The Sirenia ( ), commonly referred to as sea cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The extant Sirenia comprise two distinct families: 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) with a total of three species.
The Protosirenidae (Eocene sirenians) and Prorastomidae (terrestrial sirenians) families are extinct. Sirenians are classified in the clade Paenungulata, alongside the elephants and the hyraxes, and evolved in the Eocene 50 million years ago (mya). The Dugongidae diverged from the Trichechidae in the late Eocene or early Oligocene (30–35 mya).
Sirenians grow to between in length and in weight. The recently extinct Steller's sea cow was the largest known sirenian to have lived, reaching lengths of and weights of .
Sirenians have a large, fusiform body which reduces drag through the water and heavy bones that act as ballast to counteract the buoyancy of their blubber. They have a thin layer of blubber and consequently are sensitive to temperature fluctuations, which cause large-scale migrations when water temperatures dip too low. Sirenians are slow-moving, typically coasting at , but they can reach in short bursts. They use their strong lips to pull out seagrasses, consuming 10–15% of their body weight per day.
While breathing, sirenians hold just their nostrils above the surface, sometimes standing on their tails to do so. They typically inhabit warm, shallow, coastal waters, or rivers. They are mainly herbivorous, but have been known to consume animals such as birds and jellyfish. Males typically mate with more than one female and may gather in leks to mate. Sirenians are K-selected, displaying parental care.
The meat, oil, bones, and skins of sirenians are commercially valuable. Mortality is often caused by direct hunting from humans or by other human-induced causes, such as habitat destruction, entanglement in fishing gear, and watercraft collisions. Steller's sea cow was finally driven to extinction due to overhunting in 1768.
Taxonomy Etymology Sirenia, commonly sirenians, are also referred to by the common name sirens, deriving from the sirens of Greek mythology.What are sirenians? Sirenian International - Manatee & Dugong Research, Education, & Conservation
Classification Sirenians are classified within the cohort Afrotheria in the clade Paenungulata, alongside Proboscidea (elephants), Hyracoidea (hyraxes), Embrithopoda, Desmostylia, and Afroinsectiphilia. This clade was first established by George Gaylord Simpson in 1945 on the basis of anatomical evidence, such as testicondy and similar fetal development. The Paenungulata, along with the Afrotheria, are one of the most well-supported mammalian clades in molecular phylogeny. Sirenia, Proboscidae, and Desmotylia are grouped together in the clade Tethytheria. On the basis of morphological similarities, Tethytheria, Perissodactyla, and Hyracoidea were previously thought to be grouped together as the Altungulata, but this has been invalidated by molecular data.
Sirenia families † = Extinct
Family Dugongidae:
Genus Dugong D. dugon Genus †Anisosiren †A. pannonica Genus †Indosiren †I. javanense Genus †Bharatisiren †B. indica Genus †Callistosiren †C. boriquensis Genus †Crenatosiren †C. olseni Genus †Corystosiren †C. varguezi Genus †Dioplotherium †D. allisoni †D. manigualti Genus †Domningia †D. sodhae Genus †Kutchisiren †K. cylindrica Genus †Nanosiren †N. garciae †N. sanchezi Genus †Rytiodus †R. capgrandi †R. heali Genus †Xenosiren †X. yucateca Genus †Caribosiren †C. turneri Genus †Halitherium †H. alleni †H. schinzii Genus †Paralitherium †P. tarkanyense Genus †Priscosiren †P. atlantica Genus †Sirenavus †S. hungaricus Genus †Metaxytherium †M. albifontanum †M. arctodites †M. crataegense †M. floridanum †M. krahuletzi †M. medium †M. serresii †M. subapenninum Genus †Dusisiren †D. dewana †D. jordani †D. reinharti †D. takasatensis Genus †Hydrodamalis †H. cuestae †H. gigas
Family Trichechidae:
Genus Trichechus T. manatus T. senegalensis T. inunguis Genus †Anomotherium †A. langewieschei Genus †Miosiren †M. canhami †M. kocki Genus †Potamosiren †P. magdalenensis Genus †Ribodon †R. limbatus
†Family Protosirenidae:
Genus †Ashokia †A. antiqua Genus †Libysiren †L. sickenbergi Genus †Protosiren †P. eothene †P. fraasi †P. minima †P. sattaensis †P. smithae
†Family Prorastomidae:
Genus †Pezosiren †P. portelli Genus †Prorastomus †P. sirenoides
+ Extant order Sirenia – two genera, four speciesCommon name Genus Scientific nameStatusDistributionPictureWest Indian manatee Trichechus (manatees)T. manatus Linnaeus, 1758African manatee Trichechus (manatees)T. senegalensis Link, 1795Amazonian manatee Trichechus (manatees)T. inunguis Natterer, 1883Dugong DugongD. dugon Müller, 1776 Distribution
The warm shallow waters of the equator have been the center of sirenian habitation. The northernmost living population, the Florida subspecies of the West Indian manatee (T. manatus latirostris), inhabits the coast and frequents freshwater springs, power plants, and canals in Florida to stay warm during the winter. Individuals may migrate north in the warm summer months, some up to 1,000 kilometers (about 621.37 mi) from their winter range. The Antillean subspecies (T. manatus manatus) occurs in the Caribbean, South America, and Central America and frequent drowned cays, mangroves, lagoons, and sea grass beds.
The Amazonian manatee (T. inunguis) has been documented in all parts of the Amazon River Basin in South America. River channels that connect allow easy travel to other waterways where food may be plentiful. The Amazonian manatee lives only in freshwater.
The West African manatee (T. senegalensis) lives in murky isolated inland mangroves and coastal flats in West Africa. It is found in waters above , and its range spans Senegal to Angola.
The dugong (Dugong dugong), the closest living relative of Steller's sea cow, lives in the Indo-West Pacific Ocean in more than 40 different countries. They are coastal animals supported by wide protected sea grass meadows.
Steller's sea cow was discovered in 1741 around islands in the Bering Sea and was specialized for cold subarctic temperatures. It ranged from Alaska through the Amchitka and Aleutian Islands, and even to Japan. Steller's sea cow was reported to have congregated in shallow, sandy areas along coastline and mouths of rivers and creeks to feed on kelp.
Evolution
The evolution of sirenians is characterized by the appearance of several traits that are found in all sirenians. The nostrils are large and retracted, the upper-jaw bone contacts the frontal bone, the sagittal crest is missing, the mastoid fills the supratemporal fenestra (an opening on the top of the skull), there is a drop-like ectotympanic (a bony ring that holds the ear drum), and the bones are pachyosteosclerotic (dense and bulky).
Sirenians first appeared in the fossil record in the Early Eocene and diversified throughout the epoch. They inhabited rivers, estuaries, and nearshore marine waters. Sirenians, unlike other marine mammals such as cetaceans, lived in the New World. In Western Europe the first and oldest sirenian remains have been found in a new paleontological site, in Santa Brígida, Amer (La Selva, Catalonia, SpainFuentes-Buxó, R., Fuentes-Buxó, A. 2016. Finding of sirenian remains in the Lutetian (Middle Eocene) of Santa Brígida (Amer, La Selva, Girona). Treballs del Museu de Geologia de Barcelona, 22: 19-24 .). One of the earliest aquatic sirenians discovered is Prorastomus, which dates back to 40 million years ago, and the first known sirenian, the quadruped Pezosiren, lived 50 million years ago. An ancient sirenian fossil of a petrosal bone was found in Tunisia, dating back to approximately the same time as Prorastomus. This is the oldest sirenian fossil to be found in Africa and supports molecular data suggesting that sirenians may have originated in Africa. Prorastomidae and Protosirenidae, the earliest sirenian families, consisted of pig-like amphibious creatures who died out at the end of the Eocene. With the appearance of the Dugongidae at this time, sirenians had evolved the characteristics of the modern order, including an aquatic, streamlined body with flipper-like fore limbs and no hind limbs, and a powerful tail with horizontal caudal fins which uses an up-and-down motion to move them through the water.
The last of the sirenian families to appear, Trichechidae, apparently arose from early dugongids in the late Eocene or early Oligocene. In 1994, the family was expanded to include not only the subfamily Trichechinae (Potamosiren, Ribodon, and Trichechus), but also Miosireninae (Anomotherium and Miosiren). The African manatee and the West Indian manatee are more closely related to each other than to the Amazonian manatee.
Dugongidae comprises the subfamilies Dugonginae and Hydrodamalinae and the paraphyletic Halitheriinae. The tusks of modern-day dugongs may have originally been used for digging, but they are now used for social interaction. The genus Dugong probably originated in the Indo-Pacific.
Description Adaptations
The tail fluke of a dugong is notched and similar to those of dolphins, whereas the tail fluke of manatee is paddle-shaped. The fluke is pumped up and down in long strokes to move the animal forward, or twisted to turn. The forelimbs are paddle-like flippers which aid in turning and slowing. Unlike manatees, the dugong lacks nails on its flippers, which are only 15% of a dugong's body length. Manatees generally glide at speeds of , but can reach speeds of in short bursts. The body is fusiform to reduce drag in the water. Like those of cetaceans, the hind limbs are internal and vestigial. The snout is angled downwards to aid in bottom-feeding. Sirenians typically make two- to three-minute dives, but manatees can hold their breath for up to 15 minutes while resting and dugongs up to six minutes. They may stand on their tails to hold their heads above water.
Much like elephants, manatees are polyphyodonts, continuously replacing their teeth from the back of the jaw. Adults lack incisors, canines, and premolars, and instead have eight to ten cheek teeth. Manatees have an unlimited supply of teeth moving in from the back and shedding in the front; these are continuously formed by a dental capsule behind the tooth row. These teeth are constantly worn down by the abrasive vascular plants they forage, particularly aquatic grasses. Unlike those of manatees, the dugong's teeth do not continually grow back via horizontal tooth replacement. The dugong has two tusks which emerge in males during puberty, and sometime later in life for females after reaching the base of the premaxilla. The number of growth layer groups in a tusk indicates the age of a dugong.
Sirenians exhibit pachyostosis, a condition in which the ribs and other long bones are solid and contain little or no bone marrow. They have among the densest bones in the animal kingdom. These may act as ballast, countering the buoyancy of their blubber and helping them remain suspended slightly below the water's surface. Manatees do not possess blubber per se, but rather have thick skin and consequently are sensitive to temperature changes. They often migrate to warmer waters whenever the water temperature dips below . The lungs of sirenians are unlobed; along with the diaphragm, these extend the entire length of the vertebral column, helping the animals control their buoyancy and reducing tipping in the water.