Plectranthias raki: A new species of Perchlet from the Maldives.

The term 'Perchlet' applies to a wide variety of small Perciform Fish. The genus Plectranthias is a member of the Serranidae, the family which also includes Sea Bass and Groupers, amongst other groups. It currently contains 66 described species from mesophotic reef environments in the Atlantic, Pacific, and Indian Oceans. Plectranthias are small Fish, typically 5-10 cm long with the largest species reaching about 20 cm, which live in holes or crevices, from where they ambush small, mobile invertebrates. The small size and cryptic nature of Plectranthias means that they are not well studied, with most species described from a very small number of specimens.

In a paper published in the journal ZooKeys on 16 January 2024, Bart Shepherd of the Steinhart Aquarium at the California Academy of Sciences, Hudson Pinheiro of the Department of Ichthyology at the California Academy of Sciences, and the Center for Marine Biology at the University of São Paulo, Ahmed Najeeb, also of the Department of Ichthyology at the California Academy of Sciences, and of the Maldives Marine Research Institute, Claudia Rocha, again of the Department of Ichthyology and of the Department of Microbiology at the California Academy of Sciences, and Luiz Rocha, once again of the Department of Ichthyology at the California Academy of Sciences, describe a new species of Plectranthias from the Kuramathi Outer Reef on Rasdhoo Atoll in the Maldives.

The new species is described from two specimens collected by hand-netting at a depth of 118 m, in December 2022, and confirmed as a new species by gene-sequencing. It is named Plectranthias raki, where 'raki' means 'feeling shy to confront people' in the Dhivehi language which is spoken in the Maldives.

Living specimen (not retained) of Plectranthias raki photographed at 110 m depth at Dhaalu Atoll, Maldives. Luiz Rocha in Shepherd et al.  (2025).

The two specimens of Plectranthias raki are 66.15 and 70.41 mm long, and pinkish white in colour with a series of irregular orange-red patches, these being more red towards the tail and more yellow towards the head. Both have dorsal fins with fifteen rays, anal fins with seven rays, pectoral fins with thirteen rays, and tail fins with nine upper rays and eight lower rays.

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Sueviota aethon: A new species of Dwarf Gobi from the Red Sea coast of Saudi Arabia.

First described in 1988, the Dwarf Gobi genus Sueviota is distinguished from the closely related Eviota on the structure of its pelvic fins. The genus currently contains eight species, found from Papua New Guinea and the northwestern coast of Australia through to the Red Sea.

In a paper published in the journal ZooKeys on 12 September 2024, Viktor Nunes Peinemann and Lucía Pombo-Ayora of the Red Sea Research Center of King Abdullah University of Science and Technology, Luke Tornabene of the School of Aquatic and Fishery Sciences and Burke Museum of Natural History and Culture of the University of Washington, and Michael Berumen, also of the Red Sea Research Center of King Abdullah University of Science and Technology, describe a new species of Sueviota from the Red Sea coast of Saudi Arabia.

The new species is named Sueviota aethon, where 'aethon' derives from Aethon, one of the four Horses which drew the chariot of the Sun God Helios in Greek mythology; it is so named due to its similarity to the previously described Sueviota pyrios; Pyrios having been another of the four Horses. The species is described from ten specimens collected from exposed offshore reefs on the Saudi Arabian Red Sea coast, at depths of between 10 m and 30 m, although Nunes Peinemann et al. note that another specimen was observed at a depth of 53 m.

Holotype specimen of Sueviota aethon (UW 203365), shortly after being collected. Nunes Peinemann et al. (2024).

Specimens of Sueviota aethon are between 9.2 mm and 16.7 mm in length, and most known specimens are dark red in colour (one was a yellow-orange colour). The first dorsal fin is rounded-to-square in shape, with the second and third spines longer than the first. The rays of the second dorsal fin are commonly branched (at least some of these are branched in all known specimens). The body is covered by ctenoid (comb-edged) scales, but these are absent from the head and breast. Two rows of irregularly spaced conical teeth are present on both the lower and upper jaws. Both jaws also have enlarged canine teeth, with these forming part of the outer tooth-row in the upper jaw and the inner tooth-row in the lower jaw.

Micro-CT scan of Sueviota aethon (UW 203365, holotype) showing its osteological characters. (a) Close-up of head showing the enlarged canines on the upper jaw, (b) dentary, showcasing two enlarged canines in the internal row of teeth, (c) lateral view of the complete skeleton. Nunes Peinemann et al. (2024).

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Acanthaster benziei: A new species of Crown-of-thorns Starfish from the Red Sea.

Crown-of-thorns Starfish, Acanthaster spp., are highly distinctive Starfish found across the tropical Indo-Pacific region from the east coast of Africa to the west coast of Mexico, which get their popular name from the covering of long, venomous spines found in most species. They are typically corallivorous, feeding on Coral Polyps by extruding their stomachs and digesting them externally. Notably, Crown-of-thorns Starfish can undergo sudden rapid population increases, known as outbreaks, which can lead to large areas of Coral Reefs being denuded of their living Polyps, something of great concern to conservationists at a time when Coral Reefs are facing a range of other threats, which has led to them being one of the most extensively studied groups of Marine Invertebrates.

Crown-of-thorns Starfish were first described by the German naturalist Georg Eberhard Rumphius in 1705, and given their own generic name, Acanthaster, by the French palaeontologist François Louis Paul Gervais  in 1841. For a long while, only two species were described within the genus, Acanthaster planci, the typical, long-spined, venomous, corallovorous form, and Acanthaster brevispinus, a shorter-spined, non-venomous form, which does not feed on Corals. However, genetic studies carried out within the past three decades have shown that Acanthaster planci is in fact a species cluster, made up of a number of physically very similar species (cryptospecies), which are nevertheless genetically distinct, which often appear to have diverged from one-another a long time ago. 

Based upon this, it was suggested that the original species should be split into four different species, each inhabiting a different geographical area; the Pacific, the Southern Indian Ocean, the Northern Indian Ocean and the Red Sea, which each of these species probably needing further division into several subspecies. Subsequent studies have indeed confirmed that the Pacific, North Indian Ocean, and South Indian Ocean populations are in fact separate species, although genetic material from the Red Sea population has not, until now, been available.

In a paper published in the journal Zootaxa on 17 November 2022, Gert Wörheide of the Department of Earth and Environmental Sciences Palaeontology and Geobiology, and the GeoBio-Center at Ludwig-Maximilians-Universität München, and the Bavarian State Collection of Palaeontology and Geology, Emilie Kaltenbacher and Zara-Louise Cowan, also of the Department of Earth and Environmental Sciences Palaeontology and Geobiology at Ludwig-Maximilians-Universität München, and Gerhard Haszprunar, also of the GeoBio-Center at Ludwig-Maximilians-Universität München, and of the Bavarian Zoological State Collections, describe the Red Sea population of Crown-of-thorns Starfish as a new population.

The new species is named Acanthaster benziei in honour of marine biologist John Benzie, for his extensive work on Crown-of-thorns Starfish. The description is based upon four specimens collected from species within the territorial waters of Saudi Arabia by  Sara Campana and OliverVoigt in 2017.

Typical colouration of Acanthaster benziei. (A) GW4081 (Paratype, hiding during the day under a crevice), Al-Lith, Saudi Arabia, (B)–(D) Thuwal Reefs, Saudi Arabia. Approximate diameter of specimens is 25–30 cm. Oliver Voigt & Gert Wörheide in Wörheide (2022).

Acanthaster benziei is a large Starfish with a convex disk and 11-14 arms (the range for the genus being 10-25), of uneven lengths, and tapering to a point. Each arm has two rows of ambulacral tube feet, which have flattened tips and lack suckers. The central disk of the species is 28-65 mm across, with an aboral (upper surface) covered in papulae (pimples) arranged in an apparently random manner. Both surfaces are covered in calcareous ossicles (plates) and spines. These Starfish are grey-green to grey-purple in colour, although the aboral spines are orange or red. The papulae on the aboral surface of the central disk can form darker patterns, giving this surface a 'bulls-eye' appearance.

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Terpios hoshinota: Killer Sponge found to be invading the Coral Reefs of the Lakshadweep Archipelago.

Coral killing Sponges have the potential to overgrow live Corals, eventually killing the Coral polyps, and thus leading to an epidemic. The Cyanobacteria-symbiotic Sponge, Terpios hoshinota, also known as the Black Disease, was first reported from Guam in 1973, and later described from the coral reefs of the Ryukyu archipelago (Japan). It is identified by its gray to blackish encrustations. Since its first occurrence, it has been observed in several coral reef localities around the globe, viz., the Great Barrier Reef, Papua New Guinea, Taiwan, Philippines, Indonesia, South China Sea, Thailand, Palk Bay and the Gulf of Mannar on the southern tip of India, Maldives, and Mauritius.

In a paper published in the Journal of Threatened Taxa on 26 October 2020, Rocktim Ramen Das of the National Centre for Sustainable Coastal Management Forest and Climate Change in Chennai, Tamil Nadu, and the Graduate School of Engineering and Science at the University of the Ryukyus, and Chemmencheri Ramakrishnan Sreeraj, Gopi Mohan, Kottarathil Rajendran Abhilash, Vijay Kumar Deepak Samuel, Purvaja Ramachandran, and Ramesh Ramachandran, also of the National Centre for Sustainable Coastal Management, confirm that the species has further extended its habitat into the pristine atolls of the Lakshadweep Archipelago in the Arabian Sea, and requires urgent attention.

 

Bangaram & Thinnakara atoll (Inset, red star). Das et al. (2020).

During the coral reef surveys conducted at Lakshadweep in November 2016, Terpios hoshinota was observed overgrowing on several colonies of Acropora muricata, Isopora palifera, Cyphastrea sp., Dipsastraea lizardensis and Porites lutea in the atoll encircling Bangaram and Thinnakara Islands. Out of 34 sites surveyed, six exhibited the presence of Terpios hoshinota. The Coral colonies on the atoll were patchy and the depth of the atoll varied between 2 and 12 meters. As depth increased, (i.e. deeper than 5 m) large boulder Corals were observed whereas the shallow regions (shallower than 5 m) had greater Coral diversity. Certain areas consisting of large Acropora beds, rocks, rubbles, and dead reef were also observed. The affected Corals displayed grayish/blackish encrustations of Terpios hoshinota forming a mat-like layer on live Corals taking the shape of the Coral in all cases. The osculum in the Sponge, a primary character with a radiating network of canals, was clearly visible and the thickness of the mat was less than 1mm. It was observed that the encrusting Sponges were propagating laterally and infecting the other live Coral colonies. Other associated communities such as Ascidians and Clams remain unaffected; calcareous Serpulid tubes were overgrown by the Terpios, although the Animal was unharmed. Furthermore, in some colonies along with Terpios hoshinota, Algal presence was noted, but the Sponge was absent in the colonies which were completely covered with Turf Algae. Environmental parameters assessed with a multiparameter water quality probe revealed that the area was unpolluted with an optimum level of dissolved oxygen (5.04-8.21 mg per litre), and low turbidity (0.3 to 0.8 Nephelometric Turbidity Units). Sea surface temperature during the survey was 28.2°-30.1°C. It is important to note that, Bangaram and Thinnakara is one of the few atolls in Lakshadweep where tourism is permitted, as a result, limited amounts of diving and other water-related recreational activities can be seen in the area.

 

(A) Encrustations of Terpios hoshinota on Acropora muricata, (A1) erpios hoshinota exhibiting osculum with radiating networks. (B) Encrustation on Isopora palifera, (B1) Terpios hoshinota mat, (B2) Bleached ring, (B3) Live Coral. (C) Terpios hoshinota taking shape of a Coral (Cyphastrea sp.). (D) Terpios hoshinota overgrowing calcareous serpulid tubes, (D1) Animal unaffected. Das et al. (2020).

Previous studies suspected that the outbreak of Terpios hoshinota is related to increased water turbidity or due to high anthropogenic stress/pollution its close proximity to mainland, as reported in the south eastern reefs of India (about 800 km from Lakshadweep), Guam, and in Green Island, Taiwan. A similar conclusion, however, cannot be applied in the case of Lakshadweep because of its isolated geography and with comparatively less anthropogenic activities. As a result, Das et al.'s observation contradicts this hypothesis and is more in line with the findings of Qi Shi, Gou Hui Liu, Hong Qiang Yan, and Hui Ling Zhang, who observed Terpios hoshinota outbreak in unpolluted areas of Yongxing Island (South China Sea), highlighting the difficulty in establishing a negative co-relationship between water quality and Black Disease outbreak. In terms of host selectivity, the Killer Sponge has affected several Coral species in different parts of the world and in the reefs of Palk Bay, it has affected all genera surveyed. In Vaan Island, Gulf of Mannar, the dominant genus Montipora was the most susceptible. Das et al.'s observation though could not reveal any specific host coral selectivity, Das et al. speculate that the dense branching Acropora Coral beds in site 3, 5 and 6 were more easily overgrown because the Killer Sponge prefers branching Corals as reported from Mauritius. Das et al. further conclude that the Coral composition in any specific location may play an important role in determining its host.

 

Acropora colonies (Site 3): (A) (A1) Terpios hoshinota (A2) Algae. (B) Acropora colonies (Site 5) completely over grown by Turf Algae, Killer Sponge/Black Disease absent. Das et al. (2020).

Terpios hoshinota is a belligerent contender for space and is known to overgrow corals from its base where it interacts with Turf Algae. Branching Acropora beds in site 3, 5 and 6 consisted both Algae (e.g. Dictyota sp.) and the Killer Sponge. Additionally, a massive Turf Algae which covered area of about 0.35 km in the Terpios hoshinota occurrence site highlights a complex ecological scenario. Such complex interactions between Sponges, Corals and Algae can be only understood through long term monitoring. Manuel González-Rivero, Laith Yakob, and Peter Mumby stated that Sponges can act as a potential group that can facilitate and influence Coral-Algal shifts by acting as a 'third antagonist' as observed in Glover’s atoll (Belize).

Based on their knowledge of the life history of Terpios hoshinota Das et al. hypothesize site 5/6 scenario as follows: (1) Terpios hoshinota invades and overgrows the Acropora beds (2) The Coral dies which is followed by the death of the Killer Sponge (3) Turf Algae takes over. Moreover, reports of Turf Algae being a dominant component in the atolls might indicate a faster transition. Globally Elevated sea surface temperature is a major threat to Coral Reefs, and the reefs of India, including the atolls, are no different. With reports indicating that elevated sea surface temperature has already depleted the Coral ecosystem of Lakshadweep, which was evident during 1998, 2010, and 2016, mass bleaching events, it can provide an opportunity for Sponges to invade. The dynamics of waterflow may also play a crucial role in this regard.

Das et al.'s findings confirm that the infestation of Terpios hoshinota on the coral colonies of Lakshadweep is currently limited to only Bangaram and Thinnakara as it was not observed in the other atolls surveyed. Although there is a possibility that the Killer Sponge could invade nearby atolls as seen in other regions, large-scale damage cannot be concluded at this stage. This is in fact the first documentation of Terpios hoshinota on the reefs of Lakshadweep and can be regarded as a baseline for subsequent studies. Further, to protect the reefs of Lakshadweep, a long term Coral health monitoring program is required which will allow us to understand the nature of occurrence, distribution, the impact and the causative factors of the Killer Sponge and to understand it’s larger threat to the reefs. Black Disease along with other Coral associated diseases needs enlarged emphasis according to which various Coral Reef management plans can be initiated.

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Lophelia pertusa: Cold-water Corals found living in strongly anoxic conditions off the coast of Angola.

Being ecosystem engineers, framework-forming Scleractinian Cold-water Corals provide habitat for thousands of deep-sea species, revealing equally remarkable levels of biodiversity as found in tropical Coral Reefs. Lophelia pertusa is the dominant reef-forming Cold-water Coral in the Atlantic, and based on its distribution correlated with ocean conditions, upper and lower tolerable limits for basic oceanographic parameters were proposed for this species. Among them, dissolved oxygen concentrations can exert control on its biogeographic distribution. However, lowest dissolved oxygen concentrations inhabited by this species apparently differs between the northeast Atlanatic, where it can tollerate levels as low as about 2 millilitres of oxygen per litre of seawater, and the northwest Atlanatic, where it can tollerate levels as low as about 3.7 millilitres of oxygen per litre of seawater. These observations are corroborated by laboratory experiments, revealing that Lophelia pertusa individuals collected from waters on the the Scottish margin in the northeast Atlantic, where there is a dissolved oxygen concentration of about 6 milligrams per litre of seawater, Atlantic, were unable to maintain normal aerobic functions at dissolved oxygen concentrations of less than 3.2 millilitres per litre of seawater. Moreover, for Lophelia pertusa specimens collected from areas of the Gulf of Mexico with dissolved oxygen concentrations of about 2.8 millilitres per litre of seawater, a 7-day exposure to a dissolved oxygen concentration of 1.5 millilitres per litre of seawater proved fatal. However, discoveries of Lophelia pertusa in the oxygen minimum zones of the subtropical eastern Atlantic have hinted at an even wider tolerance of Lophelia pertusa to low dissolved oxygen concentrations. Nevertheless, the limited capability of Lophelia pertusa to thrive under dissolved oxygen concentrations (artificially) reduced below those of their natural environment questions its ability to cope with the global change-induced ocean deoxygenation expected for the coming century.

In a paper published in the journal Coral Reefs on 6 April 2020, Dierk Hebbeln and Claudia Wienberg of the MARUM Center for Marine Environmental Sciences at the University of Bremen, Wolf-Christian Dullo of the GEOMAR Helmholtz Centre for Ocean Research, André Freiwald of the Marine Research Department at Senckenberg am Meer, Furu Mienis of the NIOZ Royal Netherlands Institute for Sea Research and Utrecht University, Covadonga Orejas of the Centro Oceanogra´fico de Baleares of the Instituto Español de Oceanografía, and Jürgen Titschack, also of the MARUM Center for Marine Environmental Sciences at the University of Bremen, and the Marine Research Department at Senckenberg am Meer, present the discovery of Lophelia pertusa-dominated Cold-water Coral reefs thriving in the hypoxic oxygen minimum zone off Angola in the southeast Atlantic. The regional adaptation of the Angolan Cold-water Corals to such extreme conditions sheds new light on their potential capability to cope with expected future environmental changes in the ocean.

During RV Meteor expedition M122 in January 2016, in situ oceanographic parameters such as dissolved oxygen concentrations and temperature were recorded off Angola. Data were collected during eight dives with the Remotely Operated Vehicle Marum Squid, carried out three benthic lander deployments, and 17 conventional conductivity, temperature, and depth instrument casts. The conductivity, temperature, and depth instrument was additionally equipped with a non-calibrated fluorescence sensor only providing relative values shown as means per water depth averaged from all conductivity, temperature, and depth instrument casts.

Multibeam bathymetry map showing the distribution of coldwater Coral reefs off Angola. Locations of conductivity, temperature, and depth (CTD) casts, benthic lander deployments, and remotely operated vehicle (ROV) dives are indicated. Hebbeln et al. (2020).

Remotely operated vehicle video observations revealed the presence of Cold-water Coral Reefs dominated by Lophelia pertusa, which colonise the slopes and summits of up to 100 m high Coral mounds. While dispersed Cold-water Coral colonies were found in a depth range of 250–500 m, large aggregates of healthy colonies were restricted to 330-470 m water depth. The observation of over 50 cm high colonies clearly evidenced the continuous proliferation of Cold-water Coral off Angola for many years.

Thriving Cold-water Corals observed in the oxygen minimum zone off Angola. (a), (b) Lophelia pertusa reefs in the center of the  oxygen minimum zone (350 m water depth). (c) Transported but alive Lophelia pertusa colony in the lower oxygen minimum zone (500 m depth). (d) Lophelia pertusa colony with many living polyps (439 m depth) (ROV images). Hebbeln et al. (2020).

The available oceanographic data revealed water temperatures of 6.8–14.2°C around the Cold-water Corals at depths of 250–500 m. The corresponding dissolved oxygen concentrations of 0.6-1.5 millilitres per litre of seawater are the lowest ever obtained from waters bathing flourishing Lophelia pertusa colonies.

To gain insight into the seasonal variability of  dissolved oxygen concentrationsoff Angola, as the M122 data only represent an 8.5-day snapshot from January 2016, Hebbeln et al. included further 21  conductivity, temperature, and depth instrumentcasts obtained within the mapped area off Angola between 1995 and 2013. These data, spanning from March to September, almost completely correspond to the M122 data or reveal even lower  dissolved oxygen concentrations. Interestingly, even in this hypoxic environment, most prolific Cold-water Coral Reefs are bound to the center of the Angolan oxygen minimum zone where lowest dissolved oxygen concentrations prevail, which coincide with enhanced water-column fluorescence pointing to an increased availability of relatively fresh organic matter.

Based on field observations in the northwest and northeast Atlantic, the assumed lower limit of Lophelia pertusa’s oxygen tolerance ranges around 2-3.7 millilitres per litre of seawater. This has recently been challenged by very low dissolved oxygen concentrations of 1.1-1.4 millilitres per litre of seawater reported from Cold-water Coral sites off Mauritania, which, however, are associated with only sporadic occurrences of small Lophelia pertusa colonies. The new Angolan data documented for the first time Lophelia pertusa’s ability to develop thriving reefs even under dissolved oxygen concentrations of under 1 milligram per litre of seawater.

In addition, off Angola Lophelia pertusa lives at temperatures of up to 14.2° C, which are among the highest temperatures ever observed for this species. Thus, off Angola, the partly high temperatures could act as a second stressor since respiration rates of Lophelia pertusa increase with increasing temperature. 

Stress induced by low dissolved oxygen concentrations and relatively high temperatures is energetically a challenge for the metabolism of most marine species, but can be compensated by the availability of large quantities of high-quality organic matter. The Angolan and Mauritanian margins belong to highly productive upwelling systems triggering extensive oxygen minimum zones. Also at many other Atlantic reef sites, Lophelia pertusa is most abundant at depth intervals with highest oxygen depletion, most likely linked to highest concentrations of suspended food particles in this layer, which also applies to Angola. Comparing ambient dissolved oxygen concentrations and temperature with site-specific net primary productivity, used as a food supply indicator, for several Atlantic Cold-water Coral sites, it appears plausible that the negative effects of hypoxia and high temperatures on Lophelia pertusa seemingly could be compensated by significantly enhanced food supply.

With respect to Lophelia pertusa preferring regional oxygen minima, ambient dissolved oxygen concentrations cannot provide any information about its capability to also cope with lower dissolved oxygen concentrations. However, some information is provided by the aforementioned laboratory experiments. Lophelia pertusa collected in the northeast Atlantic and the Gulf of Mexico could not withstand dissolved oxygen concentrations of less than 40–50% of the ambient values. Consequently, the range of low dissolved oxygen concentrations tolerable by Lophelia pertusa, also beyond its natural environment, might depend on the conditions the corals are acclimated to, thus pointing to a possible genotypic adaptive capacity of Lophelia pertusa. Thus, although on a global scale the tolerable dissolved oxygen concentration limits for Lophelia pertusa range from less than 1 to more than 6 millilitres per litre of seawater, smaller ranges define these limits on regional scales.

Cold-water Coral Reefs are vulnerable marine ecosystems that are partly protected within marine protected areas. These can safeguard Cold-water Corals from destructive Human impacts (e.g., bottom trawling, hydrocarbon exploration), but offer no sustainable protection against global change-induced threats. In concert with ocean acidification and warming of intermediate waters, eoxygenation is expected to become a major stressor for  Cold-water Corals. However, Lophelia pertusa’s general capacity to thrive under well-oxygenated as well as hypoxic bottom waters reveals a rather high oxygen tolerance, although individual Lophelia pertusa populations appear to have limited adaptive capabilities to cope with reductions of 40–50% of ambient dissolved oxygen values. Consequently, the expected decrease in oxygenation of about 2% along the Atlantic continental margins by 2100 by itself might not exert a serious threat to Lophelia pertusa, except for already hypoxic settings like the Angolan margin. However, palaeotological studies revealed that during the last approximately 20 000 years regional changes in water column structure caused the collapse of Lophelia pertusa dominated ecosystems due to decreasing  dissolved oxygen concentrations. Thus, unlike a small overall decrease in dissolved oxygen concentrations, major regional reductions in dissolved oxygen concentration driven by global change-induced changes in ocean circulation have the potential to eradicate regional Lophelia pertusa populations.

Even if smaller decreases in dissolved oxygen concentration alone might not pose a serious threat to Lophelia pertusa reefs, these have to be considered in concert with other changing environmental parameters that might form additional stressors (e.g. temperature and pH) with largely unknown consequences for the Coral’s biological functions. Moreover, the flux of particulate organic carbon from the surface ocean might decline by about 30% by 2100 along the Atlantic margins, resulting in a lower food supply to the Cold-water Corals and deep-sea organisms in general, thus reducing their capacity to cope with increasing stress.

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Acanthaster solaris: Using Environmental DNA to track the Crown-of-Thorns Starfish.

The Great Barrier Reef on Australia’s east coast is the world’s largest marine protected area, a World Heritage Site and a biodiversity hotspot of global importance. Despite this, the reef is estimated to have lost more than 50% of its Corals during the past three decades. Much of this is due to global warming, and the accompanying acidification of the seawater, but other factors are important. One of these has been repeated outbreaks of the Crown-of-Thorns Starfish, Acanthaster solaris, a Coral-consuming Echinoderm credited with having caused 42% of Coral loss on the Great Barrier Reef prior to the bleaching events of 2016-17. The Crown-of-Thorns Starfish has entered a boom-and-bust population cycle since the 1960s, with outbreaks producing huge numbers of Starfish that consume all the available food (i.e. Coral) then die out due to starvation. The precise cause of these booms is unclear, but probably linked to the life-cycle of the Starfish, which produce planktonic larvae, with the most likely explanation being greater numbers of larvae surviving because of increased food availability due to nutrients from agricultural runoff, or increased larval survival due to a reduced number of predators caused by overfishing. This makes Starfish booms of great interest to conservationists trying to protect the Great Barrier Reef, who need to detect new outbreaks as quickly as possible in order to take remedial action.

 

 An adult Crown-of-Thorns Starfish predating Coral. Hall et al. (2017).

 

In a paper published in the journal Coral Reefs on 12 September 2018, Sven Uthicke of the Australian Institute of Marine Science, Miles Lamare of the Department of Marine Science at the University of Otago, and Jason Doyle, also of the Australian Institute of Marine Science, describe the results of a trial of a method which used environmental DNA to track populations of the Crown-of-Thorns Starfish.

Environmental DNA (or eDNA) is DNA shed into the environment by an organism via shed skin cells, and excretion of mucus, urine or faeces. The detection of eDNA has become a standard methodology for detecting invasive of endangered species in freshwater environments, but the much larger volume of the oceans, which means that the eDNA will be significantly more diluted by the water, makes detecting eDNA in marine environments considerably harder, and the technique has yet to be successfully applied in this setting.

In order to establish the amount of eDNA produced by Crown-of-Thorns Starfish a single individual was placed in a 10 000 litre seawater tank at the Australian Institute of Marine Science’s National Sea Simulator. This tank had continuous through-flow of water at a rate that would replace all the water twice a day, and the Starfish was kept in it and monitored for one week. This was then repeated with two Starfish, then three, up to a maximum of sixteen, in order to calibrate the methods used for eDNA detection.

Seawater was then collected on four field trips between June 2016 and August 2017, covering reefs in the Cooktown, Innisfail and Ingham to Townsville regions, and tested for levels of Crown-of-Thorns Starfish eDNA. The areas covered included two reefs where there had previously been Starfish outbreaks, two where the Starfish had never been observed, five reefs with active outbreaks, and two reefs without outbreaks, but which were 50-65 km from a reef where and outbreak was ongoing.

No Crown-of-Thorns Starfish eDNA was detected at any site where the Starfish were not present, but it was found in the samples from all the reefs where the Starfish were observed. Furthermore, the levels of eDNA found in the samples closely reflected the known densities of Starfish on these reefs, indicating that the test is both a viable method for detecting the Starfish and a reliable way to estimate their population density.

Density estimates of Acanthaster solaris (left) and eDNA concentration on 11 reefs of the Great Barrier Reef, Australia. Uthicke et al. (2018). 

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