Lionfish photo prompts scientific puzzle

| in Community

By Abby Van Selous (COM’24)

It all began with the photograph of a lionfish beside a mangrove root in Calabash Caye, Belize, and the unsuspecting students in Associate Professor of Biology John Finnerty’s “Tropical Marine Invertebrates” 2014 marine semester course.

Finnerty’s students were characterizing organisms living in mangroves — coastal wetlands found mostly in tropical and subtropical regions — when they came across a lionfish. Finnerty snapped a photo of the fish and did not think much of it. But later, when reexamining the photo, he noticed something completely unexpected: coral hanging on a mangrove root.

He was puzzled — mangroves are not often thought of as the ideal habitat for corals because they are dark, have a lot of suspended materials in the water, such as sediments, and have high nutrient loads, unlike coral reefs, which are often in shallow waters and sunny locations.

Finnerty started to look for studies that might explain this phenomenon and came across one out of the U.S. Virgin Islands, which suggested that mangrove swamps could provide refuge to corals during times of stress on reefs — such as elevated temperatures or periods of high UV radiation. The study proposed that the mangrove canopy casts shade on the corals, which may minimize exposure to the high temperatures and UV rays.

So Finnerty set out to learn more about this phenomenon. He, along with Karina Scavo, then a PhD student and now a postdoctoral researcher at the University of Texas, and four BU undergraduate students started studying thin finger coral, one of over 30 coral species that can survive in mangroves, possibly the most tolerant, and the most abundant coral species in Calabash Caye.

The varying sizes of the thin finger corals in the mangrove suggested a range of ages, telling the researchers the population was persisting and allowing them to measure how much each individual coral grew from year to year. What was unknown to them was where the new colonies were coming from, if the thin finger corals in the mangrove and on a nearby reef were related, and where they were reproducing.

“The mangrove might just be accepting colonies that are coming in from the reef, in which case, it’s not actually self-sufficient, and if it’s not self-sufficient, it’s not a potential source of new colonies for reef populations nearby,” Finnerty said.

Many coral species reproduce through fragmentation, which occurs when coral breaks off from the colony, usually because of waves crashing against it. The fragmented coral then competes with other organisms for the same settlement locations. Once they settle, a new colony begins. This is not possible in mangroves, where corals grow on vertical roots, because the coral fragments would sink into the deep, nutrient-rich mud and die.

If a colony is particularly stressed, it can release a polyp, which are small invertebrates that are unable to swim and are negatively buoyant. When they are expelled, they sink and then grow a new colony where they land. This is a rare reproduction method, and it is not suitable for coral species in mangroves because the polyp would die in the mud bottom, Scavo said.

This left only one other reproduction method: parthenogenesis, the production of asexual larvae by female corals, which, give or take a few mutations, are identical to their mothers and, genetically speaking, clones.

Suspended on a mangrove root that grew into the water, the thin finger coral hanging in Finnerty’s photograph would have gone through this last process— reproducing asexually elsewhere and traveling only a few meters to the mangrove root, where it was photographed by Finnerty.

“Parthenogenesis seems to be the only plausible way,” Finnerty said. “You can get a parent colony living on one suspended root and a few meters away on another you have its offspring.”

Now that researchers know how thin finger coral reproduces, one potential next step is to study the other coral species in the mangrove and compare them to each other to determine if they share any principles or traits, and, if so, what that means.

“We have results from one coral in one mangrove population. We want to expand that to more mangrove populations,” Finnerty said. “We also want to expand to some more corals to understand if there’s any general principles we might learn from comparing those species.”