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The Triton that ate the Crown-of-Thorns

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04 December 2014

The crown-of-thorns starfish is known for its incredible appetite for coral and the damage that it causes on coral reefs. Dr Mike Hall, Principal Research Scientist at the Australian Institute of Marine Science, explains how the rarity of a sea snail may be one reason why the crown-of-thorns is now such a threat to the survival of the Great Barrier Reef. Research towards the development of smart control technologies to manage COTS outbreaks is part of the Australian government's National Landcare Initiative Reef Rescue Program. In one line of investigation, scientists including collaborators from the University of the Sunshine Coast, are investigating the potential of natural predators of COTS to curb populations.

Crown-of-thorns aggregation on the Great Barrier Reef Photo: AIMS LTMP

The crown-of-thorns starfish (COTS), Acanthaster planci, is a specialist, feeding only upon the flesh of live corals. This animal has several biological attributes that contribute to its ability to undergo massive population fluctuations through time. With adults consuming up to 10 m2 of live coral per year a population outbreak of hundreds of thousands to millions of COTS can deal a significant blow to coral reefs. Indeed, COTS are a major biological cause of coral loss on the Great Barrier Reef (GBR) and statistical analyses show that they are second only to the destructive power of tropical cyclones which periodically criss-cross the reef.

Surprisingly few predators feed upon the vast coral covered seascapes of the GBR but COTS are an exception. As corals are sessile organisms (fixed in one location) they have no discernible defence against an approaching aggregation of hunting COTS. There is little chance of a come-back from a COTS attack as all that is left of the once-living coral is its magnesium carbonate skeleton. However, corals do have a champion and this is a case where the hunter becomes the hunted!

A giant triton consumes a COTS on a reef Photo: AIMS

The giant triton (Charonia tritonis), named after the Greek god Triton - son of Poseidon and god of the sea, is one of the world’s largest marine snails reaching a length of up to half a metre. Due to the beauty of their shell, the giant triton has long been unsustainably harvested from coral reefs, primarily for sale to shell collectors. While the giant triton was declared a protected species in the 1960s, after a century of heavy fishing pressure, they remain quite rare on the GBR. Very little has been published about the biology or ecology of the giant triton, but their preference for feeding on COTS is well known. They are also known to eat other sea stars and echinoderms such as sea cucumbers.

Hunters, and the hunted, use their senses to detect their predator and prey. COTS have a particularly well developed sense of smell and there is nothing more alarming to a COTS then the scent of a giant triton. Indeed, the very presence of a giant triton in the vicinity of a COTS is enough to cause the starfish to rapidly move away. Equally, there are few things more enticing for a giant triton than the scent of a COTS. Once detected, a COTS will be purposefully hunted down and devoured by a hungry giant triton.

Researcher testing the reaction of a Crown-of-thorns to seawater from a Giant Triton's tank Photo: S. Clarke

COTS are the most fecund of any marine organism, with females producing hundreds of millions eggs, and males billions of sperm, during spawning seasons. Fertilisation is external and these broadcast spawners, if in very close proximity, have the highest fertilisation rate of any know marine invertebrate. However, breeding success declines rapidly as the distance between spawning COTS increases.

Giant tritons typically only eat one COTS per week so they have little application in feeding down a population of COTS numbering in hundreds of thousands. However, their very presence in the vicinity of COTS disperses aggregations. As aggregations are dispersed, and fertilization success rates decline, the likelihood of massive recruitment in a spawning season may well be reduced. Based on early results, scientists are investigating the possibility that giant tritons may play a significant role as a natural control agent for COTS outbreaks.

Watch how a Crown-of-Thorns starfish reacts to the scent of a Giant Triton