The Crown of Thorns starfish

The Crown of Thorns starfish (COTS) is a creature that in the past few years is gaining a lot of attention since is one of the responsible of the deterioration of coral reefs all around the world. In fact, the Acanthaster planci is the most important corallivore since its adaptations to the environment have permitted them to reproduce excessively and leaded to the destruction of many reefs. 3810635 An Acanthaster planci, arms covered by spikes. http://bioweb. uwlax. edu

As shown by the photo the COTS’ main physical characteristic is the presence of spiked arms, mature organisms can have up to 23 arms with two inches long and extremely sharp spikes. These spikes and the body of the starfish also contain two types of venoms: saponins, that can lead to inflammation on body tissues and nausea and which also has an unpleasant taste for predators, and Plancitoxin 1 that can kill the liver cells of the injured organism by destroying the DNA stored in the nucleus. [1] This structural and functional adaptation gives protection to the starfish making it a very hard organism to be predated.

Even if some of the limbs get damaged, COTS is able to regenerate them when the central disc is still uninjured. But the starfish isn’t just very resistant, it is also a very effective predator. A complex water vascular system makes the Crown of Thorns starfish move way faster than the other starfish (20 metres per hour): hundreds of hydraulic tube feet and an opening next to the mouth help circulate water and so let the COTS move. This is again another vital adaptation (both functional and behavioural) for the starfish but a nightmare for the coral.

When through its fast movements the Crown of Thorns reaches a coral and its coral polyps, it positions on it and start to extrude his stomach through the mouth (positioned on the ventral side) and to secrets digestive enzymes that destroy the tissues; the walls of the stomach are now able to absorb the nutrients. This behavioural and functional adaptation allows a single organism to prey on 13 m^2 of reef per year causing massive deterioration. left00038102643505 The extruded stomach of the COTS. NOAA photo by Molly Timmers.

https://www. pifsc. noaa. gov/cred/crown-of-thorns_seastar. php Tube feet of a COTS arm used for fast movements. NOAA photo by Molly Timmers. https://www. pifsc. noaa. gov/cred/crown-of-thorns_seastar. php So we have understood how powerful and resistant a COTS is, but is also very important to say that this organism has one of the higher rates of fertilization (a large female can produce almost 50 million eggs) that linked with rapid growth often leads to sudden changes in the population and as a consequence to frequent outbreaks.

In a healthy reef with the right population of these starfish (15-25 COTS for hectare) they have a fundamental niche: balancing the relationship between fast and slow growing corals by killing only certain species of coral such as staghorns and plate corals. As it will be discussed in the next paragraphs many changes in the environment caused by both abiotic (temperature variation, decreased salinity of the water) and biotic (increased population of phytoplankton) factors have affected the population growth of COTS.

Discussion Past The outbreaks are certainly the result of very different and complex factors but for sure the humans had a role in this process. , in fact demographic models of populations of massive corals suggest outbreaks of crown-of-thorns starfish may now be more frequent and more intense than at any stage in the past several hundred years. One of the main reason of the recent outbreaks is the higher level of larval survivorship originated by the nutrient enrichment of nitrogen and phosphorus.

The anthropogenic causation theory of nutrient enrichment is strongly linked to the increase inhuman activities such as sewage discharge, fertiliser runoff, and increased erosion of nutrient-rich soil. These activities lead to the enrichment of adjacent coastal waters, a higher phytoplankton density, and a shift to larger species of phytoplankton which are more suitable as COTS larval food, resulting in better rates of survivorship of COTS larvae, ultimately leading to more frequent outbreaks.

Also the climate change plays an important role in the outbreaks and in their evolution: in fact, it’s not only exacerbating existing impacts like cyclones, but also may be introducing new threats to coral reefs including more severe storms and floods, coral bleaching, increased susceptibility to disease and reduced calcication rates. The combined effect of these disturbances means that reefs get less time to recover from a crown-of-thorns outbreak.

The “Predator removal hypothesis” argues that intense fishing has leaded to a lack of predator fishes like the starry and white spotted puffer fish, triggerfish, Humphead wrasse and other type of predators such as giant tritons and consequently to the proliferation of the starfish. In fact, even if their adaptations are a great effort to survival they’re still exposed to predation; some data confirm this theory: On the GBR reefs open to fishing were seven times more likely to experience an outbreak of crown-of-thorns starfish

(57% of reefs affected) compared to reefs effectively closed to fishing within no-take marine reserves (8% of reefs affected). [2] Present Coral reefs are vital for the humanity. 25% of fish depends on the reef ecosystem and also coral is crucial for human life since it provides protection from the waves by slowing them down: without the safety of reefs many coastal settlements would get destroyed. This is why scientists are researching to find the best ways to fight running outbreaks and we’ve already done many progresses.

The short term strategies consist in 3 process: surveillance of COTS’ population, cull through injection and examining the results and creating a huge database. The surveillance is made mainly using “manta tow”, a system that involves a snorkeler towed by a boat capable of giving general description very rapidly. In the past to kill COTS the divers had to inject them more than 20 times or to put them in freshwater, now the cull process has become much easier thanks to a single injection based on bile salts that by dissolving fats destroy cell membranes of starfish.

Bile salts are very effective but in the meantime also very expensive, that’s why a marine biologist at James Cook University is testing a new and much cheaper liquid to inject: vinegar. Crown of Thorn starfish aren’t able to regulate their blood’s pH since internally they’re basically compost by the only water and so even a small amount of vinegar is capable of acidificate the starfish and to kill them within 24-48 hours. [5] While the short term strategies are about controlling the outbreaks, the long term ones focus on prevention and researches.

Many biologists and different organization and universities are working together to better identify the causes of the outbreaks, finding new ways to fight the COTS and to sponsor initiatives like the Reef Water Quality Protection Plan and the Reef Programme that aspire to improve water quality. The results of the “Report Card 2015” of the Reef Plan are promising and shows clearly that the government is taking care of the issue in the right way: 402 graziers and 862 sugarcane growers have been involved in industry Best Management Practise programs with the purpose of reducing water pollution. [6]

Future Predicting the future of the crown of thorns starfish is very hard, there are many encouraging researches currently underway such as the use of a repellent made of a substance that is present in the mucus of the giant triton (charonia tritonis) to scary COTS and preserve specific areas of reefs or the employment of robots to replace human divers. Many tourist sites such as Green Island in Cairns are being protected and are now closed to fishing, but it’s necessary to extend the marine protected area in order to study the outbreaks and their causes excluding anthropological impediments.

Even if the Australian government is working hard to face water pollution,3 other reefs continue to badly suffer this issue and also Global Warming and temperature rising is a trend that doesn’t seem to want to stop. Conclusion The allegiance on joining the battle versus the loss of reefs should concern everyone: from the farmer that uses toxic fertilizes to all the governments of the world.

The only way to stop Acanthaster Planci’s outbreaks is the technological progress supported by the institution and the will of the people to stop Global Warming and overfishing due to their impact of the reef’s environment. If nobody acts in this way, the condition for the crown of thorns starfish will continue to get better and soon there will be no reef at all.

References:

Robinson B. <http://bioweb. uwlax. edu/bio203/s2013/robinson_brya/index. htm> (many different references ) [Accesed 3/08/2017] Cowan, Z. L. , Pratchett, M. , Messmer, V., & Ling, S. (2017). Known predators of crown-of-thorns starfish (Acanthaster spp. ) and their role in mitigating, if not preventing, population outbreaks. [internet] Diversity. Available from:< http://www. mdpi. com/1424-2818/9/1/7/htm>[Accesed 3/08/2017] Russell C. Bck , Jeffrey M. Dambacher, Elisabetta B. Morello, Eva E. Plaganyi, Keith R. Hayes,Hugh P. A. Sweatman, Morgan S. Pratchett,. (2016). Assessing Different Causes of Crown-of-Thorns Starfish Outbreaks and Appropriate Responses for Management on the Great Barrier Reef [internet] PlosOne.

Availablefrom:<http://journals. plos. org/plosone/article? id=10. 1371/journal. pone. 0169048> [Accesed 3/08/2017] Kayal et al. Kayal M, Vercelloni J, De Loma TL, Bosserelle P, Chancerelle Y, Geoffroy S, Stievenart C, Michonneau F, Penin L, Planes S, Adjeroud M. ,. (2012). Predator crown-of-thorns starfish (Acanthaster planci) outbreak, mass mortality of corals, and cascading effects on reef fish and benthic communities. [internet] PLOS ONE. ;7:e47363 doi: 10. 1371/journal. pone. 0047363 [Accesed 3/08/2017]

Australian Government,. (2017),. What is the short-term strategy? [internet]. Australian Government,Townsville. Avaiable from:<http://www. gbrmpa. gov. au/about-the-reef/animals/crown-of-thorns-starfish/what-is-the-short-term-strategy>[Accesed 3/08/2017] Australian Government,. (2017),. What is the long-term strategy? [internet]. Australian Government,Townsville. Avaiable from:< http://www. gbrmpa. gov. au/about-the-reef/animals/crown-of-thorns-starfish/what-is-the-long-term-strategy> [Accesed 3/08/2017]