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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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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Coral decline on the Great Barrier Reef.

Coral Reefs are generally considered to be among the most important of marine ecosystems, providing protection to storm battered tropical shores and forming biological hotspots. However in the late twentieth and early twenty first centuries reef ecosystems have undergone a decline around much of the world, something generally assumed to be a result of human activity.

In a paper published in the Proceedings of the National Academy of Sciences  on 1 October 2012, Glenn De’ath, Katharina Fabricius and Hugh Sweatman of the Australian Institute of Marine Science and Marji Puotinen of the School of Earth and Environmental Sciences at the University of Wollongong, look at Coral growth and dieback at 214 monitored sites of the Great Barrier Reef in Australia over a 27 year period, from 1985 to 2012.

Map of the Great Barrier Reef showing mean average Coral coverage over the period 1985-2012 as a percentage (background colouring) and overall increase or decrease at each monitored site during this period (circles). De'ath et al. (2012).

De'ath et al. found that during the period 1985-2012 the Great Barrier Reef suffered an overall loss in Coral cover from 28.0% to 13.8%; a loss of 50.7% of initial cover, but that this varied strongly at a local scale, with 67.8% of the study sites suffering an overall loss in coral coverage, and 32.2% of the sites having an overall gain in coverage during the study period.

They then looked at the cause of dieback over the effected areas, and found that 48% of the losses were caused by tropical cyclones, 42% by predation by the Crown of Thorns Starfish (Acanthaster planci) and 10% by bleaching events (death of symbiotic algae due to excessive sea temperature). 

Tropical cyclones and bleaching events are both associated with global warming, an important issue but one beyond the immediate reach of local wildlife managers, however the Crown of Thorns Starfish could potentially be tackled at a local level, with the potential to have a genuine impact on the decline in Coral on the Great Barrier Reef. De'ath et al. also note that Coral left to its own devices expands its coverage at a rate of 2.85% per year, from which they calculate that without the Crown of Thorns Starfish, Coral coverage on the Reef would actually of expanded by 0.89 per year over the study period (though this figure cannot safely be applied to future years, when the effects of global warming could potentially be worse).

The Crown of Thorns Starfish (Acanthaster planci). Marine Science Today/NOAA/David Burdick.

Increases in the population of the Crown of Thorns Starfish are believed to be linked to increasing nutrient runoff from agricultural land into rivers and estuaries and eventually to seawater. The larvae of the Starfish has been shown to have a better survival rate in water with increased nutrient levels, and increased nutrients reaching the Barrier Reef favors the growth of algae there, which is the main food of the Starfish. Raised nutrient levels do not directly harm the corals, but they do promote the growth of algae, which in turn supports higher population levels in the Starfish, which eat the algae and the Coral.

Thus De'ath et al believe that controlling levels of agricultural runoff reaching the waters of the Reef is one of the most important measures to be taken in order to protect the Great Barrier Reef.

See also To cull or not to cull? The Goliath Grouper in Floridan waters, A new species of Pufferfish form French Polynesia, The biology of pumice rafts, A Brittle Star from the Late Jurassic of Southern Germany and Drilling for oil on the Ningaloo Reef.

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