recent paper outlining their research, "What makes a winner? Symbiont and host dynamics determine Caribbean octocoral resilience to bleaching," was published on Nov. 22, in Science Advances by the American Association for the Advancement of Science (AAAS).

Coffroth is the lead author on the study she conducted between 2015 and 2017 with graduate student Louis Buccella, undergraduates Katherine Eaton and Alyssa Gooding and technician Harleena Franklin. Howard Lasker, professor emeritus in the departments of Environment and Sustainability and Geology, also contributed to the study.

Algae helps corals survive heat waves

Both hard and soft coral depend on a nutritional symbiosis with single-celled algae living within their tissues. Warmer waters can cause the symbiosis to break down, resulting in a loss of the algal symbionts, which turns the corals white, a phenomenon known as bleaching.

"Bleaching can lead to coral death," said Coffroth, who has studied coral reefs in the Florida Keys since 1998, including a more recent study in 2020-21. "It's unclear if the algae leave or are ejected from the coral.

"In this study, we examined possible mechanisms that contribute to the heightened resistance and resilience of three octocoral species in the face of the recurring marine heat waves leading to bleaching events," Coffroth said, noting that this is the first study that follows both symbiont genetic makeup and density in Caribbean octocorals before, during and after a major heat wave.

By and large, Caribbean octocorals harbor symbionts within the genus Breviolum, she said. And this symbiont is helping to make the octocoral better able to handle the rising heat.

"The Breviolum densities declined during the heatwaves but recovered quickly," she explained. "Octocoral mortality was low compared to their scleractinian relatives."

2014 El Niño prompted research

When Coffroth saw bleached corals during the 2014 El Niño and knew that a similar event was predicted for the following summer, she applied for a Rapid Response Research (RAPID) grant from the National Science Foundation. She was awarded $56,305 and with her master's student, Buccella, conducted the study in the Keys, following the fate of the octocorals and their symbionts for 28 months.

She and other members of the team made trips to the Keys Marine Lab at the Florida Institute of Oceanography to study the octocorals in the spring and fall of 2015 and 2016 and spring and summer of 2017, recording coral coloration and taking samples to study density of the symbionts and their genetic identity.

"We knew it was critical to follow individual colonies across an event with long-term monitoring of both host and symbiont responses," she said, "and to examine the response at least at the level of symbiont species, if not the genotype, to identify potentially resilient species."

Climate change moving faster than coral evolution

Although the study began almost a decade ago, Coffroth said the findings are extremely relevant because they mirror what is happening right now, with the continuing warming of ocean waters, increased storms and major bleaching events across the globe.

"There is evidence that corals are withstanding higher temperature now than they did in the 1960s," she said. "That signals evolution, but the problem is that climate change is moving too fast, faster than evolution."

In addition to their beautiful aesthetics, coral reefs provide many benefits to the planet and its inhabitants, including barriers to coastal regions that are susceptible to hurricanes and other tropical storms; habitat for large fish such as grouper and snapper; a tourist destination for snorkeling, fishing and diving; and a source for bioactive compounds used in drugs to treat inflammation and certain kinds of cancer.

"If you a see picture of coral reefs when I started diving in the 1970s and compare it with one now, it makes you want to cry," she said. "The change is just amazing."

While she noted that this study has some important observations, further study is needed to better understand what is happening to the ecosystem.

"I'm seeing species bleach that have never bleached before but also ones showing more resilience," she said. "There is a lot of variation within both the animal and symbiont genera. We need to understand the variation."

The hope is to continue research into coral reef relationships and the durability of the symbiotic algae while also taking steps to halt the damage to the environment by human action, such as overfishing and the burning of fossil fuels.

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Warming oceans have decimated marine parasites -- but that's not a good thing

More than a century of preserved fish specimens offer a rare glimpse into long-term trends in parasite populations. New research from the University of Washington shows that fish parasites plummeted from 1880 to 2019, a 140-year stretch when Puget Sound -- their habitat and the second largest estuary in the mainland U.S. -- warmed significantly.

The study, published the week of Jan. 9 in the Proceedings of the National Academy of Sciences, is the world's largest and longest dataset of wildlife parasite abundance. It suggests that parasites may be especially vulnerable to a changing climate.

"People generally think that climate change will cause parasites to thrive, that we will see an increase in parasite outbreaks as the world warms," said lead author Chelsea Wood, a UW associate professor of aquatic and fishery sciences. "For some parasite species that may be true, but parasites depend on hosts, and that makes them particularly vulnerable in a changing world where the fate of hosts is being reshuffled."

While some parasites have a single host species, many parasites travel between host species. Eggs are carried in one host species, the larvae emerge and infect another host and the adult may reach maturity in a third host before laying eggs.

For parasites that rely on three or more host species during their lifecycle -- including more than half the parasite species identified in the study's Puget Sound fish -- analysis of historic fish specimens showed an 11% average decline per decade in abundance. Of 10 parasite species that had disappeared completely by 1980, nine relied on three or more hosts.

"Our results show that parasites with one or two host species stayed pretty steady, but parasites with three or more hosts crashed," Wood said. "The degree of decline was severe. It would trigger conservation action if it occurred in the types of species that people care about, like mammals or birds."

And while parasites inspire fear or disgust -- especially for people who associate them with illness in themselves, their kids or their pets -- the result is worrying news for ecosystems, Wood said.

"Parasite ecology is really in its infancy, but what we do know is that these complex-lifecycle parasites probably play an important role in pushing energy through food webs and in supporting top apex predators," Wood said. She is one of the authors of a 2020 report laying out a conservation plan for parasites.

Wood's study is among the first to use a new method for resurrecting information on parasite populations of the past. Mammals and birds are preserved with taxidermy, which retains parasites only on skin, feathers or fur. But fish, reptile and amphibian specimens are preserved in fluid, which also preserves any parasites living inside the animal at the time of its death.

The study focused on eight species of fish that are common in the behind-the-scenes collections of natural history museums. Most came from the UW Fish Collection at the Burke Museum of Natural History and Culture. The authors carefully sliced into the preserved fish specimens and then identified and counted the parasites they discovered inside before returning the specimens to the museums.

"It took a long time. It's certainly not for the faint of heart," Wood said. "I'd love to stick these fish in a blender and use a genomic technique to detect their parasites' DNA, but the fish were first preserved with a fluid that shreds DNA. So what we did was just regular old shoe-leather parasitology."

Among the multi-celled parasites they found were arthropods, or animals with an exoskeleton, including crustaceans, as well as what Wood describes as "unbelievably gorgeous tapeworms:" the Trypanorhyncha, whose heads are armed with hook-covered tentacles. In total, the team counted 17,259 parasites, of 85 types, from 699 fish specimens.

To explain the parasite declines, the authors considered three possible causes: how abundant the host species was in Puget Sound; pollution levels; and temperature at the ocean's surface. The variable that best explained the decline in parasites was sea surface temperature, which rose by 1 degree Celsius (1.8 degrees Fahrenheit) in Puget Sound from 1950 to 2019.

A parasite that requires multiple hosts is like a delicate Rube Goldberg machine, Wood said. The complex series of steps they face to complete their lifecycle makes them vulnerable to disruption at any point along the way.

"This study demonstrates that major parasite declines have happened in Puget Sound. If this can happen unnoticed in an ecosystem as well studied as this one, where else might it be happening?" Wood said. "I hope our work inspires other ecologists to think about their own focal ecosystems, identify the right museum specimens, and see whether these trends are unique to Puget Sound, or something that is occurring in other places as well.

"Our result draws attention to the fact that parasitic species might be in real danger," Wood added. "And that could mean bad stuff for us -- not just fewer worms, but less of the parasite-driven ecosystem services that we've come to depend on."

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Genetic barriers, a warming ocean, and the uncertain future for an important forage fish

In the vast oceans, one would assume their inhabitants can travel far and wide and, as a result, populations of a species would mix freely. But this doesn't appear to be the case for a vital forage fish called the sand lance.

Sand lance are small schooling fish impressively rich in lipids, which makes them a fantastic and significant food source for at least 70 different species ranging from whales and sharks to seabirds, says UConn Associate Professor of Marine Sciences Hannes Baumann.

The Northern sand lance can be found from the waters off New Jersey all the way north to Greenland. Researchers, including Baumann and Ph.D. student Lucas Jones, were interested to see if sand lance constitute a massive, homogenous population, or whether there are genetically distinct groups. Their findings are published in the ICES Journal of Marine Science.

Baumann explains these are important questions to answer when considering conservation and sustainable management of the species, especially since the regions where sand lance live are warming faster than many areas of the planet due to climate change.

Sampling fish from such a broad range is no small task, but two years ago, Baumann and Jones began reaching out to other researchers to see if they had tissue samples to spare. Baumann credits the work to the international group of colleagues who contributed samples including co-authors from Canada and Greenland, and who helped sequence and analyze the data including co-authors from Cornell University.

In all, Baumann, Jones, and the team were able to sequence and analyze nearly 300 samples from a variety of locations across the sand lance's range using a technique called low-coverage whole genome sequencing. They also sequenced the first reference genome for sand lance.

In a nutshell, Baumann says they found an area on the Scotian Shelf, off the coast of Nova Scotia, where a genetic break occurs. The researchers distinguished two distinct groups, one north and one south of the divide, with parts of the genome differing quite dramatically -- namely on chromosomes 21 and 24. Without obvious physical barriers like a mountain range separating the groups, Baumann says it's logical to ask how these differences are possible.

"That is the scientific conundrum," says Baumann, and the answer, it appears, lies in the currents.

"When fish from the north reproduce and drift south, they are genetically less adapted to warmer southern waters, even if it's five or six degrees warmer in the winter, they are just not surviving," Baumann says. "These populations may be linked by the ocean currents, but the realized connectivity is basically zero."

This finding is a first for the sand lance, but it has been shown in other species such as lobsters, cod, and scallops, and this research adds further evidence to an apparent temperature divide at the Scotian Shelf, and helps demonstrate that temperature is an important factor in survival.

"Example after example shows that the ocean is not as homogeneous a place as expected, and there are all kinds of things that prevent that constant mixing,"Baumann says. "We found another striking example of that."

When researchers find adaptation in an environment where mixing is continuous, like in the ocean, Baumann says, the question is how it is possible that groups stay different, even though they are constantly encountering other genotypes. That is where powerful genomic methods, like the ones used in this paper, come in handy.

"Parts of the genome in many species have what we call a 'genetic inversion,' which means that the genes on the chromosome from one parent have a certain order and the genes on the same chromosome that come from the other parent that code for the same thing, and they're the same area, but they're flipped," Baumann says.

These inversions mean recombination cannot occur; therefore, the genes are passed down through the generations and play an important role in adaptation.

"We discovered on chromosomes 21 and 24 there are whole regions that are completely different and that is like the trademark signature of what we call an inversion because there's no recombination going on."

Baumann says that knowing there are genetic and ecological barriers on the Scotian Shelf is important, because with climate change, this barrier may move north and while that may be good news for southern fish, it's bad news for the fish currently there.

The researchers were also a little relieved in finding two clusters, because had there been many smaller clusters, it could make management and conservation more challenging, especially considering scenarios like the construction of offshore wind parks. Areas potentially well situated for wind turbines can also be habitats for sand lance, and construction disrupts habitats. If there were many, smaller population clusters, a single construction project could pose the risk of completely wiping out a cluster, whereas with more widely dispersed populations, though the local population may be temporarily disturbed, it will not be long before they are able to re-establish after construction is completed.

Baumann plans to focus further research on studying the genetic basis of the thermal divide.

"We want to make sure that this fish is productive and resilient, despite climate change, so we should make sure these areas where they are occurring are protected," Bauman says. "These decisions should include experts to ensure if there's an area that is very critical to sand lance, that any disturbance is temporary."

It isn't an unsolvable conflict, but it is something that we need to do, says Baumann, who also notes that it is possible that sand lance north of the thermal divide are already suffering more from warming because the region is warming faster.

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