Interactions

Predation: The sharp spines that cover Acanthaster planci along with its saponin filled tissues makes this species incredibly difficult to prey on; furthermore, these spines house two chemical defense mechanisms (see Venom) the saponins and plancitoxin I. When the spines enter a predator they release these toxins, the saponins (having a foul taste and serving as an irritant) and the plancitoxin I which attacks liver cells. The Crown-of-Thorns starfish also has the ability to regenerate lost body parts, which makes it a very hard to kill and an unattractive food source; however, despite its incredible defensive mechanisms there are still predators out there who have adapted and are able to feed upon the starfish.

Some predator examples:
•The triton, a gastropod mullosc utilizes their file-like radula to break down Acanthaster planci.
•A shrimp species, Hymenocera picta, and a polychaete worm, Pherecardia striata, have been seen to prey on the starfish together.
•The Pherencardia striata as well as other organisms have been found to only prey on damaged Acanthaster planci, ignoring the distastefulness of the saponins.
•One of the most potent predators to the Crown-of-Thorns starfish is the polyp-like creature in the genus Pseudocorynactis, which can wholly ingest a starfish of similar size.

Image of Hymenocera picta taken from Wikipedia. Author: Chad Ordelheide (left). Triton (right) feeding on a starfish.

Human Interaction: Swarms of predatory Acanthaster planci have a huge impact on coral reefs and because of this there are many human interactions taking place to both study Acanthaster planci and to aid in coral reef protection. Large swarms of these starfish, also known as an outbreak, cause devastating blows to coral populations and coral reef destruction. Some species of coral can be wiped out of a coral reef system in just one outbreak. The destruction of coral reefs can have a huge impact on humans due to reefs being able to slow down and weaken waves. Below is an example of a coral reef's impact on wave intensity.

Gif taken from Wikipedia. Original uploader: Lachaume

As the waves to the left travel over the reef slope the waves slow down and the height increases. This is incredibly important in the control of tidal wave intensity as the strength and speed of a tidal wave can be drastically slowed by coral reefs. Without the aid of coral reefs, tidal waves would decimate coastal villages and strong waves would make seafaring transportation very dangerous.

Biocontrol: People seeking to preserve coral reefs have used induced pathogenesis to control Acanthaster planci outbreaks. By inducing pathogenesis in this coral eating starfish, control of outbreaks and the protection of reef communities has increased. A study was taken to observe the reactions of an induced compound, called Thiosulfate-citrate-bile-sucrose agar (TCBS), on Acanthaster planci. Of the 9 TCBS compounds tested, 4 of the compounds caused allergic reactions in the starfish and eventually caused death. Researchers found that TCBS caused allergic reactions and not pathogenic reactions; furthermore, because the compounds are protein-based extracts they are considered safer than using other chemicals. Further research indicated that this method of biocontrol is cost effective and less harmful to the environment than other forms of biocontrol against Acanthaster planci (Journal of Experimental Marine Biology and Ecology 1-6).

Someone holding an Acanthaster planci starfish (left). A starfish lounging on Acroporal coral (right). Images taken from Wikipedia. Author of the image on the right: Richard Ling

Ecosystem Impacts: Coral reefs are home to thousands of organisms who depend on the reefs for protection. Researchers found that living coral reefs can house around 62 species as oppose to dead coral housing 43 and only 22 on coral debris.

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