Sirenia

Sirenia (commonly known as sea cows, from the Sirens of Greek mythology) is an order of fully aquatic, obligate herbivorous placental mammals within the superorder Afrotheria. Together with their terrestrial sister order Proboscidea (elephants) and the Hyracoidea (hyraxes), Sirenia forms the clade Paenungulata / Tethytheria, having diverged along the tropical shores of the ancient Tethys Ocean during the Early Eocene (~55–50 million years ago).

Sirenians are the only extant marine mammals that feed almost primarily on aquatic plants (principally marine seagrass meadows and freshwater vascular macrophytes). Like cetaceans (Cetacea), sirenians evolved a fusiform, hairless body lacking external hindlimbs, paddle-like or fluke-shaped horizontal tail fins, and obligately aquatic birth and nursing; however, their anatomy and physiology are specialized for slow, shallow-water benthic grazing rather than high-speed pelagic pursuit.

Only four extant species survive across two families (all listed as Vulnerable or Endangered on the IUCN Red List), alongside one historically extinct North Pacific giant:

1. Family Trichechidae (Manatees; 3 extant species): The West Indian manatee (Trichechus manatus, including the Florida manatee T. m. latirostris and Antillean manatee T. m. manatus), the Amazonian manatee (Trichechus inunguis), and the African manatee (Trichechus senegalensis).
2. Family Dugongidae (Dugongs and Sea Cows; 1 extant + 1 recently extinct species): The Indo-Pacific dugong (Dugong dugon) and the giant sub-Arctic kelp-eating Steller's sea cow (Hydrodamalis gigas, which reached 8 to 10 meters in length and 8 to 11 tonnes before being hunted to complete extinction in 1768, just 27 years after its scientific description by Georg Wilhelm Steller on Bering Island).

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The fossil record preserves an unbroken transition from amphibious four-legged Eocene waders to fully marine grazers:

Order Sirenia (Tethytheria / Afrotheria; Early Eocene origin ~52 Ma) ├── †Family Prorastomidae (*Prorastomus*, *Pezosiren portelli*; Early–Middle Eocene of Jamaica, ~50–47 Ma) │ └── 2.1-m pig-sized quadrupeds capable of walking on land with four functional legs, a long tail, and a multicuspid │ dentition, but already possessing **pachyosteosclerotic heavy ribs** and retracted dorsal nostrils for bottom wading! │ ├── †Family Protosirenidae (*Protosiren*; Middle Eocene of Egypt, India, & North America; reduced pelvic girdle) │ └── Crown Sirenia (Late Eocene–Recent: hindlimbs internal/vestigial; horizontal caudal fin) ├── Family Trichechidae (Manatees; Oligocene–Recent, South American freshwater/coastal cradle) │ └── Genus *Trichechus* (*T. manatus*, *T. inunguis*, *T. senegalensis*): rounded paddle tail, **6 cervical vertebrae**, │ and continuous **"marching molars"** (up to 30 supernumerary molars replaced horizontally across life) │ └── Family Dugongidae (Dugongs, Halitheriines, & Hydrodamalines; Eocene–Recent) ├── Subfamily Dugonginae (*Dugong dugon*): deeply notched **whale-like tail fluke**, sharply **down-turned (70°) rostrum**, │ 7 cervical vertebrae, and erupting ivory tusks in males └── †Subfamily Hydrodamalinae (*Dusisiren* → *Hydrodamalis gigas* [Steller's sea cow, †1768]): └── Evolved in the cooling Miocene–Pleistocene North Pacific to graze on cold-water **[kelp forests](/wiki/phaeophyceae)**; fully **toothless** (grinding kelp between keratinized palatal pads), 10-cm blubber, and bark-like skin *`

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Because marine plants and seagrasses are restricted to the sunlit shallow euphotic zone (1 to 15 meters depth), where trapped lung air creates strong positive upward buoyancy, sirenians evolved a unique passive hydrostatic ballast system rather than expending muscular energy to stay on the sea floor:

1. Pachyosteosclerosis (Swollen, Solid-Cortical Bone): Already present in 50-million-year-old Pezosiren, sirenian ribs and long bones exhibit pronounced pachyostosis (thickened, banana-like swollen cross-sections) combined with osteosclerosis (complete replacement of the internal spongy cancellous bone and bone marrow cavities with dense, avascular Haversian cortical bone, reaching a mineral density of 2.1–2.3 g/cm³). Because their ribs and long bones lack marrow cavities, hematopoiesis (red and white blood cell production) in sirenians is restricted almost entirely to the vertebral bodies and sternum!
2. Dorsal Horizontal Lungs and Hemidiaphragmatic Buoyancy Trim: Unlike terrestrial mammals (where the diaphragm separates the anterior thoracic cavity from the posterior abdomen transversely), the sirenian diaphragm runs horizontally along the length of the dorsal body cavity. Two separate, elongated 1-meter lungs lie flat along the dorsal spine directly beneath the back, while the heavy pachyosteosclerotic ribs hang ventrolaterally below them like a ship's keel. By selectively contracting the muscular hemidiaphragms to compress air anteriorly or posteriorly along the horizontal lungs, a manatee or dugong can pitch its body angle up, down, or hover motionless in neutral buoyancy with zero flipper movement.

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• The Orfacial Muscular Hydrostat ("Prehensile Mustache"): Closely homologous to the elephant trunk (Proboscidea), the sirenian upper lip is a split, bilobed muscular hydrostat studded with ~600 thick, stiff, innervated vibrissae (whiskers)—the only hairs on the sirenian body, each connected to a dense complex of Follicle-Sinus mechanoreceptors (up to 30 times more nerve fibers per follicle than typical mammalian whiskers). Manatees use the left and right halves of the upper lip like opposable, prehensile fingers to grasp and pull seagrass blades into the mouth at rates of 10–15% of their body weight in wet vegetation per day (40–60 kg/day).
• Endless Horizontal Molar Progression in Manatees: Because coastal seagrasses are coated in abrasive quartz sand and siliceous epiphytic diatoms (Bacillariophyta) that rapidly grind down tooth crowns, manatees (Trichechus) evolved indeterminate polyphyodont horizontal tooth replacement: new molars form continuously inside a dental crypt at the back of each jaw, migrate forward at ~1 to 2 mm per month as interdental bone septa resorb in front of them and re-ossify behind them, and drop out at the front of the mouth once worn down to the roots (20 to 30+ molars per jaw quadrant over a 60-year lifespan).
• Giant Hindgut Cecum–Colon Fermentation: Despite consuming a fibrous cellulose/hemicellulose diet, sirenians are hindgut fermenters (like elephants and horses) rather than foregut ruminants: their simple stomach (bearing a specialized cardiac gland that secretes concentrated pepsin and hydrochloric acid while protecting the main stomach mucosa) empties into a elongated 15- to 45-meter-long large intestine (cecum and colon) where cellulolytic bacteria ferment seagrass into volatile fatty acids (acetate, propionate, butyrate) over a 5- to 9-day gut transit time—yielding a digestive efficiency for cellulose exceeding 80%, the highest recorded in any herbivorous mammal!

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Because seagrass is a low-energy food source, Florida manatees (Trichechus manatus latirostris) maintain an exceptionally low basal metabolic rate (only 25–36% of the Kleiber prediction for a 500-kg mammal) combined with a high lower essential water temperature (~20 °C / 68 °F) and low thermal conductance insulation. Whenever winter water temperatures drop below 20 °C for more than a few days, manatees cannot upregulate metabolic heat production and suffer lethal manatee cold-stress syndrome (peripheral vasoconstriction, epidermal necrosis, lymphoid depletion, and secondary pneumonia), forcing winter aggregations at natural 22 °C artesian karst springs (such as Crystal River and Blue Spring) or warm-water coastal power-plant outfalls. Combined with watercraft propeller strikes, entanglement, red-tide (Karenia brevis) brevetoxin blooms, and nutrient-driven seagrass die-offs (as in the Indian River Lagoon), habitat protection of warm-water refugia and seagrass meadows remains essential to sirenian survival.

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1. Domning, D. P. (2001). The earliest known fully quadrupedal sirenian. Nature, 413(6856), 625–627. Google Scholar
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• Proboscidea
• Mammalia
• Seagrass Meadow
• Phaeophyceae
• Cetacea