Dugongs descended from terrestrial mammals that browsed in shallow grassy swamps during the Eocene and their closest modern relative is the elephant. Their smooth skin is slate-grey in colour and their body is stream-lined, with a fluke-shaped tail and a rounded head. Adults are large and can grow to 3.5 meters long and weigh up to 400 kg. Sensitive bristles covering the upper lip and a broad flat muzzle are used to uproot seagrasses, which form their main diet. They surface to breathe every few minutes using paired, valve-like nostrils positioned on the top of the head. Their lifespan is estimated to be about 70 years.
Dugongs are slow to reproduce. Both males and females become sexually mature at about 10 years of age although some females mature as late as 17. A dugong cow produces a single calf every 2.5 to 5 years, after a gestation period of 14 months. Calving occurs in the shallow waters of tidal sandbanks. A newborn calf measures 1.2 m in length, weighs approximately 30 kg and relies primarily on its mother’s milk for up to 18 months.
In the 1960’s and 70’s dugongs were seen regularly along the Tanzania coast in herds of up to 30 – 40 animals. However, after years of subsistence hunting, populations have decreased dramatically. Dugongs are now extremely rare in Tanzania and the Rufiji Delta - Mafia Island seascape is their last known refuge. Their habitat requirements and slow rate of reproduction render them particularly vulnerable to anthropogenic activities. Incidental capture in gill nets poses the biggest single threat to their survival in Tanzania, together with degradation of critical seagrass habitat casued by destructive fishing practices and pollution from land based sources.