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. Durvillaea, commonly known as southern bull kelps, occur on rocky, wave-exposed shorelines and provide a habitat for numerous intertidal organisms. 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. 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.
Common name and etymology The common name for Durvillaea is southern bull kelp. This is often shortened to bull kelp, which can generate confusion with the North Pacific kelp species Nereocystis luetkeana.
The genus is named after French explorer Jules Dumont d'Urville (1790-1842).
Description
Durvillaea species are dioecious. Based on the concentration of pigments in thalli, males and females of D. antarctica do not significantly differ in colouration. Holdfasts of males and females can be joined together.
Durvillaea species are characterised by their prolific growth and plastic morphology.
Three species, Durvillaea incurvata, D. antarctica, D. poha are buoyant due to a honeycomb-like structure in the fronds of the kelp that holds air. When these species detach from the seabed, this buoyancy allows for individuals to drift for substantial distances, permitting long distance dispersal. In contrast, species as D. willana lack such 'honeycomb' tissue and are non-buoyant, preventing the individuals from moving long distances.
Ecology Durvillaea bull kelp grow within intertidal and shallow subtidal areas, typically on rocky wave-exposed coastal sites. D. antarctica and D. poha are intertidal, whereas D. willana is subtidal (to 6 m depths). Intertidal species can grow at the uppermost limit of the intertidal zone if there is sufficient wave wash. Species can withstand a high level of disturbance from wave action, although storms can remove individuals from substrates.
Research on the bacteria associated with Durvillaea indicates that local environmental conditions shape external microbiome diversity significantly. Further research suggests that the diversity of bacterial microbiomes associated with D. antarctica is influenced by the density and connectivity of the host kelp populations.
Epibionts, parasites and rafting
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. In New Zealand, species that inhabit Durvillaea include the sea-star Anasterias suteri, crustaceans such as Parawaldeckia kidderi, P. karaka, and the gribbles Limnoria segnis and L. stephenseni, as well as the molluscs Cantharidus roseus, Onchidella marginata, Onithochiton neglectus, and Sypharochiton sinclairi, and the spider Desis marina.Vink, C., McQuillan, B., Simpson, A., & Correa-Garhwal, S. (2017). The marine spider, Desis marina (Araneae: Desidae): new observations and localities. The Weta, 51, 71-79. Retrieved from http://publications.ento.org.nz/index.php/weta/article/view/167
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. 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. 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. Rafts of D. antarctica are more likely to disperse offshore if individuals detach during outgoing tides during autumn and winter. Rafts of Kelp-associated invertebrates inside holdfasts and external epiphytes can be transported by rafting individuals, potentially leading to long-distance dispersal and a significant impact upon the population genetic structure of the invertebrate species. Attached male and female individuals can raft together, providing an opportunity for reproduction. Rafts of D. antarctica currently reach Antarctica, although freezing negative impacts the buoyancy and photosynthetic activity of D. antarctica tissue, which may affect the viability and reproductive capability of rafts.
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, 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. Rafting on D. antarctica appears to have influenced the dispersal and phylogeography of these non-buoyant species. 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.
Fronds of D. antarctica can be infected by an endophytic, phaeophycean algal parasite Herpodiscus durvillaeae (Lindauer) G.R. South. Fronds can also be infected by Maullinia, a genus of intracellular, protistan parasites. Based on genetic evidence, both H. durvillaeae and Maullinia have probably been dispersed across the Southern Hemisphere via rafting bull kelp.
Environmental stressors Increased temperatures and heatwaves, increased sedimentation, and invasive species (such as Undaria pinnatifida) are sources of physiological stress and disturbance for members of the genus.
A marine heatwave in the summer of 2017/18 appears to have caused the local extinction of multiple Durvillaea species at Pile Bay, on the Banks Peninsula. Once the kelp was extirpated, the invasive kelp Undaria pinnatifida recruited in high densities.
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. Increased sedimentation following landslides caused by earthquakes is also detrimental. 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.
Intertidal species of Durvillaea can be used to estimate earthquake uplift height, with comparable results to traditional methods such as lidar. 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.
Chile The 2010 Chile earthquake caused significant coastal uplift (~0.2 to 3.1 m), particularly around the Gulf of Arauco, Santa María Island and the Bay of Concepción. This uplift caused large scale die offs of D. antarctica and dramatically affected the intertidal community. The damage to infrastructure and ecological disturbance caused by the earthquake was assessed to be particularly damaging for seaweed gatherers and cochayuyo harvest.
New Zealand Akatore Duvillaea bull kelp diversity appears to have been affected by uplift along the Akatore fault zone. Phylogeographic analyses using mitochondrial COX1 sequence data and genotyping by sequencing data for thousands of anonymous nuclear loci, indicate that a historic uplift event (800 – 1400 years before present) along the fault zone and subsequent recolonisation, has left a lasting impact upon the genetic diversity of the intertidal species D. antarctica and D. poha, but not on the subtidal species D. willana. Such a genetic impact may support the founder takes all hypothesis. Further genetic analysis has revealed that the population structure of two epifaunal species, the gribble L. segnis and the chiton O. neglectus, closely matches the pattern observed in the intertidal host species of Durvillaea along the Akatore fault zone. However, no matching pattern was observed for another epifaunal species, the amphipod P. karaka, most likely because this species has better swimming potential and can rely upon other host seaweeds.
Kaikoura A substantial die off of Durvillaea bull kelp occurred along the Kaikoura coastline following the 2016 Kaikoura earthquake, which caused uplift up to 6 metres. The loss of Durvillaea kelp caused ecological disturbance, significantly affecting the biodiversity of the local intertidal community. Aerial drone imaging two years after the earthquake indicated that Durvillaea abundance remained low on reefs with significant uplift, but it revealed offshore refuge populations less frequently detected by field researchers.
A genetic analysis indicated that some of the Durvillaea that subsequently reached the affected coastline (i.e. potential colonists) came from areas >1,200 kilometres away. For another study, researchers sampled dying D. antarctica immediately after the earthquake to capture the 'pre-uplift' population genomic diversity and they subsequently sampled new recruits of D. antarctica within the newly formed intertidal zone to estimate 'post-uplift' population structure. Population genomic analysis indicated little change in genetic diversity within four years of recolonisation by D. antarctica. Based on those genomic results, combined with field observations of the recovery of Durvillaea throughout the uplift zone, and oceanographic connectivity modelling, it was hypothesised that the surviving populations of D. antarctica (typically sparse and lower within the intertidal zone) have dominated the early recolonisation. The researchers argued that the newly formed coastline has not yet been fully recolonised though, and the population structure and genomic diversity of D. antarctica in the Kaikoura region is likely to change over coming decades.
A genetic study investigated bacteria associated with D. antarctica and found that the 2016 earthquake changed the diversity of the microbiome in disturbed, uplifted populations. Specifically, the analysis showed that disturbed bull kelp populations supported higher functional, taxonomic and phylogenetic microbial beta diversity than non-disturbed populations. It was hypothesised that this change was induced by the sudden decline in the host D. antarctica population following the earthquake.
Rarangi The D. antarctica sampled from Rarangi, near Blenheim in Marlborough has been found to be genetically distinct from nearby geographic populations, and the kelp is most closely related to D. antarctica populations sampled over 300 km to the south on Banks Peninsula. Independently, based on LiDAR mapping and field observations, geologists have discovered a zone of uplifted rocky coastline at the same location. By combining the above genomic and geological evidence, researchers have hypothesised that a small section of coastline at Rarangi was uplifted by one of four major earthquakes between 6000 and 2000 years ago, which was sufficient in height and sudden enough to extirpate the original population of D. antarctica. Under this hypothesis, the present-day population was founded by rafts of kelp that were not related to the nearby geographic populations, leading the to current genetically distinct population present at Rarangi.
Wellington and the Wairarapa