Sirenia

The Sirenia (/saɪˈriːniə/), 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 Sirenia currently 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 four species.[2] 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).[3][4]

Sirenia
Temporal range: Early Eocene – Holocene,
Clockwise from upper left: West Indian manatee, African manatee, Steller's sea cow, dugong, Amazonian manatee
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Superorder: Afrotheria
Clade: Paenungulata
Clade: Tethytheria
Order: Sirenia
Illiger, 1811
Families
  • Dugongidae
  • Trichechidae
  • †Prorastomidae
  • †Protosirenidae
West Indian in green, Amazonian in red, African in orange, dugong in blue, Steller's sea cow circled (yellow)
Synonyms[1]
  • Halobioidea Ameghino, 1889
  • Herbivorae Gray, 1821
  • Phycoceta Haeckel, 1866
  • Sirenoidea van Beneden, 1855
  • Sireniformes Kinman, 1994
  • Trichechiformes Hay, 1923

Sirenians grow to between 2.5 and 4 metres (8.2 and 13.1 feet) in length and 1,500 kilograms (3,300 pounds) in weight. The historic Steller's sea cow was the largest known sirenian to have lived, and could reach lengths of 10 metres (33 feet) and weights of 5 to 10 tonnes (5.5 to 11.0 short tons).[2]

Sirenians have a large, fusiform body to reduce drag through the water. They have 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 migrations when water temperatures dip too low. Sirenians are slow-moving, typically coasting at 8 kilometres per hour (5.0 miles per hour), but they can reach 24 kilometres per hour (15 miles per hour) in short bursts. They use their strong lips to pull out seagrasses, consuming 10–15% of their body weight per day.

While breathing, they hold just their nostrils above the surface, sometimes standing on their tails to do so. Sirenians 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 (polygyny), and may participate in lek mating. Sirenians are K-selected, and display parental care.

Dugong skeleton displayed at Philippine National Museum

The meat, oil, bones, and skins are valuable items sold in markets. Mortality is often caused by direct hunting by humans or other human-induced causes, such as habitat destruction, entanglement in fishing gear, and watercraft collisions. Steller's sea cow was 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.[5][6] This comes from a legend about their discovery, involving lonely sailors mistaking them for mermaids.[7] Seekoei (sea cow) is also the name for a hippopotamus in Afrikaans.[8]

Classification

Sirenians are classified within the cohort Afrotheria in the clade Paenungulata, alongside Proboscidea (elephants), Hyracoidea (hyraxes), Embrithopoda, Desmostylia, and Afroinsectiphilia.[9][10][11][12] This clade was first established by George Gaylord Simpson in 1945 based on 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.[13] Sirenia, Proboscidae, and Desmotylia are grouped together in the clade Tethytheria. Based on morphological similarities, Tethytheria, Perissodactyla, and Hyracoidea were considered to be grouped together as the Altungulata, but this has been invalidated by molecular data.[10]

Afrotheria
Afroinsectiphilia
Tubulidentata

Orycteropodidae

 
Afroinsectivora
Macroscelidea

Macroscelididae

 
Afrosoricida
 

Chrysochloridae

 
Tenrecomorpha
 

Potamogalidae

 
 

Tenrecidae

 
 
 
 
 
Paenungulata
Hyracoidea

Procaviidae

 
Tethytheria
Proboscidea

Elephantidae

 
Sirenia
 

Dugongidae

 
 

Trichechidae

 
 
 
 
 
A cladogram of the Sirenia within Afrotheria based on molecular evidence[9]

Sirenia families

† = Extinct

Family Dugongidae:[14]

  • Genus Dugong
    • D. dugon
  • Genus †Anisosiren
    • †A. pannonica
  • Genus †Indosiren
    • †I. javanense
  • Genus †Bharatisiren
    • †B. indica
  • Genus †Callistosiren[15]
    • †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[16]
    • †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:[17]

  • Genus Trichechus
  • Genus †Anomotherium
    • †A. langewieschei
  • Genus †Miosiren
    • †M. canhami
    • †M. kocki
  • Genus †Potamosiren
    • †P. magdalenensis
  • Genus †Ribodon
    • †R. limbatus

†Family Protosirenidae:[18]

  • Genus †Ashokia
    • †A. antiqua
  • Genus †Libysiren
    • †L. sickenbergi
  • Genus †Protosiren
    • †P. eothene
    • †P. fraasi
    • †P. minima
    • †P. sattaensis
    • †P. smithae

†Family Prorastomidae:[19]

  • Genus †Pezosiren
    • †P. portelli
  • Genus †Prorastomus
    • †P. sirenoides
Extant order Sirenia – two genera, four species
Common nameGenusScientific nameStatusDistributionPicture
West Indian manatee Trichechus (manatees) T. manatus Linnaeus, 1758 VU IUCN
African manatee Trichechus (manatees) T. senegalensis Link, 1795 VU IUCN
Amazonian manatee Trichechus (manatees) T. inunguis Natterer, 1883 VU IUCN
Dugong Dugong D. dugon Müller, 1776 VU IUCN

Distribution

The warm waters of the equator have been the center of Sirenian habitation and tend to live in warm shallow waters.[20] Florida is the most Northern territory of the Florida Manatee subspecies because of temperature barriers limiting its range. To regulate temperature in cold winters the Florida Manatee frequents springs, power plants, and warm canals. The Florida Manatee is a costal animal that travels into freshwater rivers, springs, and bays. Individuals may migrate North in the warm summer months, some up to 1,000 kilometers (about 621.37 mi) from their normal wintering site.[21] The Antillean manatee (Trichechus manatus manatus) are Caribbean, South America, and Central America and frequent drowned cays, mangroves, lagoons, and sea grass beds.[22] The Amazonian Manatee (Trichechus inunguis) has been documented in all parts of the Amazon River Basin in South America. River channels that connect allow for easy travel to other waterways where food may be plentiful. The Amazonian manatee lives only in fresh bodies of water.[23] The West African Manatee live in murky isolated inland mangroves and costal flats in West Africa. They are found in waters above 18 °C and their range spans from Senegal to Angola.[24]

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.[25] Steller's sea cow was discovered in 1741 along the Bearing Islands and was specialized in cold sub arctic temperatures of the Bering Sea. Steller's sea cow was found to have ranges from Alaska, along the Amchitka and Aleutian Islands, and even Japan.[26] Steller's sea cow was reported to have congregated in shallow, sandy areas along coastline and mouths to rivers in creeks to feed on kelp.[26]

Evolution
Cladogram showing the estimated times of divergence between sirenian taxa
Anatomical changes of sirenian lineages

The evolution of sirenians is characterized by the appearance of several traits, which are found in all sirenians (monophyly). 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), a drop-like ectotympanic (a bony ring that holds the ear drum), and pachyosteosclerotic (dense and bulky) bones.[10]

Sirenians first appeared in the fossil record in the Early Eocene and significantly diversified throughout the epoch. They inhabited rivers, estuaries, and nearshore marine waters.[27] Sirenians, unlike other marine mammals such as cetaceans,[28] lived in the New World. 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.[27] An ancient sirenian fossil of a petrosal bone was found in Tunisia, dating back to approximately the same time as Prorastomus.[29] This is the oldest sirenian fossil to be found in Africa and supports molecular data suggesting that sirenians may have originated in Africa.[29] Prorastomidae and Protosirenidae, the earliest sirenian families, consisted of pig-like amphibious creatures who died out at the end of the Eocene. When the Dugongidae appeared at this time, sirenians had evolved the characteristics of modern variety, including an aquatic streamlined body with flipper-like front legs with no hind limbs, and a powerful tail with horizontal caudal fins which uses an up-and-down motion to move them through the water.[30]

The last of the sirenian families to appear, Trichechidae, apparently arose from early dugongids in the late Eocene or early Oligocene. It is a monophyletic taxon. In 1994, the family was expanded to include not only the subfamily Trichechinae (Potamosiren, Ribodon, and Trichechus),[31] 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.[10]

Dugongidae comprises the subfamilies Dugonginae and Hydrodamalinae (which are both monophyletic) 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 area.[10]

Description

Adaptations
See also: Aquatic locomotion
The paddle-shaped fluke of a manatee (left) vs. that of a dugong (right)

The tail fluke of a dugong is notched and similar to those of dolphins, whereas the tail fluke of manatees is paddle-shaped.[10] The fluke is raised 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.[30][32] Unlike manatees, the dugong lacks nails on its flippers, which are only 15% of a dugong's body length.[33] Manatees generally glide at speeds of 8 kilometres per hour (5 mph), but can reach speeds of 24 kilometres per hour (15 mph) in short bursts.[34] The body is fusiform to reduce drag in the water. Like cetaceans, the hind limbs are internal and vestigial. The snout is angled downwards to aid in bottom-feeding.[35] Sirenians typically make two- to three-minute dives,[36] but manatees can hold their breath for up to 15 minutes while resting[34] and dugongs up to six minutes. They may stand on their tails to hold their heads above water.[37]

A manatee skull (left) vs. a dugong skull (right)

Much like elephants, manatees are polyphyodonts, and continuously replace their teeth from the back of the jaw. Adults lack incisors, canines, and premolars, and instead have 8 to 10 cheek teeth in their mouth. Manatees have an infinite supply of teeth moving in from the back and shedding in the front, which 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 in manatees, the dugong's teeth do not continually grow back via horizontal tooth replacement.[38] 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.[33] The number of growth layer groups in a tusk indicates the age of a dugong.[39]

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.[40] Manatees do not possess blubber, per se, but rather have thick skin, and, consequently, are sensitive to temperature changes. Likewise, they often migrate to warmer waters whenever the water temperature dips below 20 °C (68 °F). The lungs of sirenians are unlobed;[41] they, along with the diaphragm, extend the entire length of the vertebral column, which help them control their buoyancy and reduce tipping in the water.[42][43]

Extant sirenians grow to between 2.5 and 4 metres (8.2 and 13.1 ft) in length and can weigh up to 1,500 kilograms (3,300 lb). Steller's sea cow was the largest known sirenian to have lived, and could reach lengths of 9 metres (30 ft),[41] and could weigh in at 8 to 10 tonnes (8.8 to 11.0 short tons).[44] A dugong's brain weighs a maximum of 300 grams (11 ounces), about 0.1% of the animal's body weight.[33] The body of sirenians is sparsely covered in short hair (vibrissae), except for on the muzzle, which may allow for tactile interpretation of their environment.[45] Manatees are the only known organism with uniformly vascularized corneas. This may be the result of irritations from or protection against their hypotonic freshwater environment.[46]

Diet
Dugongs sift through the seafloor in search of seagrasses.

Sirenians are referred to as "sea cows" because their diet consists mainly of seagrass. Dugongs sift through the seafloor in search of seagrasses. it use their sense of smell to find the seagrass because their eyesight is poor.[47] They ingest the whole plant, including the roots,[48] although they will feed on just the leaves if this is not possible.[39] Manatees, in particular the West Indian manatee, are known to consume over 60 different freshwater and saltwater plants, such as shoalweed, water lettuce, muskgrass, manatee grass, and turtle grass. Using their divided upper lip, an adult manatee will commonly eat up to 10–15% of their body weight, or 50 kilograms (110 lb), per day, which requires the manatee to graze for several hours per day.[49] However, 10% of the diet of the African manatee is fish and mollusks.[50] Manatees have been known to eat small amounts of fish from nets.[51] As opposed to bulk feeding, dugongs target high-nitrogen grasses to maximize nutrient intake, and, although almost completely herbivorous, dugongs will occasionally eat invertebrates such as jellyfish, sea squirts, and shellfish. Some populations of dugongs, such as the one in Moreton Bay, Australia, are omnivorous, feeding on invertebrates such as polychaetes[48] or marine algae when their supply of seagrasses is low. In other dugong populations in western and eastern Australia, there is evidence that dugongs actively seek out large invertebrates.[39] Populations of Amazonian manatees become restricted to lakes during the July–August dry season when water levels begin to fall, and are thought to fast during this period. Their large fat reserves and low metabolic rates—only 36% of the usual placental mammal metabolic rate—allow them to survive for up to seven months with little or no food.[52]

Feeding Behavior

Perioral bristles are not only used to sense things, they can be used to grasp and manipulate food. The microvibrissae is how they manipulate the food and the macrovibrissae is how they detect food by its size. They can be used to break off leaves and even undesirable parts while the manatee is feeding. They also have muscular hydrostat which is a biological structure that relies on muscular pressure and muscle contractions to be able to manipulate and move food. An example of this is not only a manatee's snout but an elephant's trunk. The way they use their perioral bristles is by using their large upper bristles to do a grasping motion, then they perform a flare which tightens their muscular hydrostat while their large upper bristles get pushed out and the lower jaw drops and sweeps the vegetation in by closing. This is then repeated.

Reproduction

Despite being mostly solitary, sirenians congregate in groups while females are in estrus. These groups usually include one female with multiple males. Sirenians are K-selectors, so, despite the longevity, females give birth only a few times during their lives and invest considerable parental care in their young. Dugongs generally gather in groups of less than a dozen individuals for one to two days. Since they congregate in turbid waters, little is known about their reproductive behavior. The males are often seen with scars, and the tusks on dugongs grow in first for males, suggesting they are important in lekking. They have also been known to lunge at each other. The age when a female first gives birth is disputed, ranging anywhere from six to seventeen years.[33] The time between births is unclear, with estimates ranging from 2 to 7 years.[53][39] In Sarasota, Florida, 113 manatees were observed of known sex; of these, 53 were females, which produced at least 55 calves during a five-year period of observation.[54]

Manatees can reach sexual maturity as early as two to five years of age.[54] Manatee gestation length is around one year, and then they lactate for one to two years. West Indian manatees and African manatees can breed year-round, and a female will mate with multiple male partners.[55] Amazonian manatees have a breeding season, usually mating when the river levels begin to rise, which varies from place to place.[56]

Manatees in captivity
See also: Manatee § Captivity

Manatees may be taken into captivity after being found stranded to facilitate their recovery,[57] and there are many instances of manatees being successfully rehabilitated and released into the wild.[58] As all extant sirenian species are considered to be 'vulnerable', these rehabilitation programs present a useful means to support these species. However, their vulnerability also means that the taking of manatees from the wild for commercial purposes is a conservation issue.

Diet in Captivity

Manatees, in comparison to other marine species, tend to do well in a captive environment, with incidences of manatees thriving in captivity.[59] However, it can be difficult to replicate the conditions of their natural environment to the extent necessary to maintain a manatee at their healthiest, the typical diet fed to captive manatee populations may contain insufficient quantities of the nutrients they need.[49]

Manatee captive fed diets vary greatly from the manatee's diet in the wild. In captivity manatees are fed 70-80% of leafy green vegetables, 10-20% of dried forage, and 5% vegetables and fruits.[60] Dried forage are foods such as hay and timothy grass which are often the diets of different equine and cattle. The vegetables and fruits that are fed to manatees consist of romaine lettuce, carrots, apples, etc.... Manatees in captivity have done well off of this varying diet, but the exact nutritional needs of a manatee's diet are unknown.

When compared to the natural diet of manatees in the wild, there is a significant difference. In their natural habitat approximately ½ of the manatee's diet is marine or estuarine plants; in comparison to romaine lettuce, their nutritional value is quite different.[61] When compared to the captive diet, aquatic plants have an increase in dry matter, soluble neutral detergent fiber, and a decrease in digestible nutrients.[61] Although it may seem to be a better diet for manatees to have more easily digestible nutrients, their gastrointestinal tract is adapted to the wild diet through microbial processes of fermentation.[61] Although, we are still unsure if the nutrients given in the captive diet is sufficient since there is positive digestive feedback with the captive diet.

Threats and conservation
West Indian manatees in a conservation project in Brazil

The three extant manatee species (family Trichechidae) and the dugong (family Dugongidae) are rated as vulnerable on the IUCN Red List of endangered species. All four are vulnerable to extinction from habitat loss and other negative impacts related to human population growth and coastal development.[50][62][63][64] Steller's sea cow, extinct since 1768, was hunted to extinction by humans.[65]

The meat, oil, bones, and skin of manatees are valuable items. In some countries, such as Nigeria and Cameroon, African manatees are sold to zoos, aquariums, and online as pets, sometimes being shipped internationally. Though illegal, lack of law enforcement in these areas induce poaching. Some residents of West African countries, such as Mali and Chad, believe that the oil of the African manatee can cure ailments such as ear infections, rheumatism, and skin conditions.[50] Hunting is the largest source of mortality in Amazonian manatees, and there are no management plans except for in Colombia.[66] Amazonian manatees, especially calves, are sometimes illegally sold as pets, but there are several institutions that care for and rescue these orphans, with the possibility of their releasing into the wild.[62] The body parts of dugongs are used as medicinal remedies across the Indian Ocean.[39]

Manatees have also faced threats in Cuba, an area that is not always known for its manatee population. Manatees in Cuba have faced poaching, entanglement and pollution. The area has some of the most extensive and best manatee habitat in the Caribbean, but the population has been unable to thrive there for numerous reasons.[67] Existing information about manatees in Cuba is limited and scarce, this makes it difficult to spread awareness which therefore enhances the situation of illegal poaching and entanglement within fishing nets in a majority of the coastal communities.[68] Poaching of the manatees has been a significant issue since the 1970s when it was initially reported that the hunting was taking its toll on the manatee population in Cuba.[67] In 1975 it was recorded that the manatees' status in Cuba was rare and declining at an alarming rate due to pollution and hunting. In 1996 manatees were placed under protection through the Fishery Decree law 164. This law provided penalties against those that manipulated, harm, or injured manatees. However, it was seen that the hunting of manatees in Cuba in the 1990s may have been the result of economic hardships in this country and the manatees were an alternative source of protein.[67] Although there have been efforts made to protect the population of manatees in Cuba, it has not proven to be helpful or as impactful as those working to protect the population had hoped. Many of these areas are seen as "paper parks" or parks or protected areas that only exist due to their name and nothing else, and they do not have a significant impact on conversation and protections.[69]

Rehabilitation of an Amazonian manatee calf by the Brazilian Institute of Environment and Renewable Natural Resources (IBAMA)

Environmental hazards induced by humans also puts sirenians at risk. Sirenians, especially the West Indian manatee, face high mortality from watercraft collision, and about half of all West Indian manatee deaths are caused by watercraft collisions. An increased usage of hydroelectric power and subsequent damming of rivers increase waterway traffic, which can lead to vessel collisions, and manatees may become entangled in navigational locks. The urbanized coastline of areas such as the Caribbean, Florida, and Australia can result in the decline of seagrass populations. Seagrass meadows are also highly susceptible to pollution, and are currently among the top threatened ecosystems on earth.[70] Reliable areas of warm water in Florida are generally the result of discharge from power plants, but newer plants with more efficient cooling systems may disrupt the pattern of warm water refuges, and an increased demand for artesian springs for water, the natural source of warm water, decreases the number of warm water refuges. Congregating in the warm waters of industrial areas of Florida can expose manatees to pollutants and toxins at a time of year when their immune systems are already compromised.[71]

Sirenians can be caught as bycatch from fisheries, and they can be seen as pests with the interference of local fishermen and the destruction of their nets.[50][62][63][64] African manatees have also been known to venture into rice paddies and destroy the crops during the rainy season, and these confrontations with locals may lead to intentional culling of the manatees.[72] Red tide, scientifically known as Karenia brevis is a harmful algae bloom that releases toxins into the water killing many marine species. In 1982, numerous sick manatees were accounted for and researchers believe this was due to the accumulation brevetoxins in filter-feeding organisms attached to seagrass blades, which are a popular diet for manatees. Manatees can also inhale these brevotoxins from the surface of the water as they come up for air, leading to respiratory symptoms and even drowning.[71]

Manatees have been negatively impacted by plastics and other debris that makes its way into the ocean and other waterways that the manatees frequent. The plastic and debris can cause many issues for the manatee including entanglement, ingestion, amputation or even death. Manatees in all areas are affected by litter in the waterways. When a manatee ingests plastic, it is often not known until after death and a necropsy is completed. It is then that debris is found in the manatee's GI tract. The CMA (Clearwater Marine Aquarium) Manatee Reintroduction Program in the Central and South American coast has rescued and released 40 Antillean Manatees. Of the 40 manatees that were rescued, rehabilitated, and released, four manatees were found to have plastic in their GI tract. Treatment was completed and the manatees were released. Later, three of the four released manatees were found dead, two were a direct result of plastic ingestion and the third had plastic pieces in its GI tract. In the deceased manatee's GI tract condoms, plastic bags, Raschel knit fabric (a type of polyester), unknown plastic debris and ice cream and sanitary product wrappers were found.[73]

A study at the University of Miami in Florida stated that between the years 1978 and 1986, 439 manatee carcasses were recovered and necropsied. Of these 439 carcasses, 63 (14.4%) had ingested debris. Of the 63 carcasses found, 4 deaths were directly related to ingestion of plastic or other debris. Some of the debris that was found in the animals GI tract included monofilament fishing line (most common item found), plastic bags, string/twine/rope, fishhooks, wire, paper, cellophane, synthetic sponges, rubber bands and stockings.[74]

Manatee die-offs from exposure to red tide toxins were recorded by the Florida Fish and Wildlife Conservation Commission in southwest Florida in 2002, 2003, 2005, 2007, and most recently in 2013. As of June 20, 2018 the current red tide bloom spreads from Pasco county to Collier County off the West coast of Florida.[75] As of January 2018 there have been a total of 472 manatee deaths caused by this red tide along with water crafts, cold stress, and other factors.[76]

Infectious diseases may also play an important role in morbidity and mortality in manatees. Although only Florida manatees have been found to be infected with viruses, parasites and bacteria have been observed in at least three of the four sirenian species.[77][78][79][80] The three viruses that have been detected in Florida manatees include trichechid herpesvirus 1 (TrHV-1) and manatee papillomavirus 1 (TmPV 1 & 2) and 2 (TmPV 3 & 4).[81][82][83] Mycobacteriosis has reportedly led to mortality in captive Florida manatees and illness in Amazonian manatees while bacteria such as Vibrio, Pasteurella, Pseudomonas, Streptococcus, and Clostridium have been cultured from dead dugongs in Australia.[84][85][86] Salmonellosis has been associated with mortality in dugongs since at least 1981.[78] Although not well studied, the Senegal manatee is known to host the nematode Heterocheilus tunicatus just as its sister species, the West Indian manatee, does.[87][80] There is still a great deal to learn about the threat that infectious diseases pose on both wild and captive populations of manatees as evidenced by the lack of scientific literature on the subject. The relationship between the presence of certain potential pathogens, including those listed above, and their effect on disease in individuals is still largely unknown, although many wild manatees are found to be positive for papillomavirus with no known negative health effects. Immunosuppressed individuals that test positive for papillomavirus can sometimes develop cutaneous lesions, however cutaneous papillomatosis is not always correlated with a papillomavirus infection, and further study is warranted.[88]

In Florida, agricultural runoff can negatively affect the manatee habitat, and during the rainy season, over 50 counties practice fertilizer bans to try and limit the pollutants that end up in the water.[89] Weather disasters and other natural occurrences are also sources of mortality. The West Indian manatee and Dugong face risks from hurricanes and cyclones, which are predicted to increase in the future. These storms can spread pollutants [90] and may also damage seagrass populations.[64][63] Climate change is a growing concern for manatees, as changes in temperature can affect sea levels, pH, precipitation, salinity, and the circulation patterns of coastal ecosystems.[90] Climate change is also predicted to make winter months even colder, leading to increased instances of cold stress in manatees.[91] Warming ocean temperatures can cause harmful algal blooms, which can choke out the light needed for growth of benthic vegetation.[92] Reduced seagrass beds means that more manatees end up congregating in smaller areas to feed, increasing competition for resources and the spread of pathogens.[71] Exposure to brevetoxin from Karenia brevis during a red tide event are also sources of mortality; they may be able to be exposed to brevetoxin after a red tide has subsided, as it could settle on seagrasses.[63] The act of eating vegetation also stirs up sediment, resulting in the ingestion of contaminants trapped in the mud.[71] African manatees can become stranded during the dry season when rivers and lakes become too small or dry up completely.[50]

All sirenians are protected by 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).[93] In addition to this, the four species are further protected by various specialty organizations. The Dugong is listed in the Convention on Biological Diversity (CBD), the Convention on Migratory Species, and the Coral Triangle Initiative.[64] In Florida, manatees are protected by the Florida Manatee Sanctuary Act of 1978, which implements actions such as the limitation or prohibition of watercraft speeds where manatees exist.[94] Marine mammal rehabilitation programs have been underway and regulated in the United States for more than 40 years. In 1973 injured and distressed manatees were rescued or aided within Florida. Eventually, the program was formalized into the Manatee Rescue, Rehabilitation, and Release Program managed by the USFWS. In 2012 the program became the Manatee Rescue & Rehabilitation Partnership (MRP) with permitting and oversight by the USFWS. From 1973 through 2014, this program rescued 1,619 manatees and 526 Florida manatees have been released.[95]

See also
  • Dwarf manatee
  • Cetacea
  • Marine mammal

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