Baby Corals Are as Vulnerable to Disease as Adults

Baby corals are as susceptible to a deadly disease as their adult counterparts, a new study finds.

Stony coral tissue loss disease (SCTLD) first hit coral populations along Florida’s Coral Reef in 2014. The lethal disease was part of a widespread outbreak. More than 95% of Florida’s Coral Reef has been affected and nearly half of Florida’s 45 stony coral species are impacted by the disease.

Since its initial outbreak, the disease has been documented spreading throughout Florida and the Caribbean.

“Before now, these surveys and studies of SCTLD have not included baby corals, so we had no idea how the disease may have been affecting them. In order to understand how a disease spreads and might impact a population, we need to consider all demographics within that population,” lead author Olivia (Liv) Williamson, a Ph.D. candidate in the Department of Marine Biology and Ecology at the University of Miami, tells Treehugger.

“In addition, reef restoration projects in Florida and the Caribbean are actively breeding baby corals and planting them onto reefs, but without knowledge of how those babies might be impacted by SCTLD, we could not weigh the risks they might be facing once planted.”

Researchers were focused on studying “baby corals,” which are corals in the first year of life.

“Corals are born very small—about the size of a grain of quinoa or couscous—and slowly grow to the size of a dime or quarter by one year old (depending on the species and environmental conditions),” Williamson says.

In earlier studies, SCTLD surveys have not included corals that were less than 5 centimeters in diameter, Williamson points out, which is about the size of a plum.

“Although growth varies between coral species, for the most part, that means that most corals that are less than 2 years old have not been included in disease surveys, and as such, we had no idea how SCTLD was impacting them,” she says.

About Stony Coral Tissue Loss Disease

SCTLD is possibly the most lethal coral disease ever documented, says Williamson. Researchers have found it can infect and kill at least 23 species of Caribbean corals, which is about half of the species on Florida’s reefs.

“It seems to be highly contagious, often afflicting high proportions of colonies on a reef, and within weeks or months can completely kill large corals that were hundreds of years old,” Williamson says. “The disease begins as a lesion on the edge of a coral colony, and travels across the colony rapidly, stripping away coral tissue and leaving behind a bare, white skeleton.”

Mortality rates have been reported from 60% to 90% for certain species

“These numbers are staggering, and indicate that SCTLD poses an existential threat to many brain and star coral species,” Williamson says.

Why Baby Corals Matter

Because baby corals have been excluded from disease surveys, researchers have likely underestimated the extent of the disease’s devastating impact.

As with any disease, researchers need to study it in order to find ways to treat and prevent it. Williamson compares it to the COVID-19 pandemic, where scientists are studying how infectious disease affects different members of a population because those with different traits can react differently. The same is true for diseases in non-human animals.

Earlier, researchers have analyzed how adult corals of various species in different locations have been impacted by the disease, but they didn’t know whether SCTLD was affecting coral babies.

“That’s sort of like ignoring how or whether COVID-19 infects children. Without this information, we are missing a key piece of the puzzle, leaving a large portion of the population vulnerable, and not getting a full picture of the harm caused by the disease and the future risks it poses.”

But while COVID-19 appears to be less severe and deadly for children than older adults, that’s not what researchers found for baby corals.

Baby Corals vs. Disease

For their study, researchers exposed lab-raised baby corals of two species to water containing active SCTLD for four weeks. Both 4-month-old boulder brain coral (Colpophyllia natans) and 8-month-old grooved brain coral (Diploria labyrinthiformis), began to develop lesions within 48 hours after they were exposed to the disease.

Nearly 60% of the boulder brain corals died within 2 to 8 days of developing lesions, whereas the grooved brain corals had slower tissue loss and fewer died. Larger corals and those that were in groups were more likely to survive, while smaller corals and those that were alone had the lowest survival rates.

After 20 days, the remaining corals were exposed to a second round of SCTLD. They all died within six days.

The findings were published in the journal Frontiers in Marine Science.

“Sadly, we found that baby corals are just as susceptible to this terrible, deadly disease as their adult counterparts. This unfortunately means that our estimates of the extent of damage caused by SCTLD on Florida's reefs—the number of colonies that have become infected and died - are likely underestimates; untold numbers of young corals have likely succumbed to the disease without our knowledge,” Williamson says.

“However, also like adult colonies on the reef, some of them are lucky enough to avoid becoming infected at all, suggesting that some corals harbor a degree of resistance—or are at least, relatively less susceptible—to disease. This study showed that larger babies, and those fused together into groups, were significantly less likely to get diseased and die. So, at least in this study: size matters, and there is safety in numbers!”

Knowledge Is a First Step

While the results were unfortunate, knowing how the disease appears in baby corals will help scientists eventually find ways to stop or slow its spread and create preventions and treatments to help coral recover. It’s also important to understand how the disease threatens baby corals, Williamson says.

“Damaged populations can only truly ‘recover’ if new generations establish themselves and survive on a reef,” she says. “If a disease outbreak wipes out most of the adult corals on a reef, but it also kills many new baby corals that settle there, the population has very little chance of recovering. Alternatively, if the baby corals survive, that population might bounce back after a disturbance.”

Coral reefs provide habitat for marine life, protect the coast from erosion, and encourage and support tourism. So much depends on these declining ecosystems.

“Even before SCTLD started in 2014-2015, Florida’s Coral Reef was already in trouble due to climate change. Warming ocean temperatures had already caused many species to dwindle. Now, this deadly and unprecedented disease, which infects half of the coral species on our reefs, has caused rapid, severe declines in almost two dozen species, with brain corals and star corals hit especially badly,” Williamson says.

“We don’t know much about the disease, so it is critical that we study it to better understand what makes it so deadly and how it spreads among corals, so that we can come up with intervention strategies. If we don’t take swift action, we risk losing the majority of corals left on Florida’s coral reef and throughout the Caribbean, thus also losing the critical ecosystem services they provide for us.”