Sponges, Porifera, form an important part of many benthic marine communities, both for their contribution to the structure of reefs and their forming of symbiotic relationships with a variety of other organisms, including Prokaryotes, Shrimp, Worms, Hydroids, Zoantharians, and Fish. However not all Sponges are beneficial to the communities that host them. The Heteroscleromorph Demosponge Terpios hoshinota forms symbiotic relationships with a variety of Cyanobacteria, spreading rapidly by photosynthetic growth. This enables it to grow at a rate of several milometers per day, forming a thin black or grey crust that rapidly overgrows and kills Hard Corals such as Lobophylia, Montipora, Acropora, Merulina, and Goniastrea.
The first known outbreak of Terpios hoshinota was recorded on Guam in 1973, since when it has spread to the Northern Mariana Islands, Western Caroline Islands, the Philippines, American Samoa, southern Taiwan, the Great Barrier Reef, Sulawesi, Java, the Maldives, Mauritius and the Ryukyu Islands. The first outbreak of the Sponge in the Ryukyu Islands hit the island of Tokunoshima in 1985-86, where it rapidly spread across the reefs of the Yonama Coast, eventually covering 87.9% of the reefs, and gaining the name 'Black Death Sponge'.
A colonoy of Terpios hoshinota overgrowing a Coral on Guam. Florida Museum.
The Sponge was detected on the reefs of Okinoerabu-jima Island, about 50 km to the southwest of Tokunoshima in 2010, raising concerns about the fate of the reefs there, however a survey carried out at this time found that Terpios hoshinota had completely disappeared from Tokunoshima, where reefs were now dominated by Hard Corals of the genus Acropora, suggesting that the appearance of the Sponge does not necessarily represent the end of a Coral Reef community.
In a paper published in the journal Zoological Studies on 19 April 2017, Masashi Yomogida, Masaru Mizuyama, and Toshiki Kubomura of the Molecular Invertebrate Systematics and Ecology Laboratory at the University of the Ryukyus, and James Davis Reimer, also of the Molecular Invertebrate Systematics and Ecology Laboratory, and of the Tropical Biosphere Research Center at the University of the Ryukyus, describe the results of a long-term study of the Terpios hoshinota outbreak on Okinoerabu-jima Island, based upon a series of surveys carried out between March 2010 and September 2014.
Yomogida et al. carried out a series of transect studies on the Yakomo Coast of Okinoerabu-jima Island, with each survey examing the surface covering of an area of reef measuring 10 m by 1 m. Each survey divided the covering of the reef into nine categories: (1) Terpios hoshinota, (2) Macroalgae (Seaweed) except the Sponge Weed Ceratodictyon spongiosum, (3) the Sponge Weed Ceratodictyon spongiosum, (4) Cyanobacterial mats, (5) living reef-building Corals, (6) dead Coral, (7) other benthic organisms, including Soft Corals, Giant Clams, Sea Cucumbers, and Sea Urchins, (8) sand and gravel, and (9) anything that could not be identified.
(A) Location of Okinaerabu-jima Island, Kagoshima, Japan in the northwestern Pacific and, (B) map of Yakomo coast on Okinoerabu-jima Island. Red dotted box shows the Terpios hoshinota survey area, white dotted lines show the approximate area of Terpios hoshinota along the coast, and red solid lines approximate locations of permanent transects. Google Earth/Yomogida et al. (2017).
Terpios hoshinota covered over 24% of the reef at the outset of the study (March 2010), and remained this high until October of that year, but fell to 17.6% coverage in December 2010. In June 2011 the species underwent a catastrophic die-back, falling to a covering of only 0.02% of the reef. Sponge levels remained low for the next year, having reached only 0.3% coverage by May 2012, but did eventually begin to recover, reaching 11.4% coverage by September 2014.
Coverage of the reef by Macroalgae remained below 10% in all surveys except one, in May 2012, when it reached 13.6%. Cyanobacteria were completely absent from the reef in all surveys except one, in October 2011, when it covered 39.9% of the reef. Sand and gravel remained the dominant coverings of the reef throughout the survey, with coverage varying between 50.2% and 89.4%; none of the other categories ever climbed above a 5% coverage on the reef.
Clearly some event significantly reduced the coverage of Terpios hoshinota in 2011, and came close to removing the Sponge from the reef altogether. Yomogida et al. suggest that the most likely culprit was Typhoon Songda, which passed close to the island on 28 May 2011, and which is recorded as having generated windspeeds of up to 139 kilometres per hour, and wave heights of up to 10.22 m. This event could have removed the Sponge encrustation either by directly tearing it from the reef or covering it in sand or other soft sediments.
This suggests that typhoons could play a major role in inhibiting the ability of Terpios hoshinota to dominate ecosystems, and are likely to have been the cause of the disappearance of the Sponge from Tokunoshima Island. However Yomogida et al. also note that tropical storms may also play a role in the dispersal of Terpios hoshinota, as the larvae of Cyanobacteria-hosting Sponges tend to have rather limited dispersal capacities, suggesting that something else has aided the apparent rapid dispersal of this species. They also note that Terpios hoshinota is now found in both tropical and subtropical seas, and that tropical storms are a feature of only subtropical seas, with areas such as Indonesia and the Maldives, where the Sponge has become established, not effected by these storms.
Yomogida et al. also note that an outbreak of Terpios hoshinota on Pagan Island in the Mariana group was strongly linked to a volcanic eruption on that island, with a large patch of the Sponge appearing with the onset of volcanic activity in 2010, and disappearing when volcanic activity stopped in 2012. They suggest that this might be connected to the deposition of volcanic ash into the waters around Pagan Island, which would have increased the levels of nutrients, particularly iron, to the Sponge and its symbiotic Cyanobacteria. This raised the possibility that Human activities may be facilitating the spread of Terpios hoshinota, if these activities result in extra nutrients being released into the water.
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