To protect threatened species, it is important to understand more about their populations and how different areas are connected. Until recently, scientific studies on manta rays have been heavily focused on photo-identification and tagging to answer questions such as “how big is this population?” or “how far do individuals travel?”. However, genetic techniques are also a powerful and effective tool for gaining insight into populations.
In a new study, published today in the journal Heredity, researchers from the Marine Megafauna Foundation (MMF), the University of Western Australia and the University of Queensland’s Project Manta used genetic markers to understand population connectivity of reef manta rays in the coastal waters of Mozambique’s Inhambane Province. This is the first population genetics study on reef manta rays to be published in the scientific literature.
Infograph showing the interconnectivity of mantas in Southeastern Africa, described in this new DNA research
MMF scientists have been studying the manta rays in Mozambique for almost two decades, a program which was pioneered by Dr. Andrea Marshall, MMF co-founder and principal scientist for the organization. “This is now one of the longest running and most comprehensively studied populations of mantas in the world,” notes Dr. Marshall. Over the years the team has used a variety of research techniques to better understand this population. For the most part, MMF’s previous studies had suggested limited movements between different areas of the Mozambican coastline and site preference to certain regions. However, seeing as reef manta rays are a highly mobile species capable of long-distance movements, the team was not convinced. The current study was developed as a way to use cutting edge genetic analysis to ask the same question in a different way.
“We wanted to test whether the patterns we had observed in the photo-ID and tagging studies were reflected genetically, or if the DNA told a different story” explained lead author Dr. Stephanie Venables, MMF Senior Scientist. “Understanding whether the coastline supports a single manta ray population, or a number of smaller, sub-populations is particularly useful for developing effective conservation strategies,” she continued.
This is especially important in the Inhambane Province, where in 2013 MMF published a landmarked study showing an 88% decline in reef manta ray sightings in this region over a single decade. “Such drastic declines are worrying for a species whose low reproductive rates make them susceptible to over-exploitation. Clear and effective conservation plans are urgently needed to prevent further decline, which could lead to localized extinction” said Dr. Marshall.
Aerial view of the Mozambican coastline, one of the research sites for this study. (c) Dr. Stephanie Venables, co-author of the study
For this study researchers collected small tissue samples from 120 wild manta rays at different sites along the south-east African coast, from the Bazaruto Archipelago in Mozambique, down to Port St Johns in South Africa. They extracted the DNA from these samples and sequenced thousands of genetic markers known as Single Nucleotide Polymorphisms (or simply, SNPs). “Next-generation sequencing technologies are becoming much more accessible and affordable, enabling scientists to sequence thousands of markers across an animal’s genome and providing a powerful technique to investigate genetic connectivity and relatedness” explained Dr. Venables.
Using these markers, researchers examined the genetic variability amongst individuals and found them to belong to one intermixing population, which can be considered a single unit for the sake of management and conservation. “Interestingly, our analysis placed the one manta ray sampled in South Africa within the larger Mozambican population. While we must be cautious of this because it is only a single individual, it indicates the Mozambican mantas are likely venturing into South African waters” said Dr. Marshall. “More research is needed but the results support the need for joint management plans between the two nations.”
In 2014 Dr. Marshall and colleagues worked hard to get reef manta rays listed on the appendices of the Convention for Migratory Species (CMS), an environmental treaty of the United Nations, providing a global platform for the conservation of migratory animals and their habitats and laying the legal foundation for internationally coordinated conservation measures throughout a migratory range. “We suspected connectivity between Mozambique and South Africa and this study provides further evidence to support this” Dr. Marshall says.
A group of manta rays traveling along the Mozambican coastline. (c) Dr. Andrea Marshall, co-author of the study
To understand just how far these highly mobile rays might travel, the team worked with collaborators in Australia to examine the connectivity between Mozambican manta rays and those in Western Australia. “We found a high level of genetic differentiation between the two locations, showing that manta rays are not crossing the deep Indian Ocean basin and that these populations are not intermixing,” said Amelia Armstrong, a researcher at Project Manta and the University of Queensland and a co-author of the study. While this result was expected, given the almost 8000 km distance between the two coastlines, it is an important step to confirm this
Understanding broad-scale connectivity can provide researchers with an insight into how reef manta rays may be connected in other locations around the world. “Our findings show evidence of high genetic connectivity along continuous coastlines but indicate limited gene flow across large bodies of water,” concluded Dr. Venables
Reef manta rays are among the largest of the living rays. As filter feeders with no stinging barb, they are gentle giants and their graceful and inquisitive nature makes them a favorite of SCUBA divers. Sadly, due mainly to target and bycatch fisheries, reef manta rays are listed as ‘vulnerable’ and decreasing on the IUCN Red List of Threatened Species. Understanding more about remaining populations can help develop science-based recommendations for their conservation and management.
The study entitled “Genome-wide SNPs detect no evidence of genetic population structure for reef manta rays (Mobula alfredi) in southern Mozambique” was published in the journal Heredity on 01 October 2020 and is available here.
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Notes to Editors:
The full study is available online at Heredity (https://rdcu.be/b7XS3)
For questions about this research, please contact:
Dr. Stephanie Venables, Senior Scientist, Marine Megafauna Foundation
The Marine Megafauna Foundation (MMF) was created in 2009 to research, protect and conserve the populations of threatened marine megafauna around the world. ‘Megafauna’ are large marine species such as sharks, rays, and sea turtles. For further details, please see www.marinemegafauna.org or follow us on Twitter, Facebook, Instagram and LinkedIn.