Photograph by Jackie Reid, Flower Garden Banks NMS, NOAA
Published October 15, 2010
SPECIAL SERIES | DEEP IMPACT
Deciphering the unseen, underwater effects of the Gulf oil spill.
The Gulf of Mexico's mysterious manta rays could face invisible and long-lasting threats from the BP oil spill, experts say.
In the weeks after the April 20 disaster, aerial photos and reports from boaters placed at least some mantas in the thick of the surface spill. But it's the oil's unseen impacts, deep underwater, may be even more troubling, especially as preliminary studies suggest the spill isn't going away.
(Related: "Whale Sharks Killed, Displaced by Gulf Oil?")
Mantas are filter feeders that reach huge sizes in part by taking in seawater and ejecting it through their gills, retaining plankton or other tiny creatures, according to Rachel Graham, lead shark scientist with the Wildlife Conservation Society's Ocean Giants Program.
"Gill filaments, which enable mantas to extract oxygen from the water, are very vulnerable to any kind of toxin or oil coverage," Graham said. "If they are covered, they will likely die."
What's more, there's so little known about Gulf mantas that scientists aren't sure if the 20-foot-wide (6-meter-wide) fish belong to one of two known manta species—or if the Gulf mantas are their own species altogether.
Oil and Mantas Don't Mix
After burning and sinking last April, the damaged Deepwater Horizon wellhead released nearly five million barrels of oil into the northern Gulf of Mexico (map). Each barrel equals 42 gallons, or 159 liters, of oil.
The oil could affect the "ways mantas live their day-to-day lives for years," said marine biologist Andrea Marshall of the Mozambique-based Foundation for the Protection of Marine Megafauna. "It won't clear up over a few months."
For instance, oil—and dispersants used to break it up—might hurt the rays' plankton food sources, as well as the "cleaning stations" where mantas go to have their parasites eaten by smaller fish.
Oil could also disrupt mantas' migrations throughout the Gulf, or even their reproduction—no one knows where the animals give birth, according to Mexican marine biologist Silvia Hinojosa Alvarez of the Mexican Caribbean Manta Project.
"The main problem is that we know [very little] about their biology," Alvarez said. "So how can we predict with accuracy what will happen?"
(Related picture: "First Giant Manta Ray Born in Captivity Dies.")
Complicating matters is the Gulf's role as a manta hot spot.
Fortunately, well-known Gulf manta haunts such as Flower Garden Banks National Marine Sanctuary were not in the direct path of the oil, said the Wildlife Conservation Society's Graham.
However, "we have this huge threat of a deepwater oil spill, but [we] don't know any population sizes, and we have no baseline," she added. "So it's very difficult to estimate the impact of something like this."
In addition, scientists believe that mantas are likely found throughout the Gulf. "So while those at the Flower Gardens may not be directly impacted, they may also move into areas affected by the spill," said Tim Clark, a marine ecologist with the National Park of American Samoa.
Gulf Mantas: Hybrids or Unique Species?
In 2008 the megafauna foundation's Marshall discovered that the manta ray is actually two separate species, and her work left the door open for the existence of a third species.
According to Marshall's work, the smaller, more familiar manta species, Manta alfredi, tends to stay near coasts as a year-round coral reef resident with a small home range.
The larger species, Manta birostris can top 4,400 pounds (2,000 kilograms) and tends to wander widely on deep-ocean migrations through a variety of marine habitats.
Scientists in the Gulf have been studying the region's mantas—informally called Yucatán mantas—to see where these stingray cousins fit on the family tree.
The Wildlife Conservation Society's Graham has studied Gulf mantas closely and conducted acoustic tagging research at Flower Garden. She said Gulf mantas share physical and behavioral elements of both confirmed species, including varied sizes, mixed markings, and colors. Also, the Gulf mantas' travel habits lie somewhere between the two extremes.
"It's almost like they are a hybrid," Graham said. "Interestingly, at Flower Garden Banks, [Gulf mantas] tend to be really small, while in the southern Gulf, the ones I recently tagged are really big, birostris-sized animals.
(See the satellite tracking online.)
“What I’ve seen from tags is that they seem to be spending a fair amount of time in the southern Gulf aggregation area but occasionally move over 56 miles [90 kilometers] before returning to the aggregation site. Why? [That’s] another mystery to uncover.”
After diving with Gulf mantas recently, Marshall, who first indentified the two known manta species, said she thinks that they may represent a third species but might also be hybrids or even M. birostris with regional color variations.
At this stage there is "tempting evidence" for each of these hypotheses, she said.
Gulf Manta Gene Study Underway
The Mexican manta project's Alvarez and colleagues are also unsure what to make of the Gulf animals.
"When we saw all the differences between them, [including] color patterns, presence or absence of [tail] spines, behavior, habitat, [and] skin, we could not classify our Yucatán mantas" as either known manta species, Alvarez said.
"Our Yucatán mantas shared morphological [structural] characteristics with [each of] the two mantas previously [described] by Dr. Marshall."
American Samoa's Clark did some early genetic work on manta species nearly a decade ago and suspected that the animals in the Gulf could be a new species.
"The Gulf mantas are genetically distinct," he said. "If M. alfredi and M. birostris are [separate] species—which Andrea [Marshall's] morphological work shows is true—then there is strong genetic support for a third species in the Gulf of Mexico."
Alvarez and colleagues have also begun a genetic study of the Gulf species, though there are no results yet, she said.
"The main thing, and what is important, is that scientific knowledge of this species is poor and needs to be supported so [we] can continue research with this species.
"We are working in a marine protected area, and as scientists [we] need to know what kind of mantas we have in the area." (See pictures of U.S. marine protected areas.)
Manta Research May Boost Protection
Classifying the new manta species could also aid efforts to protect the fish by revealing more about how they use the Gulf and its resources, scientists say.
To this end, the Wildlife Conservation Society's Graham is working with the National Marine Sanctuary's Emma Hickerson on programs such as the Flower Garden Banks' Manta Catalog.
The site has solicited photos from divers, boaters, scientists, and others who have spotted animals on the site—and already identified more than 70 individual Gulf mantas.
"One of the things that we've found in the photo catalog—which is also supported by the acoustic tag work—is that a lot of mantas certainly ... [stick] to these coral-covered salt domes and seamounts on the edge of the continental shelf," Graham said.
"Are these mantas staying in this particular area for their entire lifetime, or are they moving away, possibly to the southern Gulf and then returning to the northern Gulf banks?”
Solving such mysteries will shed light on where and how the animals live and how best to protect them.
Graham believes that's an obvious goal for anyone who has ever encountered the graceful giants close-up.
"A manta will interact with you," she said. "They are unbelievably smart animals—they have the largest brain-size-per-weight ratio of all the sharks and rays." (See stingray pictures.)
"When you are swimming alongside one and look into its eyes, you can see that they are very cognizant."
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