Crown of Thorns

Figure 7-The Crown of Thorns Starfish (Nhobgood, 2005)

The crown of thorns (COT) starfish, Acanthaster planci, (Figure 7) is a destructive corallivore. COT was first recorded in 1705 but was not named and described until 1758 by Linnaeus (Vine, 1973). There is a background level of COT present throughout reefs in the pacific, typically one starfish per hectare (Glynn, 1973). When present in low densities COT has little effect on the ecosystem. However during an outbreak in 1967 COT consumed more than 90% of coral along the coastline of the US Territory of Guam (Chesher, 1969) and many outbreaks have since been recorded.

COT feeds by everting its stomach through its mouth and digesting the soft coral tissue (Chesher, 1969). It leaves behind the skeleton of the coral which looks as if it has been bleached. COT is a nocturnal feeder and it is rarely seen throughout the day, except during outbreaks (Vine 1973). It can feed on and hence destroy an area twice the size of its disk (the main body) over a day (Chesher, 1969). COT has been shown to prefer feeding on Acropora sp and Montipora sp over Porties sp and Favita sp (Pratchett, 2007) which explain patterns in the destruction of corals.

Little is known about why these outbreaks occur. There are two different kinds of outbreaks, primary and secondary (Fabricius, 2010).  Secondary outbreaks have been attributed to the large number of gametes produced during a primary outbreak (Dight et al., 1990).  There are currently two main hypotheses for the cause of the primary outbreaks, the terrestrial runoff hypothesis and the predator removal hypothesis. The terrestrial runoff hypothesis states that high amounts of sewage/fertiliser runoff from land contain enough nutrients to cause large phytoplankton blooms, which can support large numbers of COT larvae (Birkeland, 1982). Food availability is strongly linked with the survival of COT larvae (Lucus, 1982). The predator removal hypothesis suggests that COT juvenile mortality is lowered because of the removal of predators through human overexploitation (Dulvy et al., 2004). Recent research using molecular data by Vogler et al. (2008) has shown that Acanthaster planci is in fact four separate species which could have implications regarding the management of the starfish and could explain why reasons for outbreaks have not been determined.

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