Report to GBRMPA (1988) – John Brewer and Grub Reefs – Paterson and Poulsen (1988b)
The foraging activities of the Giant Triton (Charonia tritonis) may limit aggregation formation and fertilization success of the Crown-of-Thorns Starfish (Acanthaster planci)
Tritons (Charonia) are confirmed predators of many species of starfish (Endean, 1969; Chesher, 1969; Laxton, 1971; Noguchi et al., 1982; Percharde, 1972). Endean (1969) discusses the possible causes of Crown-of-Thorns Starfish (Acanthaster planci) population explosions with particular emphasis on the removal of the predators of adult and juvenile starfish by humans. The Giant Triton (Charonia tritonis) is the only well documented predator of the Crown-of-Thorns Starfish.
The Giant Triton and other members of the genus Charonia have been collected by humans for most of recorded history, but it is difficult to determine the extent to which the population densities of these species have been altered by human activities. The triton has generally been regarded as uncommon on the Great Barrier Reef (Endean, 1969), but recent work (report to GBRMPA 1986) suggests that the triton may be more abundant on the reef than previously thought. This may be a result of their protection since 1969 (fisheries act 76-84, second schedule “protected species”).
Chesher (1969) notes that the triton can detect and actively seek out its prey from a distance of at least one metre. When contact is made the starfish recognizes the predator and moves away rapidly. Our observations (report to GBRMPA 1986) support this finding.
“Following the location of an Acanthaster prey, Charonia raises the anterior region of the foot sufficiently high above the substrate to allow it to pass over the spines and sensory tentacles 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. The proboscis subsequently extends in excess of 250mm as it reaches over the aboral surface and back under the oral surface in an attempt to penetrate the heavily armoured mouth of the starfish. This is achieved by a combination of radula abrasion and chemical attack. When juveniles of Acanthaster planci are attacked, the extended proboscis overturns the starfish into the raised 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 less than four arms are secured by the triton, these arms will be autotomized immediately to allow escape. If the gastropod has been able to secure more than four arms with its foot, 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 attack of Charonia tritonis elicits an escape response by the starfish which, if successful, results in rapid prey movement with the loss of only a few arms. The escape response appears to vary in its successfulness and is 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.”
Ormond and others (1973) discuss the consequences of spawning aggregations of starfish suggesting that the “increased proximity of adult starfish may enhance the chances of fertilization, especially if synchronous spawning takes place, as has been described for other echinoderms”. Furthermore, “the population density of A. planci at which aggregation into groups begins may constitute a threshold beyond which a population explosion is likely to occur. Populations of A.planci may therefor be particularly sensitive to small changes of significant environmental factors which could result in densities in the region of this threshold”.
The effect of sperm dilution, adult aggregation and synchronous spawning upon the fertilization success of echinoids was reported by Pennington (1985). Pennington concluded that significant fertilization occurred only when spawning individuals are closer than a few meters. Percharde (1972) described the attack of the caribbean triton (Charonia tritonis variegata) upon a breeding aggregation of the starfish Echinaster sentus and concluded that “this mollusc must play an important role in the ecological balance of the extensive areas of its habitat”. Similarly, the prevention or disruption of small-scale starfish aggregations by the giant triton may be significant in limiting the scale of starfish population explosions.
Tritons will prey on many species of starfish, but the preferred prey species appears to vary. Laxton (1971) states that “New Zealand species of Charonia prey upon the most common large echinoderm in the area in which they happen to be living. If, however, a choice is offered, Charonia from all habitats prefer the cushion star Patiriella regularis followed closely by Coscinasterias calamaria”. Endean found the preferred prey of the Giant Triton to be a species of Nardoa followed by the Crown-of-Thorns Starfish. Both Chesher (1969) and Endean (1969) found that one adult Giant Triton will eat an average of one adult specimen of Crown-of-Thorns Starfish per week.
During our study of starfish predators (July – Dec 1986), a total of 21 tritons were located (12 from Grub Reef, 7 from John Brewer Reef and 2 from Beaver Reef). Five tritons were placed together in one 6m by 2m by 0.5m aquarium together with 18 Crown-of-Thorns Starfish (Acanthaster planci) and 18 Blue Starfish (Linckia laevigata) collected from John Brewer Reef. After two weeks 10 of the Acanthaster and 9 of the Linckia had been consumed. Further tritons and starfish were added to the tank until it contained 7 tritons, 20 Acanthaster, 20 Linckia, 5 Nardoa, one Culcita and one Choriaster. The Culcita was attacked the first night and two tritons were observed feeding on it throughout the morning. At noon they were joined by a third. The Choriaster was observed with two large (20mm diameter) holes in non-adjacent interradii, presumably two tritons had simultaneously devoured this individual overnight. During the following two week period, at least 10 Linckia remained in the tank.
Of the 20 Acanthaster half were consumed and the remaining individuals showed increasing signs of multiple arm injury or autotomy. Another two weeks later one moderately injured Acanthaster and three Linckia remained in the tank. During the following month equal numbers of Acanthaster and Linckia were added to the tank. Each Acanthaster was attacked and comsumed entirely within 12 hours of being introduced. Some Linckia were always present throughout this period. These results demonstrate a preference for Acanthaster when a choice is offered of either Acanthaster or Linckia. Our more recent field studies (Jan – April 1988) support these findings.
A brief study of a small number of tritons was undertaken between January and April 1988 on John Brewer Reef, east of Townsville. Searches for tritons were conducted within snorkelling distance of the Reef Link pontoons, as often as weather permitted, from 27-1-1988 to 17-3-1988. Two people, each searching within visible distance of each other, covered an area approximately 2m by 100 m in a two hour dive. The total area searched in any two hour period varied according to visibility, topography, current, equipment etc. Some areas were searched regularly. A total of 46, two hour searches were undertaken, 12 with snorkelling equipment and 34 with scuba.
As tritons were located their size, sex, position and foraging activities were noted. A total of 7 tritons were located. One triton with a charactaristic shell was never relocated and subsequently never tagged. Another was tagged and released on John Brewer Reef following a stay of 12 months in an AIMS aquarium. The relocation of tagged specimens was the primary objective in the latter half of the study.
Figure 1. Tritons collected from John Brewer Reef (1988) 7| * | | 6| * | cumulative | 5| * number | | of 4| * | tritons | 3| * | | 2| * | | 1| * | |<- snorkelling> <-------------- scuba> ___________________________________________________________ 0 10 20 30 40 50 60 70 80 Time searching (Hours) Figure 2. Tritons collected from Grub Reef (1986) 12| * | 11| * | 10| * | 9| * | 8| * | 7| * | 6| * | cumulat. 5| * number | of 4| * tritons | 3| * | 2| * | 1| * |<--------------------- scuba> |__________________________________________ 0 100 200 300 Time searching (Hours) Table 1. - Tritons collected from Grub Reef (1986) Triton # Date Depth (m) Time Feeding Length (mm) 1 5-7-86 4 1400 no 385 2 6-7-86 1 - no 300 3 7-7-86 1 1400 no 285 4 7-7-86 8 1200 A. planci 255 5 9-7-86 9 1600 no 305 6 11-7-86 15 2200 no 350 7 13-7-86 1 1500 no 290 8 13-7-86 8 1430 A. planci 300 9 14-7-86 10 1115 A. planci 370 10 14-7-86 10 1100 no 330 11 14-7-86 6 1500 A. planci 385 12 15-7-86 10 1030 no 330 Table 2. - Tritons tagged at John Brewer Reef Triton # Date Depth Notes 795 13-2-88 2 found 1N, 7E 15-2-88 3 moved to 4N, 7E 16-2-88 3 unmoved 17-2-88 3 " 18-2-88 3 " 19-2-88 3 " 20-2-88 3 " 25-2-88 3 moved to 14N, 10E, tagged and replaced 26-2-88 4 moved down 1 metre in cave 8-3-88 3 moved to 4N, 8E 9-3-88 3 unmoved 11-3-88 4 moved to 2N, 8E, feeding on Acanthaster 14-3-88 3 moved to 4N, 8E, fed it an Acanthaster 16-3-88 3 unmoved, fed it an Acanthaster 17-3-88 3 moved to 5N, 8E, same cave 12-4-88 2 moved to 0N, 2E end of study 920 18-2-88 8 found 40N, 30W, in dead Acropora 19-2-88 3 transferred to 13N, 12E 20-2-88 3 unmoved, fed it a juvenile Acanthaster 22-2-88 3 unmoved 23-2-88 3 moved to 4N, 6E 25-2-88 3 unmoved, tagged and replaced, started crawling 26-2-88 4 amongst dead Acropora 4N, 1E never relocated 858 23-2-88 1 found 20S, 10E, transferred to 4N, 8E 25-2-88 3 unmoved, tagged and replaced 26-2-88 3 " 11-3-88 4 moved to 40S, 0E, Acanthaster spines in faeces 14-3-88 1 moved to 40S, 5E, feeding on Acanthaster 17-3-88 1 unmoved never relocated 672 25-2-88 3 tagged and tranferred from AIMS aquarium 26-2-88 3 unmoved, 3N, 8E 8-3-88 3 " 9-3-88 3 moved to 4N, 8E never relocated 1 13-2-88 5 found 10N, 20W, in dead Acropora never relocated 7 26-2-88 3 found site A, tagged, transferred to 12N, 12E never relocated 8 26-2-88 3 found 14N, 7E, tagged, transferred to 12N, 12E never relocated during study May 88 - relocated at site A, 200 metres away 9 16-3-88 2 found 15N, 40E, tagged, transferred to 4N, 8E 17-3-88 3 moved down half metre in cave end of study
Tritons were located at three times the rate on John Brewer Reef (Figure 1) compared with Grub Reef (Figure 2). This result does not necessarily reflect a higher abundance of tritons on John Brewer Reef. Tritons were located at Grub Reef by divers whose primary objectives were the location and destruction of starfish. Their search methods were not dedicated to the location of tritons whereas searches at John Brewer Reef were dedicated to their location and recapture. Divers at Grub Reef located mainly exposed specimens (10 of the 12 tritons were exposed) and did not search caves or other sheltered habitats. Searching at John Brewer Reef was concentrated in a much smaller area and all habitats were searched. As all tritons located at John Brewer Reef were found in cryptic habitats one might expect the number of tritons found at Grub to be a very small sample of the total population.
The total area searched on John Brewer Reef was less than one hectare whereas the area surveyed at Grub Reef was approximately 50 hectares. It is apparent from Figure 1 that new captures occurred throughout the sampling period, although many areas were searched more than ten times. It became apparent that potential refuges for tritons were very common, and the time required to thoroughly search an area increased manyfold as we became aware of their abundance. We would not regard the one hectare search as exhaustive, as the linearity of Figure 1 would lead us to expect to find more tritons in this area.
The results listed in Table 1 suggest that Acanthaster is commonly preyed upon by tritons on reefs experiencing population explosions. Although the Blue Starfish (Linckia laevigata) was also abundant on Grub Reef, tritons were not observed feeding on this species, whereas four tritons were located in the process of eating an adult Acanthaster and one was found actively hunting an Acanthaster.
The population of Acanthaster concentrated in the area of the Reef Link pontoons (Jan-April 1988) was relatively small, while the starfish Linckia laevigata was the most abundant species in the area. The depletion of most of the coral at John Brewer Reef by Acanthaster, and its subsequent overgrowth by algae may explain the high abundance of Linckia on this reef. A residual population of Acanthaster appears to have remained in the vicinity of the pontoons (area of manual starfish control). The majority of these starfish are immature specimens, predominantly cryptic and located deep within the branches of staghorn Acropora.
It can be seen from Table 2 that two of the tagged specimens of Charonia were observed feeding on Acanthaster and one had Acanthaster spines clearly visible in its faeces. Skeletal elements of an unrecognizable species (possibly Linckia) were observed in the faeces of another. Our observations of tagged Charonia suggests that the triton actively seeks out Acanthaster in preference to other species, even when they occur in low numbers as was the case at John Brewer Reef (1988). One might expect that Acanthaster (sub-adult and adult) would provide a more substantial meal than any of the smaller herbivorous starfish species such as Linckia. Tritons observed in the field appear to feed more often than those kept under laboratory conditions. Greater energy expended in searching for prey would result in larger quantities consumed. Further field studies are needed to determine the feeding rate and feeding preference of tritons in the field.
The multiple relocation of a number of tagged specimens, particularly No. 858 and No. 795 suggests strongly that tritons forage within a particular area for extended periods of time. It is possible that a home range exists for each animal. Further observations of tagged specimens are needed before this can be verified.
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