''Posidonia australis'', also known as fibre-ball weed or ribbon weed, is a species of

seagrass Seagrasses are the only flowering plants which grow in marine (ocean), marine environments. There are about 60 species of fully marine seagrasses which belong to four Family (biology), families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and ...

that occurs in the southern waters of Australia. It forms large meadows important to environmental conservation. Balls of decomposing detritus from the foliage are found along nearby shore-lines. In 2022, a single stand in

Shark Bay Shark Bay () is a World Heritage Site in the Gascoyne region of Western Australia. The area is located approximately north of Perth, on the westernmost point of the Australian continent. UNESCO's listing of Shark Bay as a World Heritage S ...

was reported by scientists to not only be the largest plant in the world, but the largest organism by square size.

Description

''Posidonia australis'' is a flowering plant occurring in dense meadows, or along channels, in white sand. It is found at depths from . Subsurface

rhizome In botany and dendrology, a rhizome ( ) is a modified subterranean plant stem that sends out roots and Shoot (botany), shoots from its Node (botany), nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from ...

s and roots provide stability in the sands it occupies. Erect rhizomes and leaves reduce the accumulation of

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...

. The leaves are ribbon-like and wide. They are bright green, perhaps becoming browned with age. The terminus of the leaf is rounded or absent through damage. They are arranged in groups with older leaves on the outside, longer and differing in form from the younger leaves they surround. The species is

monoecious Monoecy (; adj. monoecious ) is a sexual system in seed plants where separate male and female cones or flowers are present on the same plant. It is a monomorphic sexual system comparable with gynomonoecy, andromonoecy and trimonoecy, and contras ...

. The flowers appear on small spikes on leafless stems, two bracts on each spike. The plant pollinates by

hydrophily Hydrophily is a fairly uncommon form of pollination whereby pollen is distributed by the flow of waters, particularly in rivers and streams. Hydrophilous species fall into two categories: (i) Those that distribute their pollen to the surface of ...

, by dispersing in the water. ''Posidonia australis'' reproduction usually occurs through sexual or asexual methods but, under extreme conditions, by pseudovivipary. A 2013 study showed that ''P. australis'' can sequester carbon 35 times more efficiently than

rainforest Rainforests are forests characterized by a closed and continuous tree Canopy (biology), canopy, moisture-dependent vegetation, the presence of epiphytes and lianas and the absence of wildfire. Rainforests can be generally classified as tropi ...

s. In 2022, a study by the School of Biological Sciences and Oceans Institute at The University of Western Australia showed that a single plant of this species can grow vegetatively by using

s to cover an extensive area, similar to

buffalo grass Buffalo grass may refer to * Buffalo grass, sweet vernal grass or vanilla grass (''Anthoxanthum odoratum'') * Buffalo grass (''Bouteloua dactyloides'') * Buffalo grass (''Brachiaria mutica'') * Buffalo grass or sweet grass (''Hierochloe odorata'') ...

. This particular specimen has double the number of chromosomes of other studied populations (40 chromosomes instead of the usual 20).

Distribution

This species is found in waters around the southern coast of Australia. In

Western Australia Western Australia (WA) is the westernmost state of Australia. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to the south, the Northern Territory to the north-east, and South Australia to the south-east. Western Aust ...

it occurs in the

region, around islands of the

Houtman Abrolhos The Houtman Abrolhos (often called the Abrolhos Islands) is a chain of 122 islands and associated coral reefs in the Indian Ocean off the west coast of Australia about west of Geraldton, Western Australia. It is the southernmost true coral r ...

, and southward along the coast of the

Swan Coastal Plain The Swan Coastal Plain in Western Australia is the geographic feature which contains the Swan River as it travels west to the Indian Ocean. The coastal plain continues well beyond the boundaries of the Swan River and its tributaries, as a geol ...

. The species is recorded at the edge of the

Esperance Plains Esperance Plains, also known as Eyre Botanical District, is a biogeography, biogeographic region in southern Western Australia on the South_coast_of_Western_Australia , south coast between the Avon Wheatbelt and Hampton bioregions, and bordere ...

, the Archipelago of the Recherche, at the southern coast of the southwest region. The range extends to the east to coastal areas of New South Wales, South Australia, Tasmania, and Victoria. A sign of a nearby occurrence of ''

Posidonia ''Posidonia'' is a genus of flowering plants. It contains nine species of marine plants ("seagrass"), found in the seas of the Mediterranean and around the south coast of Australia. The APG system (1998) and APG II system (2003) accept this ge ...

'' is the presence of masses of decomposing leaves on beaches, forming fibrous balls.

Largest known organism

A research article in the Proceedings of the Royal Society reported in June 2022 that

genetic testing Genetic testing, also known as DNA testing, is used to identify changes in DNA sequence or chromosome structure. Genetic testing can also include measuring the results of genetic changes, such as RNA analysis as an output of gene expression, or ...

had revealed that samples of ''Posidonia australis'' taken from a meadow in

up to apart were all from a single clone of the same plant. The plant covers an area of

seafloor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

of around . This would make it the largest known organism in the world by area, exceeding the size of a colony of the ''

Armillaria ostoyae ''Armillaria'' is a genus of fungi that includes the ''Armillaria mellea, A. mellea'' species ('honey fungus') that live on trees and woody shrubs. It includes about 10 species formerly categorized summarily as ''A. mellea''. ''Armillaria'' ...

'' fungus in

Malheur National Forest The Malheur National Forest is a United States National Forest, National Forest in the U.S. state of Oregon. It contains more than in the Blue Mountains (Pacific Northwest), Blue Mountains of eastern Oregon. The forest consists of Great Basi ...

Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...

that extends , as well as a stand of quaking aspen trees in

Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...

that extends over more than . The plant is estimated to have taken at least 4,500 years to grow to this size by using

s to colonise new parts of the seafloor, assuming a rhizome growth rate of around a year. This age puts it among the oldest known clonal plants too.

Taxonomy

This species is a member of the family

Posidoniaceae ''Posidonia'' is a genus of flowering plants. It contains nine species of marine plants (" seagrass"), found in the seas of the Mediterranean and around the south coast of Australia. The APG system (1998) and APG II system (2003) accept this ...

, one of eight occurring in Australia. The ninth member, ''

Posidonia oceanica ''Posidonia oceanica'', commonly known as Neptune grass or Mediterranean tapeweed, is a seagrass species that is endemic to the Mediterranean Sea. It forms large underwater meadows that are an important part of the ecosystem. The fruit is free ...

'', is found in the Mediterranean sea. The genus name for this species, ''Posidonia'', is given for the god of the seas

Poseidon Poseidon (; ) is one of the twelve Olympians in ancient Greek religion and mythology, presiding over the sea, storms, earthquakes and horses.Burkert 1985pp. 136–139 He was the protector of seafarers and the guardian of many Hellenic cit ...

, and ''australis'' refers to the southern distribution. The species was first described by

Joseph Hooker Joseph Hooker (November 13, 1814 – October 31, 1879) was an American Civil War general for the Union, chiefly remembered for his decisive defeat by Confederate General Robert E. Lee at the Battle of Chancellorsville in 1863. Hooker had serv ...

in ''

Flora Tasmaniae The ''Flora Tasmaniae'' is a description of the plants discovered in Tasmania during the Ross expedition written by Joseph Dalton Hooker and published by Reeve Brothers in London between 1855 and 1860. Hooker sailed on HMS ''Erebus'' as assistan ...

''.

Common name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often con ...

s for the plant include fibre-ball weed and ribbon weed.

Conservation status

IUCN lists this species as "near threatened", while the meadows in New South Wales have been listed by the

Commonwealth of Australia Australia, officially the Commonwealth of Australia, is a country comprising the mainland of the Australian continent, the island of Tasmania and numerous smaller islands. It has a total area of , making it the sixth-largest country in ...

as an endangered ecological community since 2015.

References

External links

''Posidonia australis'' occurrence data
from

Australasian Virtual Herbarium The ''Australasian Virtual Herbarium'' (AVH) is an online resource that allows access to plant specimen data held by various Australian and New Zealand herbaria. It is part of the Atlas of Living Australia (ALA), and was formed by the amalgama ...

* * {{Authority control australis Flora of New South Wales Flora of South Australia Flora of Tasmania Flora of Victoria (state) Angiosperms of Western Australia Monocots of Australia Largest organisms