Dugongs could be more endangered than we thought

Dugongs, one of four species of sea cow, live near to coasts in the tropical Indo-Pacific ocean around India, East Africa, Malaysia and western Australia.

Historically, they have lived along almost all the coasts of the Indian Ocean, and around some islands in the Pacific. But the animals are suffering pressures on their coastal habitat, and in many places their numbers are plummeting.

Dugongs are herbivores, relying on seagrass that grows in shallow ocean water - a fragile habitat that is under threat from fishing activities and human occupation. They were also killed for hundreds of years for their meat and oil. One of their closest relatives, the Steller's sea cow, has already been hunted into extinction.

Conservationists want to preserve different populations of dugongs and increase their chances of being able to adapt to ocean changes in the future. It is particularly important to do this because of the vital role the mammals play in maintaining healthy coastlines.

But dugongs are tricky to study because they like to live in muddy water and they all look very similar to the untrained eye, so not much is known about where they travel and how their populations are structured. It is also difficult to get tissue samples from living animals.

In circumstances like this, museum collections can step in to help plug the knowledge gap.

Dr Stephanie Plön, a researcher at Nelson Mandela University in South Africa, led the latest DNA study on the animals, which is published in PLOS ONE.

She says, 'We need dugongs as they are important for maintaining healthy seagrass meadows. These are nursery areas for many fish species and thus important for food security in the region.

'Recent studies have also shown that seagrasses, along with mangroves, are important in removing carbon from the atmosphere and oceans, and this so-called "blue carbon" is important in mitigating climate change.'

Stephanie found that dugongs around Madagascar were particularly threatened - more so than previously thought. 

Studying DNA

The type of analysis that Stephanie did is called phylogeography. It is a way of studying how and why populations of animals and plants are spread across a certain area, done by looking at the genetics of those organisms.

Often, researchers can tell if populations of animals have migrated, or been split in two by continents moving.

Phylogeography is a relatively new way of studying nature, because it relies on DNA analysis, which has become much easier since the 1970s.

Stephanie looked into the genetic history of dugongs across their original range. She used 14 different museum collections, taking DNA samples from the bones or teeth of 162 dugongs. The oldest specimen she used dates back to 1827.

Stephanie took samples from 62 specimens from the Natural History Museum collections, along with samples from other institutions around the world.

Richard Sabin, Principal Curator of Marine Mammals at the Museum, says, 'The dugong samples from the mammal collection in the Life Sciences department made a substantial contribution to the overall study.

'Thanks to the availability of samples from all of the museum reference collections used, the researchers were able to obtain genetic sequences from 172 individual animals throughout the entire historic range of the dugong. The results now help us to understand how dugong populations are connected and how genetically different they are from each other.'

A clearer picture

In many parts of the dugong's range it may be extinct, or only small populations may be left.

Alongside a drop in their numbers comes the risk of a loss of genetic diversity. As animal populations are significantly reduced, this results in genetic bottlenecking. In other words, it is a drastic shrinking of the gene pool. When an animal population is affected by sudden habitat loss or intensive hunting, lots of gene variations are lost very quickly.

It is a problem facing most animals on the brink of extinction, such as rhinos, and it is an issue because a species needs a wide range of healthy DNA in a population to be able to cope with any changes it might face in the future. Lack of diversity makes it harder for a species to keep evolving, adapting and ultimately surviving.

But Stephanie's analysis found that it is likely that valuable genetic diversity among dugongs has already been lost during the last 150 years.

She found unique and previously unidentified dugong lineages in the Indian Ocean - meaning it is likely there were once even more unique lineages that have already disappeared.

Stephanie also found that there isn't much gene flow between some populations of dugong in different regions.

Perhaps the most important result was that all the dugongs that live around Madagascar are particularly genetically isolated, so may be even more at risk from human activity than previously thought and deserve a higher conservation status.

It is hoped that this research can feed into a reassessment of how dugongs are protected in the future.