New DNA Research Reveals Connectivity of Manta Rays in Southeastern Africa
A new study by researchers from the Marine Megafauna Foundation (MMF), the University of Western Australia and the University of Queensland's Project Manta has used DNA sampling for the first time to determine the range of reef manta ray populations.
The study, published in the journal Heredity, used genetic markers to understand 'population connectivity' of reef manta rays in the coastal waters of Mozambique’s Inhambane Province. Manta ray populations have been decimated in recent decades and understanding the movement of individuals and groups of animals is important to better preserve the species.
Reef manta rays (Mobula alfredi) are a wide-ranging species, but tracking their movement has, until recently, relied on photo identification and tagging to determine how far a particular individual may travel. Such research has shown that reef mantas do have a significant range, but is limited in scope given the small number of rays that can be tagged or that are positively identified in different locations.
MMF scientists have been studying the manta rays in Mozambique for almost two decades, a programme pioneered by MMF's co-founder and principal scientist, Dr Andrea Marshall. The team has used a variety of techniques to demonstrate a limited movement of mantas between different areas of the Mozambican coastline and certain preferred sites.
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Knowing that reef mantas are capable of travelling much longer distances, however, the team thought that the observed range of Mozambique's mantas may be larger than it first appears. The new study was developed to assess whether or not the manta-matching data was supported by genetic patterns.
'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', said Dr Stephanie Venables, the study's lead author and 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.'
Such strategies are particularly important in Inhambane Province, in which a 2013 MMF study found an 88 per cent decline in reef manta rays over the previous decade. 'Such drastic declines are worrying for a species whose low reproductive rates make them susceptible to over-exploitation,' said Dr Marshall. 'Clear and effective conservation plans are urgently needed to prevent further decline, which could lead to localized extinction.'
For the new study, researchers collected tissue samples from 120 wild manta rays at different sites along the southeast African coast, from the Bazaruto Archipelago in Mozambique down to Port St Johns in South Africa, approximately 1600km distant. The team extracted DNA from the samples and sequenced thousands of genetic markers known as Single Nucleotide Polymorphisms (SNPs, or 'snips'), which can be used to identify how closely related individual members of a particular species might be.
Using these markers to examine genetic variability between individuals, the scientists found that all the mantas sampled in the study belong to one large intermixing population, which can therefore be considered a single unit for the sake of management and conservation. 'Our analysis placed the one manta ray sampled in South Africa within the larger Mozambican population,' said Dr Marshall '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.'
Dr Marshall and her team had successfully lobbied to have reef manta rays listed on the appendices of the Convention for Migratory Species (CMS), a United Nations environmental treaty for the conservation of migratory animals and their habitats,in 2014. It is hoped that the new research will provide additional support for cross-border co-operations. 'We suspected connectivity between Mozambique and South Africa and this study provides further evidence to support this,' said Dr Marshall 'More research is needed but the results support the need for joint management plans between the two nations.'
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 a co-author of the study. 'While this result was expected, given the almost 8000km 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,' said 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 original version of this press release is published on the Marine Megafauna Foundation's blog page. 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 at https://rdcu.be/b7XS3.