New research using the National Geographic Society’s specially designed ‘Crittercams’ has revealed a new feeding strategy in manta rays.
Known as the gentle giants of the sea, manta rays are large oceanic filter feeders, measuring up to 7 meters (23 feet) in wingspan, found in subtropical and tropical oceans around the globe. One of their challenges is tracking down tiny zooplankton in a vast ocean, in order to feed and survive. However, zooplankton come and go as ocean conditions change, so how do manta rays know where to find a reliable meal?
This latest research, led by Dr. Joshua Stewart, Associate Director of the Manta Trust, shows that thermoclines - which are the transitional layers between warmer, surface ocean water and colder, deeper waters - could be part of the answer.
During the two-year study, Dr. Stewart and a team of marine biologists used ‘Crittercams,’ designed by the National Geographic Society’s Exploration Technology Lab, to capture manta rays’ feeding behaviour in thermocline layers at several sites in Mexico and the Maldives, typically between depths of 20m – 80m.
The study, which was funded with a grant from the National Geographic
Society, provided the team with a manta’s-eye-view of what happens when these enormous rays leave the cleaning stations where they’re typically observed. The small camera was attached to the upper jaw of the manta rays, using a minimally invasive suction cup and a generous dose of creamy peanut butter as sealant.
The resulting animal-bourne video footage showed that both oceanic and reef manta rays spent time foraging within or adjacent to the thermocline layer, where the density of plankton can be 10 times more concentrated than at the surface, making it a highly profitable feeding area.
“We’ve known for a while now that mantas are spending a lot of time feeding away from the surface,” says Stewart. “Basically, every technique previously used, from tags to chemical analyses of their tissues, have indicated that a huge part of their diet comes from non-surface sources. But being able to capture and confirm these feeding behaviours away from aggregation sites has been a challenge. The National Geographic team has used Crittercams to capture amazing behaviours in dozens of other species, so it was fantastic to put this innovative technology to use in manta rays for the first time.”
As scuba divers well know, the thermocline is found virtually everywhere in the world’s oceans, with its depth varying depending on latitude and oceanographic dynamics. Thermoclines have long been recognized as an oceanographic feature that shape the distribution of animals ranging from zooplankton to tuna, and other species such as seals and penguins may use the thermocline as a cue for where to find their prey.
While more research is needed, Stewart says this study indicates that the thermocline may be an important feature in making foraging profitable for manta rays. “Filter feeding isn’t cheap, from an energetics perspective. Opening up a gigantic mouth and pushing a huge sieve through the water burns a lot of calories, so filter feeders need to target dense patches of prey. There are, without a doubt, a lot of different strategies that mantas use for finding zooplankton at sufficient densities and using the thermocline to find and target prey is just one more of their tools that we now know about.”
These initial findings could aid in the protection of manta rays, whose populations are under threat from both targeted fisheries and bycatch. Previous tagging studies have shown that manta rays spend a large portion of their time close to the upper edge of the thermocline. Combined with observations from the new Crittercam study demonstrating the potential importance of the thermocline to manta ray foraging success, this information could be useful in setting guidelines to help avoid incidental capture of manta rays in fishing gear.
The cameras also captured footage that will provide insights into courtship and other social behaviors, which are currently being analyzed for a future publication.
“Most of all,” Stewart says, “this is a great example of how technology is allowing us to gain insights into the behaviors and ecology of marine species that wouldn’t have otherwise been possible.”
The full paper can be read here:
NOTES TO EDITORS:
• This study was made possible with a grant from the National Geographic Society and Wiatt Foundation. We are also particularly grateful to Thomas P. Peschak for facilitating a National Geographic research cruise to the Revillagigedo Islands, the community members of Yelapa and David Connell for assistance with deployments in Bahia de Banderas, Mexico. The research expedition in the Maldives was made possible thanks to the permission and support provided by the Maldives Ministry of Fisheries and Agriculture and the Ministry of Environment and Energy and Four Seasons Resort at Landaa Giraavaru for providing accommodation and support to the research team.
• Manta Trust: founded in 2011, the Manta Trust is a UK-registered charity, coordinating global research and conservation efforts via research projects in over 20 countries worldwide. Our mission is to conserve mobulid rays and their habitats, through research, education and collaboration. www.mantatrust.org
• Manta and Devil Rays are collectively known as mobulids and belong to the genus Mobula. In 2009 manta rays were officially split into two distinct species based on new genetic and morphological evidence, resulting in the recognition of reef manta rays (Mobula alfredi) and oceanic manta rays (Mobula birostris)