Sirenian (Order Sirenia)

Sirenian refers to any species of animal that belongs to the taxonomic order of aquatic mammals called Sirenia, which includes manatees and dugongs. Within this taxonomic order, Sirenia is split into five extinct and two living families. The two surviving families are the Dugongidae, which includes one existing species called the dugong, and the Trichechidae, into which all species of manatees are placed. Formerly, there was another species called the Steller's sea cow (Hydrodamalis gigas), which was placed into the Dugongidae family; however, this species was hunted into extinction within thirty years of its initial discovery. The animals of the order Sirenia are often referred to as sea cows.

Characteristics

Ancient mariners used to confuse these animals with mermaids and sirens, an association that gave rise to the name sirenian. Sirens were creatures of Greek mythology that resembled beautiful women and enchanted sailors with their songs, causing them to wreck their vessels onto rocky shores. While sea cows would seem to bear little resemblance to beautiful women, it is speculated that the legends of mermaids and sirens may have been inspired by early sightings of these animals by sailors who mistook them for people. In one such case, Christopher Columbus wrote that his crew had mistaken three manatees for mermaids during his first trip to the New World in 1493. The word dugong even means "lady of the sea" in the Malay language.

Sirenians are herbivorous mammals, which means they only consume plant matter. They have few natural predators outside of humans, and they are typically heavy and move slowly. The species of the order Sirenia are also the only fully aquatic herbivorous mammals in the world. All parts of their life cycle—including eating, sleeping, reproducing, and giving birth—occur in the water. As a result, these animals developed special evolutionary adaptations geared toward living in an aquatic environment. They have long, streamlined bodies suited for swimming in the shallow waters where the seagrasses and other aquatic vegetation grow in abundance. Their bones are very dense, with many parts of their skeletons lacking marrow. This evolutionary adaptation—combined with the presence of long, thin lungs that are evenly distributed along their backbones—helps them manage their buoyancy despite their enormous weight. Sirenians are also able to remain underwater for long periods thanks to nostrils equipped with valves that close underwater and create a seal.

Unlike most land mammals, sirenians have little hair. What hair they do have is specialized to assist them in their aquatic environment. The short, bristle-like hair on their snouts is like tiny fingers that allow them to sense objects underwater, while small hairs along the body enable them to detect changes in water flow and topography.

Species

There are four recognized species of surviving sirenians, and a recently discovered animal tentatively called the dwarf manatee (Trichechus pygmaeus) is being reviewed as a possible fifth species. The three species of manatee in the family Trichechidae are the West Indian manatee (Trichechus manatus), the Amazonian manatee (Trichechus inunguis), and the West African manatee (Trichechus senegalensis). The family Dugongidae contains only a single species, the dugong (Dugong dugon).

There are several differences between dugongs and manatees. Perhaps most important are their choices of aquatic environment; dugongs are strictly marine animals—meaning that they live in saltwater habitats—while the manatees all inhabit freshwater to varying degrees. Dugongs are the only members of the Sirenia order to have tusks. In addition, dugongs have pointed tail flukes like whales and dolphins (although they are not closely related), whereas manatee tails look more like rounded paddles that move up and down. Dugongs are smaller than most species of manatees and are equipped with downward-turning snouts used primarily for bottom feeding. They are also more compact and thinner than manatees. They are located in coastal environments stretching from eastern Africa to northern Australia.

Manatees are typically larger and can weigh between 800 and 1,200 pounds. Manatees also have three to four small residual nails on each flipper, while dugongs lack them entirely. The three manatee species are most obviously differentiated by their distribution. West African manatees are found along the brackish coastal areas of western Africa where fresh and saltwater meet. West Indian manatees, which are found throughout the Caribbean, also prefer brackish waters and are able to move between fresh and saltwater environs thanks to a special internal regulation system that controls how their kidneys process salt content. Amazonian manatees (and dwarf manatees, if they are eventually determined to be a separate species) live solely in the fresh waters of the Amazon River basin. Amazonian manatees are also much smaller than the other two identified species of manatee and typically only grow to be about 500 to 600 pounds.

Topic Today

Sirenians live in coastal and aquatic environments that bring them into regular contact with humans. Historically, humans have occasionally hunted sirenians for their meat and their skins. They have been seen as viable prey by some cultures due to their large size and generally docile behavior. In the twenty-first century, the greatest threats to the survival of sirenian species are habitat loss, pollution, and other human activities that damage the environment.

In the United States, 829 West Indian manatees (from a total estimated population of about 6,500 animals) died in 2013. This was the highest one-year loss since studies of their Florida population began. The majority of these animals are believed to have perished from either old age or toxins such as red tide. Still, scientists expressed uncertainty and concern about what caused the spike in deaths. Such dramatic population losses highlight the fragile nature of sirenian populations around the world. The International Union for the Conservation of Nature (IUCN) classifies all four recognized species of sea cows as vulnerable, meaning that the long-term survival of each species is at risk. Although they have long lives, sea cows are slow breeders and have trouble rebounding from sudden losses in their populations.

However, the gentle nature, charisma, and visibility of the various sirenian species have raised their profile and made them popular subjects for conservation campaigns. In 2017, the US Fish and Wildlife Service estimated that 6,620 Indian manatees were in Florida. This marked a record-high and a welcome increase from the estimated 1,300 animals recorded living in the state in 1991. Because of this increase, Florida manatee was downgraded from endangered to threatened. However, many manatees died since the species protections were reduced. In 2021, 1,100 of the animals, or more than 10 percent of the animal's population in the state, died. Most of the deaths occurred in the East Coast estuary known as the Indian River Lagoon, where pollution led to algal blooms that killed most of the seagrass the animals eat. The 2022 mortality rate was 800 manatees, of which 76 of the slow-moving mammals were determined to have been killed by watercraft. By 2023, the mortality rate had dropped to 518, with 100 of these deaths caused by red tide. However, since the mortality rate was still high, by 2024, the US Fish and Wildlife Service was considering upgrading the Florida manatee's conservation status from threatened to endangered.

Bibliography

"Dugong (Dugong dugon)." National Geographic, animals.nationalgeographic.com/animals/mammals/dugong/. Accessed 11 Oct. 2016.

Frost, Emily. "14 Fun Facts about Manatees." Smithsonian.com, 31 Mar. 2015, www.smithsonianmag.com/science-nature/14-fun-facts-about-manatees-180950308/. Accessed 11 Oct. 2016.

Gheerbrant, Emmanuel, Daryl P. Domning, and Pascal Tassy. "Paenungulata (Sirenia, Proboscidea, Hyracoidea, and Relatives)." The Rise of the Placental Mammals, edited by Kenneth D. Rose and J. David Archibald, Johns Hopkins UP, 2005, pp. 84–105.

Green, Amy. "Four Years Ago, Manatees Were Declared No Longer Endangered. Now They Are Dying at a Record Pace." WUSF Public Media, 14 Sept. 2021, wusfnews.wusf.usf.edu/environment/2021-09-14/four-years-ago-manatees-were-declared-no-longer-endangered-now-they-are-dying-at-a-record-pace. Accessed 19 Jan. 2023.

Kanai, Lang. "How Did Manatees Inspire Mermaid Legends?" National Geographic, 25 Nov. 2014, news.nationalgeographic.com/2014/11/141124-manatee-awareness-month-dugongs-animals-science/. Accessed 11 Oct. 2016.

Kanelos, Matthew James, et al. "Sirenia: Dugongs, Manatees, and Sea Cows." Animal Diversity Web, 2010, animaldiversity.org/accounts/Sirenia/. Accessed 11 Oct. 2016.

"Manatee (Trichechus)." National Geographic, animals.nationalgeographic.com/animals/mammals/manatee/#. Accessed 11 Oct. 2016.

"Manatees: An Educator's Guide." Save the Manatee, 2010, www.savethemanatee.org/Educators‗Guide‗9-10.pdf. Accessed 11 Oct. 2016.

Marsh, Helene, Helen Penrose, and Carol Eros. "A Future for the Dugong?" Marine Mammals: Fisheries, Tourism and Management Issues, edited by Nick Gales, Mark Hindell, and Roger Kirkwood, Csiro Publishing, 2003, pp. 383–99.

Marsh, Helene, Thomas J. O'Shea, and John E. Reynolds III. Ecology and Conservation of the Sirenia: Dugongs and Manatees. Cambridge UP, 2011.

van Roosmalen, Marc G.M. "Hotspot of New Megafauna Found in the Central Amazon: The Lower Rio Aripuanã Basin." Biodiversity Journal, vol. 6, no. 1, 2015, pp. 219–244.

Williams, Kylie. "The Manatee May Re-Enter Endangered Species List After Population Decline." The Independent Florida Alligator, 16 Jan. 2024, www.alligator.org/article/2024/01/manatee-may-reenter-endangered-species-list-after-population-decline. Accessed 25 Nov. 2024.