Sirenians: manatees and dugongs – January 2021

This month, we will talk about the sirenians: manatees and dugongs. These slow and peaceful “herbivores”, also called sea cows, have elephants as their closest land relatives. In this post, we highlight some mostly unknown facts about the sirenian family and hope to raise awareness about the threats they face today.

Credit: pixabay

They are not strict vegans

Their diet mainly consists of different types of seagrasses. Although almost entirely herbivorous, dugongs will occasionally eat invertebrates such as sea jellies, sea squirts, and shellfish. Some populations of dugongs, such as the one around Moreton Bay, Australia, are omnivorous and will actively seek out large invertebrates. Manatees are known to also eat small amounts of fish out of nets.

Although they mostly eat seagrass, some sirenians also hunt invertebrates and fish – Credit: Julien Willem/Wikimedia Commons

The Sirenia order includes two distinct families

The Dugongidae (the dugong and, the now extinct Steller’s sea cow) and Trichechidae (manatees; namely the Amazonian manatee, West Indian manatee, and West African manatee) families make up the Sirenia order. This is the smallest order of all marine mammals.

You can tell the difference between families by their flukes (tails). Dugongs have a notched fluke, like a dolphin’s tail. Manatees have a paddle-shaped fluke.

Why the name sirenians? As they are often seen close to shore, near the surface, they are thought to be the inspiration behind the mermaid myths. Lonely sailors back in the days would confuse sirenians with beautiful half-fish women… Interestingly, though their order carries the name siren, sirenians do not depend on sound to find their food, like dolphins use echolocation for example.

They have a large habitat range

Sirenians have a strong preference for warm waters. However, there is a difference between dugong and manatee habitats. Dugongs are exclusively marine, while manatees inhabit both marine and freshwater systems. They live in swamps, rivers, estuaries, marine wetlands, and coastal marine water. Sirenians have no dorsal fin. No need, really, when you can navigate calm freshwaters instead of strong ocean waves.

Habitat ranges of sirenian species: West Indian manatee in green, Amazonian manatee in red, African manatee in orange, dugong in blue, Steller’s sea cow circled (yellow).

Steller’s sea cows, a now-extinct species of the dugong family, was the largest sirenian and one of the few to live in cold water. They preferred waters in the Bering Sea (yellow circle above). Unfortunately, they were hunted to extinction by 1741, less than 30 years after they were first discovered.

They have some of the densest bones in the animal kingdom

Sirenians’ ribs and other long bones are solid and contain little to no bone marrow; a condition called pachyostosis. This special adaptation helps counteract the buoyancy effects from their thick skin. The balance of mass helps keep them suspended slightly below the surface where they are safer from the eyes of hunters.

On the topic of interesting anatomy, manatees do not possess blubber, per se, but rather have thick skin. As a result, they are sensitive to temperature changes and will migrate to warmer waters when the water temperature dips below 20 °C (68 °F). Also, despite being mammals, all sirenians are nearly hairless, except for the thick sensory hairs called vibrissae around their muzzle. Think of the whiskers of a cat.

Sirenians face threats today

All sirenian species are rated as vulnerable on the IUCN Red List of endangered species. By the same token, the Marine Mammal Protection Act of 1972 (MMPA), the Endangered Species Act of 1973 (ESA), and the Convention on the International Trade in Endangered Species of Wild Fauna and Flora (CITES) offer protections for all sirenians.

Sirenians have no natural predators. Humans are sadly the main threat to sirenians’ survival. Habitat loss and other negative impacts from human population growth and coastal development are the main risks. Developments from the increased usage of hydroelectric power and the damming of rivers can increase boat traffic and can lead to boat collisions.

Climate change and ocean warming also pose significant threats. Seagrass needs shallow, relatively clear water in order to grow. High temperatures translate to sea-level rise which, in turn, increases water turbidity and decreases water quality. Bad quality and murky waters could lead to less seagrass and fewer sirenians.

Manatees may also be displaced, suffer delayed health effects, or even be killed due to ecosystem changes from intense storms. Unfortunately, storms are getting stronger and more frequent due to climate change. However, there are solutions in development that could benefit both humans and sirenians. Indeed, healthy wetlands help prevent coastal flooding, filter pollutants, and protect shorelines from erosion. Investing in the health and resilience of our ecosystems is investing in the future. Every proactive step we take will help safeguard the future for sirenians and ourselves.

Credit: Pixabay

Resources and further reading

Brianna is a marine biologist and a recent graduate from the University of Auckland with a postgraduate diploma in marine science. Her research interests include identifying marine mammal vocalizations and marine conservation in the high-seas and polar regions. She is passionate about music and can't write without coffee.

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