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Oceanic dispersal is a type of biological dispersal that occurs when terrestrial organisms transfer from one
land mass A landmass, or land mass, is a large region or area of land. The term is often used to refer to lands surrounded by an ocean or sea, such as a continent or a large island. In the field of geology, a landmass is a defined section of continenta ...
to another by way of a sea crossing. Island hopping is the crossing of an ocean by a series of shorter journeys between islands, as opposed to a single journey directly to the destination. Often this occurs via large rafts of floating vegetation such as are sometimes seen floating down major rivers in the tropics and washing out to sea, occasionally with animals trapped on them. Dispersal via such a raft is sometimes referred to as a rafting event. Colonization of land masses by plants can also occur via long-distance oceanic dispersal of floating seeds.


History

Rafting has played an important role in the colonization of isolated land masses by mammals. Prominent examples include Madagascar, which has been isolated for ~120 million years ( Ma), and South America, which was isolated for much of the
Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configurat ...
. Both land masses, for example, appear to have received their primates by this mechanism. According to genetic evidence, the common ancestor of the lemurs of Madagascar appears to have crossed the Mozambique Channel by rafting between 50 and 60 Ma ago. Likewise, the
New World monkey New World monkeys are the five families of primates that are found in the tropical regions of Mexico, Central and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Ceboidea ...
s are thought to have originated in
Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
and rafted to South America by the Oligocene, when the continents were much closer than they are today. Madagascar also appears to have received its
tenrecs A tenrec is any species of mammal within the afrotherian family Tenrecidae endemic to Madagascar. Tenrecs are wildly diverse; as a result of convergent evolution some resemble hedgehogs, shrews, opossums, rats, and mice. They occupy aquatic, a ...
(25–42 Ma ago), nesomyid rodents (20–24 Ma ago) and euplerid carnivorans (19–26 Ma ago) by this route and South America its caviomorph rodents (over 30 Ma ago). Simian primates (ancestral to monkeys) and
hystricognath The Hystricognathi are an infraorder of rodents, distinguished from other rodents by the bone structure of their skulls. The masseter medialis (a jaw muscle) passes partially through a hole below each eye socket (called the infraorbital foramen) ...
rodents (ancestral to caviomorphs) are believed to have previously rafted from Asia to Africa about 40 Ma ago. Among reptiles, several iguanid species in the South Pacific have been hypothesized to be descended from iguanas that rafted from Central or South America (an alternative theory involves dispersal of a putative now-extinct iguana lineage from Australia or Asia). Similarly, a number of clades of American
gecko Geckos are small, mostly carnivorous lizards that have a wide distribution, found on every continent except Antarctica. Belonging to the infraorder Gekkota, geckos are found in warm climates throughout the world. They range from . Geckos are ...
s seem to have rafted over from Africa during both the Paleogene and Neogene. Skinks of the related genera '' Mabuya'' and '' Trachylepis'' also apparently both floated across the Atlantic from Africa to South America and Fernando de Noronha, respectively, during the last 9 Ma. Skinks from the same group have also rafted from Africa to
Cape Verde , national_anthem = () , official_languages = Portuguese , national_languages = Cape Verdean Creole , capital = Praia , coordinates = , largest_city = capital , demonym ...
, Madagascar, the
Seychelles Seychelles (, ; ), officially the Republic of Seychelles (french: link=no, République des Seychelles; Creole: ''La Repiblik Sesel''), is an archipelagic state consisting of 115 islands in the Indian Ocean. Its capital and largest city, ...
, the Comoros and Socotra. (Among lizards, skinks and geckos seem especially capable of surviving long transoceanic journeys.) Surprisingly, even burrowing
amphisbaenia Amphisbaenia (called amphisbaenians or worm lizards) is a group of usually legless squamates, comprising over 200 extant species. Amphisbaenians are characterized by their long bodies, the reduction or loss of the limbs, and rudimentary eyes. As ...
ns and
blind snake The Scolecophidia, commonly known as blind snakes or thread snakes, are an infraorder of snakes. They range in length from . All are fossorial (adapted for burrowing). Five families and 39 genera are recognized. The Scolecophidia infraorder is mos ...
s appear to have rafted from Africa to South America. An example of a bird that is thought to have reached its present location by rafting is the weak-flying South American hoatzin, whose ancestors apparently floated over from Africa. Colonization of groups of islands can occur by an iterative rafting process sometimes called island hopping. Such a process appears to have played a role, for example, in the colonization of the
Caribbean The Caribbean (, ) ( es, El Caribe; french: la Caraïbe; ht, Karayib; nl, De Caraïben) is a region of the Americas that consists of the Caribbean Sea, its islands (some surrounded by the Caribbean Sea and some bordering both the Caribbean S ...
by mammals of South American origin (i.e., caviomorphs, monkeys and sloths). A remarkable example of iterative rafting has been proposed for spiders of the genus '' Amaurobioides''. Members of this genus inhabit coastal sites and build silken cells which they seal at high tide; however, they do not
balloon A balloon is a flexible bag that can be inflated with a gas, such as helium, hydrogen, nitrous oxide, oxygen, and air. For special tasks, balloons can be filled with smoke, liquid water, granular media (e.g. sand, flour or rice), or light s ...
. DNA sequence analysis suggests that ancestors of the genus dispersed from southern South America to South Africa about 10 million years (Ma) ago, where the most basal clade is found; subsequent rafting events then took the genus eastward with the
Antarctic Circumpolar Current The Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise (as seen from the South Pole) from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feat ...
to Australia, then to New Zealand and finally to Chile by about 2 Ma ago. Another example among spiders is the species '' Moggridgea rainbowi'', the only Australian member of a genus otherwise endemic to Africa, with a divergence date of 2 to 16 Ma ago. However, oceanic dispersal of terrestrial species may not always take the form of rafting; in some cases, swimming or simply floating may suffice. Tortoises of the genus ''
Chelonoidis ''Chelonoidis'' is a genus of turtles in the tortoise family erected by Leopold Fitzinger in 1835. They are found in South America and the Galápagos Islands, and formerly had a wide distribution in the West Indies. The multiple subspecies of ...
'' arrived in South America from Africa in the Oligocene; they were probably aided by their ability to float with their heads up, and to survive up to six months without food or fresh water. South American tortoises then went on to colonize the West Indies and
Galápagos Islands The Galápagos Islands ( Spanish: , , ) are an archipelago of volcanic islands. They are distributed on each side of the equator in the Pacific Ocean, surrounding the centre of the Western Hemisphere, and are part of the Republic of Ecuador ...
. The dispersal of semiaquatic species is likely to occur similarly. The dispersal of
anthracothere Anthracotheriidae is a paraphyletic family of extinct, hippopotamus-like artiodactyl ungulates related to hippopotamuses and whales. The oldest genus, ''Elomeryx'', first appeared during the middle Eocene in Asia. They thrived in Africa and Eura ...
s from Asia to Africa about 40 Ma ago, and the much more recent dispersal of
hippo The hippopotamus ( ; : hippopotamuses or hippopotami; ''Hippopotamus amphibius''), also called the hippo, common hippopotamus, or river hippopotamus, is a large semiaquatic mammal native to sub-Saharan Africa. It is one of only two extant ...
s (relatives and possible descendants of anthracotheres) from Africa to Madagascar may have occurred by floating or swimming. Ancestors of the Nile crocodile are thought to have reached the Americas from Africa 5 to 6 Ma ago.


Observation

The first documented example of colonization of a land mass by rafting occurred in the aftermath of hurricanes
Luis Luis is a given name. It is the Spanish form of the originally Germanic name or . Other Iberian Romance languages have comparable forms: (with an accent mark on the i) in Portuguese and Galician, in Aragonese and Catalan, while is archaic ...
and Marilyn in the
Caribbean The Caribbean (, ) ( es, El Caribe; french: la Caraïbe; ht, Karayib; nl, De Caraïben) is a region of the Americas that consists of the Caribbean Sea, its islands (some surrounded by the Caribbean Sea and some bordering both the Caribbean S ...
in 1995. A raft of uprooted trees carrying fifteen or more green iguanas was observed by fishermen landing on the east side of
Anguilla Anguilla ( ) is a British Overseas Territory in the Caribbean. It is one of the most northerly of the Leeward Islands in the Lesser Antilles, lying east of Puerto Rico and the Virgin Islands and directly north of Saint Martin. The territor ...
–an island where they had never before been recorded. The iguanas had apparently been caught on the trees and rafted two hundred miles across the ocean from Guadeloupe, where they are indigenous. Examination of the weather patterns and ocean currents indicated that they had probably spent three weeks at sea before landfall. This colony began breeding on the new island within two years of its arrival. The advent of human civilization has created opportunities for organisms to raft on floating artifacts, which may be more durable than natural floating objects. This phenomenon was noted following the 2011 Tōhoku tsunami in Japan, with about 300 species found to have been carried on debris by the North Pacific Current to the west coast of North America (although no colonizations have been detected thus far).


See also

*
Allopatric speciation Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...
* Biological dispersal *
Panspermia Panspermia () is the hypothesis, first proposed in the 5th century BCE by the Greek philosopher Anaxagoras, that life exists throughout the Universe, distributed by space dust, meteoroids, asteroids, comets, and planetoids, as well as by spacecr ...


References


Further reading

* * {{evolution Biogeography Biological evolution Evolutionary biology Speciation Oceans