Sex And Mucus: Uncovering Rays’ Secrets

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“It’s not slime, its mucus!” screamed Tiana at Dr. Facilier in a scene from Disney’s The Princess and the Frog. If you’ve ever touched a ray at a touch-tank at an aquarium, you would be surprised to see that they too are also covered in mucus, even if they don’t look slimy. And Dr. Rodrigo R. Domingues of the Federal University of São Paulo with colleagues Dr. Otto Bismarck and Dr. Domingos at the State University of São Paulo, and Dr. Alexandre Hilsdorf at the University of Mogi das Cruzes, have been collecting mucus for genetic analysis of these animals, rather than cutting the fins or other parts for a small sample to run tests on. “The idea arose during another research, which also adopted non-lethal methods to study the spatial ecology of these species of batoid elasmobranchs in the north coast of São Paulo, south-eastern Brazil. As we were working in a protected marine area at that time, we opted to maximize our research effort with minimal impact on animals,” said Domingos in an e-mail.

A night dive with one of the most iconic batoids: mantas. A recent DNA study on members of the genus Manta have revealed these rays actually belong to the genus Mobula. Manta birostris (the giant, or oceanic manta) and Manta alfredi (reef manta) are now known as Mobula birostris and Mobula alfredi.

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It isn’t the first time this has been done. After all, scientists have been looking for ways to study animals more in depth outside of the procedures in place right now that can be quite invasive. But why does the DNA of these rays even matter? “To get genetic information of any taxon is of paramount importance for conservation and management action. So, in the case of […] rays, to have a source of DNA by a non-invasive method is [of great importance]. We can assess [everything from] species identification to population genetic structures by just rubbing some small amount of mucus from specimens without harming them,” said Rodrigo. The mucus layer of a ray helps protect them from disease and wound infections, and it has epidermal cells on the surface that can be used for genetic analysis. For example, they could distinguish between several species of rays, identify discrete populations, estimate their population size, and other analysis that have an impact on the conservation of these animals. “So, we showed that this source of DNA is reliable to be used in further genetic investigation with rays, without causing injuries or even death of the animal,” commented Rodrigo.

“[We performed a] phylogenetic analysis, [which] seeks to understand the evolutionary relationship among biological entities such as species, individuals, and genes, tracing an evolutionary history of the group under study. Such approach allows dating origin of evolutionary lineages, to describe the history of this lineage, to identify the ancestral origin (the common ancestor of the several lineages within a certain monophyletic group) and also to estimate the time of divergence between the lineages,” said Otto. But could other animals rubbing up against the ray and skew results in any way? “We believe so. As I said, rubbing [the] mucus we get skin cells. So […] it is possible to get an alternative DNA source,” said Alexandre.

Thirty one percent of the world’s sharks and rays are threatened with extinction according to The IUCN Red List of Threatened Species™, with ray species found to be at a higher risk than sharks.

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Scientists know very little about the population and life history of elasmobranchs (sharks, skates, and rays) in the western South Atlantic Ocean. While there have been technological advances in research methods in the last decade and many marine scientists have devoted their careers to studying these animals, they still hold many secrets. This is the ocean, and not only is securing funding tough but finding the rays is a whole other obstacle! “In the case of Brazil, about 320 scientific papers on elasmobranchs were published between in the last 10 years, among them the first was with a population genetics approach. Therefore, the use of the non-invasive technique to obtain genetic data is important, considering that some species are under threat or their data are insufficient according to IUCN criteria,” said Otto.

These sort of non-invasive procedures will help us pieces together the life history of these threatened rays worldwide. And while this team in Brazil is doing non-invasive genetic work for these rays, another from the University of Cambridge and the Manta Trust have successfully pulled off another non-invasive procedure: the first ultrasound of a pregnant wild reef manta ray! Across the world in the Republic of the Maldives in South Asia, the scientists scanned a pregnant wild reef manta ray underwater using the world’s first contactless underwater ultrasound scanner.

Sharks and rays do not just live in the ocean- many people are surprised to learn that some live in freshwater. The ocellate river stingray (Potamotrygon motoro) is a member of the freshwater stingray in the family Potamotrygonidae.

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Working with the Manta Trust, Dr. Gareth Pearce from the Department of Veterinary Medicine at the University of Cambridge and Ph.D. student Niv Froman used the new ‘Duo-Scan: Go Oceanic’ ultrasound scanner, developed by IMV-imaging, to study the reproductive ecology of manta rays. The scientists dove down to a ‘cleaning station’ manta’s go to have parasites removed by smaller fish and stay above the animal to not disturb it. Whipping out the ultrasound scanner, they position it about 5 cm above the manta and target the left side of the manta’s dorsal fin. It is here that the reproductive structures such as the ovaries and the uterus are visible. “It’s important not to cause the manta ray any stress,” explained Froman in a press release. “Using these portable scanners, we’re able to obtain ultrasound images of their internal structures, particularly their reproductive tracts, without disturbing the animal. This is the first time that this has been possible in free-swimming mantas.” Hopefully, these research projects will contribute to conserving the species for future generations. “Ultimately, our work aims to inform the conservation of manta rays both in the Maldives and other areas of the world, enabling the populations to survive and hopefully flourish,” commented Pearce.

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