THERE are four species of sea-cow of the order Sirenia, of which three are types of manatee and the fourth is the dugong. The Sirens were the classical mythological mermaids that tempted sailors to their death when their ships were wrecked on rocks. Sadly, a second species of dugong, Stellar’s sea-cow, was hunted to extinction in the 18th century in Mediterranean waters. These mammals have always mystified me and I just wonder whether they may have in the past strayed into North Atlantic waters.
Hans Christian Anderson’s children’s story is immortalised in the bronze ‘Little Mermaid’ statue in Copenhagen, in his native Denmark. In the remote, windswept, clifftop Cornish village of Zennor, only a stone’s throw from my birthplace, the mermaid of Zennor’s legend still exists. It is said that a mermaid once lured a local village lad, besotted by her beauty, into the sea to disappear forever. There today, in the rugged, roughly-hewn granite built 11th century church, this legendary mermaid is carved on the end of one its oak, church-pews. Not dissimilar tales abound in Southeast Asian folklore.
Dugongs (Dugong dugon)
Found in Indo-Pacific inshore seas, stretching from the Red Sea, along the East coast of Africa and across the Indian Ocean and the Andaman Sea to South East Asia, north to the Philippines and south to the western, northern and eastern shores of Australia, the dugong’s distribution is very wide but sadly its numbers do not reflect this.
I have always wanted to swim with a dugong and observe it at close range but my attempts whilst snorkelling in Kenyan, Northern Madagascan and East Malaysian waters have proven futile. It is thought, but yet to be proved, that Southeast Asian dugongs are a distinct sub-species. I have recently seen manatees at Singapore’s River Safari. Dugongs are quite distinctive from manatees, for the latter have a distinct fish-like notch in their rounded tails.
Structure and development
Almost cylindrical in its shaped body, tapering at both ends, a dugong is up to three metres in length with a thick, smooth skin covered in short hairs. Its lobed lips are covered in sensory bristles, which pass foraging seagrass easily into its mouth. In photographs, a dugong grazing, with its small eyes and tusks, looks exactly like its close land mammal relative, the elephant, but without the latter’s trunk.
A dugong’s vision is limited and to compensate for this, its small ears possess acute hearing. It also has an extraordinary sense of small to locate edible plants. With two breathing holes, covered by skin flaps when submerged, it can easily breathe when breaking the sea-surface as the flaps open.
On average, its weight is about 420kg, with females slightly larger than males. Its paddle-like pair of flippers are used in twisting its body and slowing down while its tail, flashing up and down, provides forward propulsion.
Habitat and food source
Dugongs are the only true strictly herbivorous marine mammal, living off only seagrass meadows (see ‘Seagrass and seahorses’ in thesundaypost – Oct 1, 2017). Preferring warm, shallow coastal bay waters up to a depth of 10 metres and found in brackish waters of river outlets and shallow river channels, they are voracious feeders, each consuming up to 40kg of seagrass per day. They grub up the seagrass rhizomes but prefer softer varieties of seagrass to more fibrous ones.
Today, their largest populations are found in Australia, with over 20,000 seen in the Gulf of Carpentaria as well as 25,000 noted in the Torres Strait and New Guinea. Worldwide however, there are numerous reports of ever shrinking populations.
Males reach maturity when their testosterone levels allow their tusks to erupt. Part of the reason why these mammals are endangered is that although they may live for 50 years or more, female dugongs only give birth to one calf after a gestation period of 13 to 15 months. The calf’s birth is in shallow waters and, just like a cow licks its afterbirth mucus from a calf’s nose, so a dugong pushes her calf up to the surface to take in its first gulp of fresh air. A newborn calf is 1.2 metres long and weighs 30kg, staying with its mother for nursing from her two teats for up to 18 months, partially for protection.
Well-preserved cave paintings in the Tambun Cave, Perak, depict these mammals – those paintings are 5,000 plus year’s old. For centuries they were hunted by early man, with their bones used in necklace beads and their tusks gracing dagger and sword handles. Their meat was devoured and various organs taken for traditional medicines. The dugong’s oil was used to preserve the hulls of wooden boats.
The International Union for the Conservation of Nature (IUCN) Red List declares these mammals as vulnerable and conservation areas have been established in which hunting and mesh fishing have been banned.
However, enforcement of such bans is difficult to practise worldwide. The dugongs’ main threat comes from trawler fisheries sweeping their very subsistence, seagrass, away from the sea-beds.
Seawater pollution in the forms of untreated sewerage, agricultural herbicides and detergents together with the excavation of sand for building material have had a major impact on seagrass meadows. The reclamation of land in shallow waters for tourist developments has, too, had an impact on dugong-habitats. Inshore propeller boats, often associated with the tourist trade, have also churned up sand, burying and smothering the eel-grass plants, thus preventing vital sunlight from reaching them. The recovery of these seagrass meadows can take more than 10 years after only one day’s damage.
Like their Asian counterparts, these marine mammals have been in existence for over 60 million years. Unlike dugongs, they spend some of their lives in fresh water, migrating up rivers. Found mostly in Caribbean and South American tropical waters they are also located along the West African coast. Their tails are rounded and their mouths are without tusks and possess molar and premolar teeth, which are replaced from the back of their gums as the frontal teeth decay with age. If only mankind had such genes.
Like the dugong, a manatee possesses a dense, massive bone structure, helping it to submerge. Its lungs lie along its back thus assisting its buoyancy when surfacing for air. Living in mostly saline, brackish and fresh water environments at a depth of two to three metres, they have smaller snouts than dugongs with many more bristles around their mouths, almost seal-like.
Slightly more prolific breeders than dugongs, they reach sexual maturity earlier and with a one-year gestation period again produce one calf.
A newly-born calf is suckled for up to 18 months, always swimming above its mother, and thus is more prone to wounds imposed by overhead boat propellers’ blades. Such propeller cuts have been responsible for 25 per cent of manatee deaths, especially in the booming water-sport industry in Florida, USA. Despite these losses, in the last 26 years, and based on aerial surveillance, there has been a six-fold increase in the population of manatees around peninsular Florida’s coastline.
With relatively limited zoological knowledge, I just wonder whether dugongs can be crossbred with manatees to help build further populations to enchant children and adults for years to come in mermaid legends. Perhaps, and with a bit of luck, I may yet see a dugong.