Durvillaea
Durvillaea antarctica and D. willana on Taieri Island
Scientific classification
Domain:	Eukaryota
Clade:	Diaphoretickes
Clade:	SAR
Clade:	Stramenopiles
Phylum:	Gyrista
Subphylum:	Ochrophytina
Class:	Phaeophyceae
Order:	Fucales
Family:	Durvillaeaceae (Oltmanns) De Toni
Genus:	Durvillaea Bory
Type species
D. antarctica (Chamisso) Hariot[1]
Species
See text

Durvillaea is a genus of large brown algae in the monotypic family Durvillaeaceae. All members of the genus are found in the southern hemisphere, including Australia, New Zealand, South America, and various subantarctic islands.^[2][3] Durvillaea, commonly known as southern bull kelps, occur on rocky, wave-exposed shorelines and provide a habitat for numerous intertidal organisms.^[4][5] Many species exhibit a honeycomb-like structure in their fronds that provides buoyancy, which allows individuals detached from substrates to raft alive at sea, permitting dispersal for hundreds of days over thousands of kilometres.^[3][6][7] Durvillaea species have been used for clothing, tools and as a food source by many indigenous cultures throughout the South Pacific, and they continue to play a prominent role in Chilean cuisine.^[3]

Common name and etymology

The common name for Durvillaea is southern bull kelp, although this is often shortened to bull kelp, which can generate confusion with the North Pacific kelp species Nereocystis luetkeana.^[8][9]

The genus is named after French explorer Jules Dumont d'Urville (1790-1842).^[10]

Description

Cross-section of D. antarctica showing the 'honeycomb' structure of the blades

Durvillaea species are characterised by their prolific growth and plastic morphology.^[11]

Two species, D. antarctica and D. poha are buoyant due to a honeycomb-like structure in the fronds of the kelp that holds air.^[3][12] When these species detach from the seabed, this buoyancy allows for individuals to drift for substantial distances, permitting long distance dispersal.^[3][13] In contrast, species as D. willana lack such 'honeycomb' tissue and are non-buoyant, preventing the individuals from moving long distances.^[13]

Ecology

Durvillaea bull kelp grow within intertidal and shallow subtidal areas, typically on rocky wave-exposed coastal sites.^[11] D. antarctica and D. poha are intertidal, whereas D. willana is subtidal (to 6 m depths).^[14] Intertidal species can grow at the uppermost limit of the intertidal zone if there is sufficient wave wash.^[15] Species can withstand a high level of disturbance from wave action,^[11] although storms can remove individuals from substrates.^[16][17][18]

Epibionts, parasites and rafting

Beachcast D. antarctica at St Kilda Beach, Dunedin

Holdfasts of D. antarctica and other species are often inhabited by a diverse array of epifaunal and infaunal invertebrates, many of which burrow into and graze on the kelp.^[4][5] In New Zealand, species that inhabit Durvillaea include the sea-star Anasterias suteri, crustaceans such as Parawaldeckia kidderi, P. karaka,^[5] and the gribbles Limnoria segnis^[5] and L. stephenseni, as well as the molluscs Cantharidus roseus, Onchidella marginata,^[19] Onithochiton neglectus,^[20][5] and Sypharochiton sinclairi,^[16][17][18] and the spider Desis marina.^[21][22]

Durvillaea individuals can detach from substrates, particularly during storms. Once detached, buoyant species such as D. antarctica and D. poha can float as rafts, and can travel vast distances at sea, driven by ocean currents.^[6][7] Specimens of D. antarctica have been found to float for up to 210 days, during which time high wind speeds transport kelp rafts up to 10,000 km.^[6][7] Environmental factors such as temperature, solar radiation and surface winds (all of which vary with latitude) affect buoyancy of southern bull kelp rafts and their rate of travel.^[6] Rafts of D. antarctica are more likely to disperse offshore if individuals detach during outgoing tides during autumn and winter.^[23] Rafts of Kelp-associated invertebrates can be transported inside of drifting kelp holdfasts, potentially leading to long-distance dispersal and a significant impact upon the population genetic structure of the invertebrate species.^{[16][17][18][19][20]}

Rafts of Durvillaea can be colonised by the goose barnacles Lepas australis and L. pectinata. Beachcast, decomposing bull-kelp is colonised and consumed by a wide variety of invertebrates including sandhoppers Bellorchestia quoyana,^[24] and kelp flies Chaetocoelopa littoralis.

Other seaweeds including Gelidium lingulatum, G. rex, Corallina officinalis var. chilensis, and Lessonia spicata also grow as epiphytes in the holdfasts of D. antarctica.^[25] Rafting on D. antarctica appears to have influenced the dispersal and phylogeography of these non-buoyant species.^[25][26] In New Zealand, Durvillaea fronds can also be infected by the obligate red algal epiphyte Pyrophyllon subtumens (J. Agardh ex R.M. Laing) W.A. Nelson 2003.^[27][28]

Fronds of D. antarctica can be infected by an endophytic, phaeophycean algal parasite Herpodiscus durvillaeae (Lindauer) G.R. South.^[29][30] Fronds can also be infected Maullinia, a genus of intracellular, protistan parasites.^[31][32][33] Based on genetic evidence, both H. durvillaeae and Maullinia have likely been dispersed across the Southern Hemisphere via rafting bull kelp.^{[30][31][33][34]}

Parasites of Durvillaea southern bull kelp

• 
  Cross-section of a D. antarctica frond, showing Pyrophyllon subtumens growing on the outer surface
• 
  Yellow galls caused by Maullinia infections in fronds of D. poha and D. antarctica
• 
  Beachcast D. antarctica kelp frond with blisters caused by an infection
• 
  A detached holdfast of D. antarctica found off Chile, colonised by the goose barnacle Lepas australis

Disturbance from earthquake uplift

Earthquake uplift that raises the intertidal zone by as little as 1.5 metres can cause Durvillaea bull kelp to die off in large numbers.^[13][37][38] Increased sedimentation following landslides caused by earthquakes is also detrimental.^[37][38] Once an area is cleared of Durvillaea following an uplift event, the bull kelp that re-colonises the area can potentially originate from genetically distinct populations far outside the uplift zone, spread via long distance-dispersal.^[39]

Intertidal species of Durvillaea can be used to estimate earthquake uplift height, with comparable results to traditional methods such as lidar.^[15] However, since Durvillaea holdfasts often grow at the uppermost limit of the intertidal zone, these uplift estimates are slightly less accurate compared to measures derived from other intertidal kelp such as Carpophyllum maschalocarpum.^[15]

Species and distribution

There are currently eight recognised species within the genus, and the type species is D. antarctica.^[1] All species are restricted to the Southern Hemisphere and many taxa are endemic to particular coastlines or subantarctic islands.

• Durvillaea amatheiae X.A. Weber, G.J. Edgar, S.C. Banks, J.M. Waters & C.I. Fraser, 2017,^[47] endemic to southeast Australia.^[3][47]
• Durvillaea antarctica (Chamisso) Hariot,^[1] found in New Zealand, Chile and various subantarctic islands including Macquarie Island.^{[2][24][3][9][11][12][42][44][46][48][49]}
• Durvillaea chathamensis C.H.Hay, 1979,^[49] endemic to the Chatham Islands.^[3][9]
• Durvillaea fenestrata C. Hay, 2019,^[3] endemic in the subantarctic Antipodes Islands.^[3][9]
• Durvillaea incurvata (Suhr) Macaya,^[3] endemic to Chile.^[3]
• Durvillaea poha C.I. Fraser, H.G. Spencer & J.M. Waters, 2012,^[12] endemic to South Island of New Zealand, as well as the subantarctic Snares and Auckland Islands.^{[3][9][12][48]}
• Durvillaea potatorum (Labillardière) Areschoug, endemic to southeast Australia.^[8][9][50]
• Durvillaea willana Lindauer, 1949,^[51] endemic to New Zealand.^{[2][3][9][13][49][51]}

Evolution

Time-calibrated phylogenetic trees using mixtures of mitochondrial and nuclear DNA markers have estimated that Durvillaea diverged from other brown algae approximately 20 to 60 million years ago.^[9][52] Given the modern distribution of extant Durvillaea species throughout the Southern Ocean, it has been suggested that the distribution may reflect vicariance following the break-up of Gondwana 40 to 50 million years ago, but this distribution can also be explained by the long-distance dispersal of buoyant Durvillaea lineages throughout the Southern Ocean.^[9][53] Based on molecular phylogenetic research, non-buoyancy is not necessarily the ancestral state for the genus,^[3][9] and non-buoyant lineages could have still been transported across the ocean when attached to rafts of different species of buoyant algae.^[9]

A phylogeny focused on the genus, based on four genes (COI, rbcL, 28S and 18S) indicates the evolutionary relationships shown in the cladogram below.^[3][9] Notably, additional unclassified lineages were estimated within D. antarctica.^[3][9] Mitochondrial introgression has been observed between two species, where some individuals with nuclear DNA of D. poha exhibited mitochondrial DNA belonging to D. antarctica.^[45]

Use of Durvillaea species

References

Further reading

External links

• Algaebase: Durvillaea Bory, 1826
• Museum of New Zealand Te Papa Tongarewa: Durvillaea (Genus)
• Critter of the Week NZ Bull Kelp (Critter of the Week)

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