Aquatic mammals and semiaquatic mammals are a diverse group of mammals that dwell partly or entirely in bodies of water. They include the various marine mammals who dwell in oceans, as well as various freshwater species, such as the European otter. They are not a taxon and are not unified by any distinct biological grouping, but rather their dependence on and integral relation to aquatic ecosystems. The level of dependence on aquatic life varies greatly among species. Among freshwater taxa, the Amazonian manatee and river dolphins are completely aquatic and fully dependent on aquatic ecosystems. Semiaquatic freshwater taxa include the Baikal seal, which feeds underwater but rests, molts, and breeds on land; and the capybara and hippopotamus which are able to venture in and out of water in search of food.
Mammal adaptation to an aquatic lifestyle vary considerably between species. River dolphins and manatees are both fully aquatic and therefore are completely tethered to a life in the water. Seals are semiaquatic; they spend the majority of their time in the water, but need to return to land for important activities such as mating, breeding and molting. In contrast, many other aquatic mammals, such as hippopotamus, capybara, and water shrews, are much less adapted to aquatic living. Likewise, their diet ranges considerably as well, anywhere from aquatic plants and leaves to small fish and crustaceans. They play major roles in maintaining aquatic ecosystems, beavers especially.
Aquatic mammals were the target for commercial industry, leading to a sharp decline in all populations of exploited species, such as beavers. Their pelts, suited for conserving heat, were taken during the fur trade and made into coats and hats. Other aquatic mammals, such as the Indian rhinoceros, were targets for sport hunting and had a sharp population decline in the 1900s. After it was made illegal, many aquatic mammals became subject to poaching. Other than hunting, aquatic mammals can be killed as bycatch from fisheries, where they become entangled in fixed netting and drown or starve. Increased river traffic, most notably in the Yangtze river, causes collisions between fast ocean vessels and aquatic mammals, and damming of rivers may land migratory aquatic mammals in unsuitable areas or destroy habitat upstream. The industrialization of rivers led to the extinction of the Chinese river dolphin, with the last confirmed sighting in 2004.
Taxonomy and evolution
Groups This list covers only mammals that live in freshwater. Although this can include both species that also live in saltwater, or that are members of otherwise marine families. For a list of saltwater mammals, see List of marine mammal species. Order Sirenia: sirenians Family Trichechidae: manatees Amazonian manatee (Trichechus inunguis) African manatee (Trichechus senegalensis) West Indian manatee (Trichechus manatus) Order Artiodactyla: even-toed ungulates Suborder Whippomorpha Family Platanistidae Ganges river dolphin, or susu (Platanista gangetica) Indus river dolphin, or bhulan (Platanista minor) Family Iniidae Amazon river dolphin, or boto (Inia geoffrensis) Araguaian river dolphin (Inia araguaiaensis) Family Lipotidae Chinese river dolphin, or baiji (Lipotes vexillifer) likely extinct Family Delphinidae Irrawaddy dolphin (Orcaella brevirostris) Tucuxi (Sotalia fluviatilis) Guiana dolphin (Sotalia guianensis) Family Phocoenidae Yangtze finless porpoise (Neophocaena asiaeorientalis) Family Hippopotamidae: hippopotamuses Hippopotamus (Hippopotamus amphibius) Pygmy hippopotamus (Choeropsis liberiensis) Order Perissodactyla: odd-toed ungulates Suborder Ceratomorpha Family Rhinocerotidae: rhinoceroses Indian rhinoceros (Rhinoceros unicornis) Order Carnivora Family Canidae Bush dog (Speothos venaticus) Family Mustelidae Subfamily Lutrinae Eurasian otter (Lutra lutra) Hairy-nosed otter (Lutra sumatrana) Spotted-necked otter (Hydrictis maculicollis) Smooth-coated otter (Lutrogale perspicillata) North American river otter (Lontra canadensis) Southern river otter (Lontra provocax) Neotropical river otter (Lontra longicaudis) Marine otter (Lontra felina) Giant otter (Pteronura brasiliensis) African clawless otter (Aonyx capensis) Oriental small-clawed otter (Aonyx cinerea) Subfamily Mustelinae European mink (Mustela lutreola) American mink (Neogale vison) Family Phocidae Baikal seal (Pusa sibirica) Ladoga seal (Pusa hispida ladogensis) Saimaa seal (Pusa saimensis) Ungava seal (Phoca vitulina mellonae) Family Otariidae California sea lion (Zalophus californianus) Steller sea lion (Eumetopias jubatus) Family Viverridae Otter civet (Cynogale bennettii) Aquatic genet (Genetta piscivora) Family Herpestidae Marsh mongoose (Atilax paludinosus)
Order Rodentia: rodents Suborder Hystricomorpha Capybara (Hydrochoerus hydrochaeris) Lesser capybara (Hydrochoerus isthmius) Coypu (Myocastor coypus) Family Castoridae: beavers North American beaver (Castor canadensis) Eurasian beaver (Castor fiber) Family Cricetidae Subfamily Arvicolinae Muskrat (Ondatra zibethicus) Round-tailed muskrat (Neofiber alleni) European water vole (Arvicola amphibius) Southwestern water vole (Arvicola sapidus) North American water vole (Microtus richardsoni) Subfamily Sigmodontinae Ucayali water rat (Amphinectomys savamis) Ecuador fish-eating rat (Anotomys leander) Ferreira's fish-eating rat (Daptomys ferreirai) Musser's fish-eating Rat (Daptomys musseri) Tingo María Fish-eating Rat (Daptomys nunashae) Oyapock's fish-eating rat (Daptomys oyapocki) Peruvian fish-eating rat (Daptomys peruviensis) Venezuelan fish-eating rat (Daptomys venezuelae) Chibchan water mouse (Chibchanomys trichotis) Web-footed marsh rat (Holochilus brasiliensis) Chacoan marsh rat (Holochilus chacarius) Amazonian marsh rat (Holochilus nanus) Brazilian Northeastern Marsh Rat (Holochilus oxe) Common marsh rat, or Amazonian marsh rat (Holochilus sciureus) Crab-eating rat (Ichthyomys hydrobates) Ecuadorean crab-eating rat (Ichthyomys orientalis) Pine's crab-eating rat (Ichthyomys pinei) Pittier's crab-eating rat (Ichthyomys pittieri) Stolzmann's crab-eating rat (Ichthyomys stolzmanni) Tweedy's crab-eating rat (Ichthyomys tweedii) Incan water mouse (Incanomys mayopuma) Incan small-eared water mouse (Icanomys parviauris) Lund's amphibious rat (Lundomys molitor) Western Amazonian nectomys (Nectomys apicalis) Magdalena-Cauca water rat (Nectomys grandis) Trinidad water rat (Nectomys palmipes) Amazonian nectomys (Nectomys rattus) South American water rat (Nectomys squamipes) Montane fish-eating rat (Neusticomys monticolus) Voss' fish-eating rat (Neusticomys vossi) Las Cajas water mouse (Neusticomys orcesi) White-bellied Rice Rat (Oryzomys albiventer) Coues's rice rat (Oryzomys couesi) Nicaraguan rice rat (Oryzomys dimidiatus) Gorgas's rice rat (Oryzomys gorgasi) Marsh rice rat (Oryzomys palustris) Brazilian false rice rat (Pseudoryzomys simplex) Mexican water mouse (Rheomys mexicanus) Goldman's water mouse (Rheomys raptor) Thomas's water mouse (Rheomys thomasi) Underwood's water mouse (Rheomys underwoodi) Argentine swamp rat (Scapteromys aquaticus) Plateau swamp rat (Scapteromys meridionalis) Waterhouse's swamp rat (Scapteromys tumidus) Alfaro's rice water rat (Sigmodontomys alfari) Family Muridae Mountain water rat (Baiyankamys habbema) Shaw Mayer's water rat (Baiyankamys shawmayeri) Earless water rat (Crossomys moncktoni) Rakali, or water-rat (Hydromys chrysogaster) Western water rat (Hydromys hussoni) New Britain water rat (Hydromys neobritannicus) Ziegler's water rat (Hydromys ziegleri) Mangrove mouse, or False water rat (Xeromys myoides) East African wading rat (Colomys goslingi) Cameroonean Wading Rat (Colomys eisentrauti) Wologizi Wading Rat (Colomys wologizi) Congo Wading Rat (Colomys lumumbai) Ethiopian amphibious rat (Nilopegamys plumbeus) Family Thryonomyidae Greater cane rat (Thryonomys swinderianus) Order Monotremata: monotremes Platypus (Ornithorhynchus anatinus) Order Afrosoricida Family Potamogalidae Giant otter shrew (Potamogale velox) Nimba otter shrew (Micropotamogale lamottei) Ruwenzori otter shrew (Micropotamogale ruwenzorii) Family Tenrecidae Aquatic tenrec (Microgale mergulus) Order Eulipotyphla Family Soricidae: shrews Malayan water shrew (Chimarrogale hantu) Himalayan water shrew (Chimarrogale himalayica) Sunda water shrew (Chimarrogale phaeura) Japanese water shrew (Chimarrogale platycephala) Chinese water shrew (Chimarrogale styani) Sumatran water shrew (Chimarrogale sumatrana) Elegant water shrew (Nectogale elegans) Mediterranean water shrew (Neomys anomalus) Eurasian water shrew (Neomys fodiens) Transcaucasian water shrew (Neomys teres) Glacier Bay water shrew (Sorex alaskanus) American water shrew (Sorex palustris) Pacific water shrew, or marsh shrew (Sorex bendirii) Family Talpidae (moles and relatives) Star-nosed mole (Condylura cristata) Russian desman (Desmana moschata) Pyrenean desman (Galemys pyrenaicus) Order Didelphimorphia: opossums Family Didelphidae: opossums Big lutrine opossum (Lutreolina crassicaudata) Massoia's lutrine opossum (Lutreolina massoia) Yapok (Chironectes minimus)
Evolution
Mesozoic
One of the first known proto-mammals similar to modern placentals was aquatic, the Jurassic mammaliaform Castorocauda. It seems to have been adapted to water much like a beaver, with teeth different in many ways from all other docodonts, presumably due to a difference in diet. Most docodonts had teeth specialized for an omnivorous diet. The teeth of Castorocauda suggest that the animal was a piscivore, feeding on fish and small invertebrates. The first two molars had cusps in a straight row, eliminating the grinding function suggesting that they were strictly for gripping and not for chewing. This feature of three cusps in a row is similar to the ancestral condition in mammal relatives (as seen in triconodonts), but is almost certainly a derived character in Castorocauda. These first molars were also recurved in a manner adapted to hold slippery prey once grasped. These teeth are very similar to the teeth seen in mesonychians, an extinct group of semiaquatic carnivorous ungulates, and resemble, to a lesser degree, the teeth of seals.
Another docodontan, the Late Jurassic Haldanodon, has been suggested to be a platypus or desman-like swimmer and burrower, being well adapted to dig and swim and occurring in a wetland environment.Thomas Martin, Postcranial anatomy of Haldanodon exspectatus (Mammalia, Docodonta) from the Late Jurassic (Kimmeridgian) of Portugal and its bearing for mammalian evolution, Forschungsinstitut Senckenberg, Senckenberganlage 25, 60325 Frankfurt am Main, Germany Received October 2004; accepted for publication March 2005
The tritylodontid Kayentatherium has been suggested to be semiaquatic. Unlike Castorocauda and Haldanodon, it was an herbivore, being probably beaver or capybara-like in habits.Anusuya Chinsamy-Turan, Forerunners of Mammals: Radiation • Histology • Biology, Indiana University Press, 18/11/2011