Saturday 30 May 2015

Analysing the distribution of shallow-water Black Corals in the Hawaiian Archipelago.

Black Corals, Antipatharia, are Anthozoan Hexacorallid Corals known from across the world’s oceans, particularly in waters below the photic zone, waters shallower than about 50 m where light levels are high and attached benthic communities are dominated by photosynthetic organisms, such as Macroalgae (Seaweeds) and Coral species which host symbiotic single-celled Algae, and have been recorded at depths below 8600 m in the Western Pacific. All Black Corals are colonial, they have largely proteinaceous skeletons with minute spines, mouths surrounded by eight non-retractable tentacles and stomachs with eight mesentery canals (single-ended digestive canals which much be emptied of waste matter by periodic eversion). Because most Black Corals live at depths where surveys by Scuba divers are impossible, very little is known about their distributions, with most species known only from single records, though in some areas where shallow-water Black Corals occur better records exist.

One area where such shallow-water Black Corals are abundant is the Hawaiian Archipelago. Here Black Corals are of some economic significance, having been used in traditional Hawaiian medicine, and with a commercial harvesting industry which collects Black Corals for the jewellery industry (Black Coral is the official State Gemstone of Hawaii). As such a large number of surveys of Black Corals at shallow depths have been carried out in Hawaii, although these are generally local in nature and concentrated around the inhabited islands in the southeast of the archipelago.

In a paper published in the journal Marine Biodiversity Records on 17 April 2015, Daniel Wagner of the NOAA’s Papahānaumokuākea Marine National Monument publishes an analysis of the distribution of shallow-water Black Coral species from the Hawaiian Archipelago, based upon collected surveys from the harvesting industry, collection records from museum specimens, and photographic and video records from submersible surveys carried out by the Hawai‘i Undersea Research Laboratory and Monterrey Bay Aquarium Research Institute as well as published records in scientific literature.

For the purpose of this study ‘shallow-water’ was defined as less than 150 m from the surface, the base of the mesophotic zone (the zone below the photic zone where photosynthesis is not carried out but the water is not in permanent darkness) in the clear waters of Hawaii. A total of eight species of Black Corals were recorded, though since the surveying was somewhat patchy in the northern part of the islands this cannot be concluded to be an exhaustive list of the species present. All species grow attached to hard substrates and favour areas of high currents; it is likely that the current affects the distribution of these Corals, but sufficient records were not available to include this in the survey.

The first species recorded is Antipathes griggi, a commercially significant species found throughout the islands from Hawai‘i to Pearl and Hermes Atoll at depths of between nine and 110 m, but most abundant between forty and fifty meters and fairly rare below sixty meters. The species was found to tolerate temperatures of 22.30-27.41˚C, and has a dense bushy crown which extends into the current flow.

(Left) Growing specimen of Antipathes griggi. (Right) The known distribution of the species within the Hawaiian Archipelago. Wagner (2015).

The second species recorded is Antipathes grandis, another commercially important species which is found from from Hawai‘i to Ni‘ihau at depths of depths of between 24 and 146 m, but which is most abundant between 90 and 110 m, and fairly uncommon shallower than 50 m or deeper than 120 m. This species also has a dense bushy crown, and lives at temperatures of 20.01-26.91˚C.

(Left) Growing specimen of Antipathes grandis. (Right) The known distribution of the species within the Hawaiian Archipelago. Wagner (2015).

The third species recorded is Cirrhipathes cf. anguina (i.e. probably Cirrhipathes anguina a specimen cannot actually be confirmed as belonging to a species without comparing it to the holotype – first specimen described – which with some older species, such as Cirrhipathes anguina which was described in 1846, is not always possible) which is found from across the archipelago from Hawai‘i to the north-west of Brooks Banks at depths of between nine and 150 m, although it is less common below 60 m. Cirrhipathes cf. anguina is a wire Coral (i.e. its colonies consist of a single wire-like strand) found at temperatures of 21.92-27.69˚C.

(Left) Growing colonies of Antipathes grandis. (Right) The known distribution of the species within the Hawaiian Archipelago. Wagner (2015).

The fourth species recorded is Stichopathes echinulata, which is found across the archipelago from Hawai‘i to Lisianski, at recorded depths of 90-150 m, and probably also deeper than it was possible to evaluate using the records available for this study. Stichopathes echinulata forms unbranching wire-shaped colonies and lives at temperatures of 19.59-22.91˚C.

(Left) Growing colony of Stichopathes echinulata being manipulated by the arm of a submersible. (Right) The known distribution of the species within the Hawaiian Archipelago. Wagner (2015).

The fifth Coral species recorded is Stichopathes? sp. (an unidentified species of Coral which probably belongs to the genus Stichopathes) which was found from Hawai‘i to French Frigate Shoals at depths of 9-58 m, though it was never abundant. This species also forms unbranching wire colonies, though insufficient data was available to determine its temperature preferences.

(Left) Growing colonies of Stichopathes? sp. (Right) The known distribution of the species within the Hawaiian Archipelago. Wagner (2015).

The sixth Coral species recorded is Aphanipathes verticillata, which has been found only in the Keyhole Pinnacle area of the Au‘au Channel, at depths of 88-130 m. This species forms flat branching colonies and is found at a temperature range of 19.88-22.96˚C. The Hawaiian population of Aphanipathes verticillata was only discovered in 2008, and is thought to be a distinct subspecies, Aphanipathes verticillata mauiensis, though it closely resembles the related Antipathes griggi, and cannot be differentiated without close examination of the polyps, making it likely that many older surveys have failed to identify this species, and that it is more widely distributed within the Hawaiian Archipelago than is currently appreciated. Other populations of Aphanipathes verticillata are known from Mauritius and Okinawa, which supports the idea of this species being more widely distributed.

(Left) Growing colonies of Aphanipathes verticillata mauiensis. (Right) The known distribution of the species within the Hawaiian Archipelago. Wagner (2015).

The seventh species of Coral recorded is Acanthopathes undulata, which is found across the archipelago from Hawai‘i to Laysan at depths of 32-150 m, although it is more common below 100 m. Acanthopathes undulata forms branching colonies. It was not possible to establish a range of temperature preferences for this study.

(Left) Growing colonies of Acanthopathes undulata. (Right) The known distribution of the species within the Hawaiian Archipelago. Wagner (2015).

The final species recorded was Myriopathes cf. ulex, which is a commercially exploited species found across the archipelago from 30-150 m; it is more commonly recorded at depths of 60 m or shallower, though this may reflect the limit of harvesting potential rather than the true distribution of colonies, and the species may also be present deeper than 150 m, the limit of the survey. Myriopathes cf. ulex forms densely branching fan-shaped colonies, and has a temperature range of 20.52-26.99˚C.

(Left) Growing colonies of Myriopathes cf. ulex. (Right) The known distribution of the species within the Hawaiian Archipelago. Wagner (2015).

All of the Coral species examined in this study had depth and temperature ranges which overlapped with other species. Nevertheless there was clear separation in the habitat preferences of these species, particularly in mean temperature (the average temperature at which colonies were found), with Antipathes griggi colonies found in waters with an average temperature of 26.25˚C, Cirrhipathes cf. anguina having a mean temperature of 26.27˚C, Antipathes grandis 24.25˚C, Myriopathes cf. ulex 22.56˚C, Aphanipathes verticillata 21.19˚C and Stichopathes echinulata 21.08 ˚C. Furthermore, where species pairs had close mean temperature preferences, such as Antipathes griggi and Cirrhipathes cf. anguina or Aphanipathes verticillata and Stichopathes echinulata, they quite often had very different polyp sizes, suggesting that they were exploiting different resources. It is very likely that the distribution of these Corals is affected by other factors, such as current speeds and food supply (i.e. the amount of edible food in the water passing over them), but these were beyond the scope of the current study.

See also…

A new species of Soft Coral from the Republic of Congo.

Soft Corals (Octocorals) of the genus Alcyonium form encrusting, lobed colonies on shallow rocky surfaces in tropical waters. These Corals typically have...

The Abrolhos Bank is an area of the Brazilian continental shelf to the south of Bahia State, noted for its large and rich coral reef fauna and unique geochemical nature, with high levels of siliclastic material...

Zoanthids are unusual Corals with similarities to both the reef-forming calcareous skeleton excreting Scleractinian Corals and the larger, free living Sea Anemones. Most species are colonial, with individual polyps connected by tissue as in the Scleractinians...

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