Just we thought we had a hold on all the threats to Coral reef ecosystems, most of which allegedly anthropogenic (ocean acidification, ocean warming and coral bleaching, overfishing and destructive fishing practices, tourism, coastal development and sedimentation); it turns out the most destructive threat to coral reef ecosystems is a ravenous sea star.
An article in an Science Daily that took me by complete surprise, I read that hordes of Crown-of-thorns sea stars (Acanthaster planci) are the principle natural enemy of reef-building corals and the most significant threat to coral reef ecosystems today.
According to Science Daily (1 Feb, 2013) these coral killing starfish are decimating entire coral reefs, & the reasons for their spread is unclear:
Outbreaks of this coral-feeding starfish occur periodically, due to reasons that remain unclear. It decimates entire reefs in the space of just a few years, as has been observed in French Polynesia since 2004. A new study conducted by Institut de recherche pour le développement (IRD) describes a population explosion [of Acanthaster] around Moorea, the “sister island of Tahiti”. The rate of living coral cover in ocean depths and lagoons alike has dropped from 50% (healthy reef) to under 5% in 2009. The ecosystem will need at least a decade to be restored to its original state
This study published in PLoS One, paints a somewhat scary story of outbreaks of a ravenous coral eating machine that systematically goes about devouring live coral, which results in a a disastrous state of affairs where reef community structure and function is disrupted (including other coral-feeding species, such as butterflyfish):
Outbreaks of the coral-killing seastar Acanthaster planci are intense disturbances that can decimate coral reefs [...] While cyclic occurrences of such outbreaks are reported from many tropical reefs throughout the Indo-Pacific, their causes are hotly debated, and the spatio-temporal dynamics of the outbreaks and impacts to reef communities remain unclear.
Based on observations of a recent event around the island of Moorea, French Polynesia, we show that Acanthaster outbreaks are methodic, slow-paced, and diffusive biological disturbances. Acanthaster outbreaks on insular reef systems like Moorea’s appear to originate from restricted areas confined to the ocean-exposed base of reefs. Elevated Acanthaster densities then progressively spread to adjacent and shallower locations by migrations of seastars in aggregative waves that eventually affect the entire reef system. The directional migration across reefs appears to be a search for prey as reef portions affected by dense seastar aggregations are rapidly depleted of living corals and subsequently left behind.
The [French Polynesian] outbreak [...] is among the most intense and devastating ever reported. Using a hierarchical, multi-scale approach, [it is possible to see] how sessile benthic communities and resident coral-feeding fish assemblages were subsequently affected by the decline of corals. By elucidating the processes involved in an Acanthaster outbreak, the IRD study contributes to comprehending this widespread disturbance and should thus benefit targeted management actions for coral reef ecosystems.
This BBC timelapse of swarming monster worms and sea stars from youtube shows just how readily seastars repond to the existence of prey. Although these stars are not Acanthaster, the nature of the response is the same.
The Great Barrier Reef in Queensland, Australia has been hit hard by outbreaks of Acanthaster planci
Outbreaks have also noted in the Indian Ocean and the Andaman sea
What are the reasons behind outbreaks of Acanthaster planci?
Unfortunately the causes of outbreaks remain unclear. It is believed that fertilizer used in agriculture, which is subsequently being released to the sea, provides food for phytoplankton, and when phytoplankton blooms it causes zooplankton to bloom as well. The larvae of the crown-of-thorns starfish begins as zooplankton, so the presence of fertilizer causes the crown-of-thorns starfish to proliferate.
[This seems to be the case] in Australia, where the pest is also rife, [where] invasions occur after years with high rainfall. Rainfall leads to the excess release of nutrients from human activities and the proliferation of algae on which echinoderm larvae feed.
However this does not seem to be a contributing factor in Polynesia:
In Polynesia, however, anthropic pressure seems too low and localised to explain such an outbreak of the starfish. The current lack of data on the subject means the phenomenon remains a mystery.
Another possible cause of the abundance of Acanthaster starfish is a lack of abundance of predator species such as the Triton sea snails, or triggerfish. However not much is understood about the predation Acanthaster other than an observation of occasional predation by some species.
- Giant Triton (Charonia tritonis) a very large gastropod mollusc, is a known predator of Acanthaster. The triton has been described as tearing the starfish ‘to pieces with its file-like radula‘. This triton attacks and consumes an Acanthaster on Beaver Reef (Mission Beach, Australia) in 2002.
- An old (1973) article in Nature observed species of puffer-fish and two trigger-fish feeding on Acanthaster in the Red Sea. It was concluded that although this predation may have some effect on the Acanthaster planci population, there was no evidence of systematic predation.
Since the causes of outbreaks remain unclear, there is limited ability to fight against Acanthaster planci. Researchers are currently studying processes to “recruit” new corals and make reefs more resilient. Without a new widespread disturbance, a coral ecosystem would need 10 to 30 years to be restored to its original state.
What is the definition of an Outbreak?
Scientists approximate if there are greater than 10 individuals in one hectare, that area is considered to have an outbreak. If there are greater than 30 individuals in one hectare, the outbreak is at a very serious level.
What can you or I do?
Well the usual practice with invasion species is direct intervention .. Isn’t it? Surely it must be acceptable for a diver remove or kill the starfish as a measure to protect and preserve affected reefs (this is subject to local law of course, and should not preclude the obtaining of a permit should one be required).
There are several methods which can be used to limit the number of crown-of-thorns starfish. You can use a knife to cut the central part of the starfish body (take care not to merely cut it in half, as the two halves can regenerate). Another option is to collect the starfish and bring them on shore to dry out.
Another method might be to sustainably harvest tritons (Charonia tritonis); or even facilitate their increased abundance.