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The Big Picture
Diving deep to detect manta ray pregnancies

Abstract

Georgina Mills explains how researchers have used cutting-edge technology to learn more about reproduction in manta rays in the Maldives.

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We can observe the stages of pregnancy and the development of the fetus

Researchers from Cambridge vet school have used the world’s first contactless underwater ultrasound scanner to scan pregnant reef manta rays in the wild.

The researchers hope their studies will lead to further understanding of annual fluctuations in breeding and discover why manta rays breed in certain areas but not others.

Manta rays are close relatives of sharks and other rays, and individuals can reach up to seven metres in width and weigh up to two tonnes. Found throughout the tropical and sub-tropical oceans of the world, manta rays never stop moving, as they must keep water flowing over their gills to breathe.

Manta ray populations are becoming increasingly threatened – not only are they caught as by catch in fisheries targeting other species, but they are also hunted for their gill plates, which are used in Chinese medicine.

To examine the rays, the team, which worked with the Manta Trust in the Republic of Maldives in south Asia, used the Duo-Scan:Go oceanic ultrasound scanner developed by IMV-Imaging. Researchers dived down to a ‘cleaning station’, which are typically 20 to 30 metres below the surface, where smaller fish remove parasites from the mantas’ skin.

Approaching from above so as not to disturb the animal, divers were able to position the scanner 4 to 5 cm above the surface of the manta, targeting the left side of the dorsal fin, which is where the reproductive structures such as the ovaries and the uterus are located.

The work enabled the team to obtain the first ever scans of wild reef manta rays, including pregnant females and mature males. Sightings of the animals in the Maldives are reliable and consistent, which allowed the researchers to take multiple images of the same animals throughout their gestational period, which lasts just over a year.

Gareth Pearce, from the Department of Veterinary Medicine at the University of Cambridge, said: ‘Using the scans, we’re able to determine the stages of maturity and when animals are becoming reproductively active. We can observe the stages of pregnancy, the development of the fetus and, importantly, whether an animal maintains that pregnancy and gives birth to a live animal.

‘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. Our hope is that this research project will contribute to conserving the species for future generations.’

Guy Stevens, co-founder and chief executive of the Manta Trust, said: ‘When the project began, none of the team knew whether scanning wild reef manta rays would even be possible. What has been achieved is beyond what we could have hoped for. Manta rays are threatened worldwide and we still know so little about their reproductive strategies. The ability to scan pregnant individuals will be invaluable in our quest to protect them.’

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