Report to GBRMPA (1986)
Starfish – Predator or Prey
The attack of the triton elicits an escape response by the starfish which, if successful, results in rapid prey dispersion with the loss of only a few arms. The escape response varies in its successfulness and is heavily dependent on (1) size and hunger of predator, (2) prey size and degree of cumulative prey injury and (3) physical composition and relief of substrate. If the escape response is unsuccessful then the predator feeds on the starfish until the prey is either consumed (large predator-small prey) or discarded (small predator-large prey). Charonia tritonis will follow the scent of an injured starfish and resume the attack and subsequent feeding if hungry. When not in the process of prey hunting, capture or feeding, Charonia tritonis can appear inert. Under turbulent water-current conditions, the sensitive olfactory organ of Charonia tritonis can be disabled temporarily giving the appearance of random searching behavior.
The distance over which Charonia tritonis can locate an uninjured prey by chemo-detection is unknown, but the osphradium (olfactory organ) is most highly developed in the Cymatiidae which actively hunt their prey or scavenge dead animals (Morton, 1958). If an Acanthaster is injured, Charonia is certainly capable of small-scale chemical prey location, but the distance over which this is effective is unknown also. When either one of the two sensory tentacles of Charonia has touched the spine of a large specimen of Acanthaster, the gastropod raises the anterior region of the foot sufficiently high above the substrate to allow it to pass over the spines of the closest three arms of the starfish. When this has occurred, the proboscis of the gastropod is extended and as the foot descends on the aboral surface of the starfish, the proboscis probes the surface between the spines, in an orderly manner, and subsequently extends in excess of 250mm as it reaches over the aboral surface and back under the oral surface while the proboscis tip attempts to penetrate the heavily armoured mouth of the starfish. This is achieved by a combination of physical radula abraision and chemical attack.
Although specimens of the asteroid Choriaster granulosa are immobilized following direct body wall proboscis penetration, Charonia seems incapable of penetrating the body wall of a complete Acanthaster planci. When specimens of Linckia laevigata, Gomophia sp. and juveniles of Acanthaster planci are attacked, the proboscis extends sufficient to overturn the specimen into the upturned anterior region of the gastropod’s foot. The engulphed asteroid is held in this manner while consumed.
The sensory tentacles at the tip of each arm of Acanthaster planci can detect the presence of Charonia tritonis. When one of these tentacles touches the foot of the gastropod, the starfish immediately moves rapidly to avoid capture. If the gastropod has its foot holding the starfish by its arms, these will be autotomized almost immediately to allow escape. If the gastropod has been able to grasp more than a few arms with its foot, then it will use the highly toothed lip of the shell to further restrain the starfish while the proboscis attempts to penetrate the oral spines. The starfish will attempt to escape by crawling laterally over the shell of the gastropod. This causes the gastropod to fall on its side and if the penetration of the starfish’s oral spines has not been achieved then escape is often successful. By this time, much damage has already been caused to the oral spines which will make future predator attack more successful.
The structural complexity of external spinulation inhibits, to varying degrees, the proboscis penetration by gastropod predators and parasites. Thickness of test, toxin and venom elaboration, along with the spatial organization of papular groups probably serve a similar purpose. The usefulness of these structures is apparant following careful examination of the method of predator attack. These defences maximize the probability of prey escape by increasing the time necessary for the predator to immobilize its prey.
In Trinidad during February large numbers of the asteroid Echinaster sentus congregate in sheltered areas to spawn. Percharde (1972) describes pairs of Charonia tritonis variegata methodically driving dozens of Echinaster sentus up a slope, attacking the outside members. He proposes that this mollusc plays an important role in the ecological balance of the extensive areas of its habitat.
Field and aquarium behavioral observations suggest that Charonia tritonis can disperse aggregations of Acanthaster planci. The reproductive success of Acanthaster planci is heavily dependent on the proximity of conspecifics during spawning. Charonia tritonis may play an important role in the dispersion of both feeding and breeding aggregations of Acanthaster planci.