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THE EFFECTS OF DRUPELLA SPP. GRAZING ON CORAL REEFS IN AUSTRALIA.
It was long thought that corals were little affected by predators thanks to the defensive action of their stinging cells or nematocysts. However, once the extensive use of scuba equipment allowed a detailed study of coral communities in their natural habitat it became clear that a large number of molluscs are associated with living corals, some of which actually feed on the coral tissue (Hadfield 1976). Of these coral feeding molluscs, the most abundant are several species in the genus Coralliophila (family: Coralliophilidae), and two species in the genus Drupella (family: Muricidae). Some 10 years ago it became clear that the two Drupella species sometimes occurred in dense aggregations capable of causing significant damage to coral communities over areas of hundreds of square metres (Moyer et al. 1982). This report from southern Japan and the Philippines suggested that these small snails were capable of causing damage to coral communities as great as that due to the grazing activities of the well known crown-of-thorns star Acanthaster planci.
There have been a number of species described in the genus Drupella but it appears that there are only two valid species (Wilson 1992). These are Drupella cornus (synonyms: D. elata, D. eburnea, D. dealbata) and D. rugosa (synonyms: D. concatenata, D. fragum). Both these species are very similar in appearance, with the major difference being in the number of spiral rows of nodules on the shell: D. cornus has 4 rows and D. rugosa has 5. Both species are white or cream in colour, sometimes with brown spots on the nodules. There is, however, a consistent difference in the size of the two species, D. cornus is larger, ranging in shell length from 30-40 mm as an adult, compared to D. rugosa which normally ranges in shell length from 20-25 mm and occasionally to 30 mm.
There were a number of reports of the presence of Drupella on coral reefs in Australia in the 1970s and early 1980s, all of these concerning small groups of snails on a single coral colony (Loch 1987). However, in 1987, during a survey of fish communities and coral communities on the Ningaloo fringing reef tract in West Australia, it was found that high density populations of Drupella cornus were causing considerable damage to reefs over a distance of about 100 km (Ayling and Ayling 1987).
The Ningaloo fringing reef system extends for 260 km along the west coast of the North West Cape Peninsula in Western Australia (WA). In preliminary surveys of a proposed marine park area made by the West Australian Museum in 1980 it was reported that the coral communities were rich and diverse and although Drupella were present they were not causing any significant coral damage (Marsh personal communication). During the 1987 survey it quickly became apparent that the rich and diverse coral cover was no longer a feature of large areas of this reef. Live coral cover measured at a range of sites over a 100 km length of this reef between Tantabiddi Creek and Winderabandi Point was less than 10%, whereas dead standing hard corals, mainly plate Acropora covered over 60% of the bottom. Many of the remaining live coral colonies were infested with large numbers of Drupella cornus that were actively grazing on the coral tissues.
Estimates of Drupella density were made by searching for all adult individuals in a number of replicate 5 x 0.5 m transects at each site visited. These counts were made in 11 areas over a 150 km length of reef and showed that Drupella was present at all sites but was especially common in the back reef habitat where mean density was over 12 per sq m (table 1). The snails were very clumped and numbers as high as 175 per sq m were recorded in single transects. Some extreme cases were observed with over 300 individuals counted on a single 20 cm diameter corymbose Acropora colony, and over 1,000 on a plate Acropora. Numbers were apparently lower in the more exposed habitats on the reef crest and front reef slope with densities of between 1 and 2 per sq m, but only single sites were surveyed in these two habitats due to access problems caused by bad weather.
The Drupella were recorded feeding on a wide variety of coral species but predominantly on all Acropora species especially plate forming species. At one site 50% of all Acropora colonies examined were infested with Drupella, as well as 60% of all Montipora colonies, 30% of pocilloporids, poritids and Astreopora colonies, 20% of mussids and less than 10% of faviids. It is interesting that the range of corals eaten almost exactly parallels those eaten by crown of thorns; this coral grazer also prefers acroporids and rejects most faviids.
During this survey of Ningaloo reefs a preliminary estimate of the feeding rate of Drupella was made by using repeated photographs of Acropora plates on the back reef over a 7 day period. The number of Drupella individuals on each colony was counted at the start and end of the 7 day period. The mean feeding rate was about 2.5 sq cm of the plate area per mollusc per day. At this rate the mean number of Drupella found per sq m in this habitat could completely eat a plate 1 m square in just over a year. Drupella is certainly capable of causing the damage that was observed.
The damage done by Drupella cornus grazing on the Ningaloo reef tract has been dramatic. These preliminary data suggest that Drupella grazing had devastated (to use a word normally applied to Crown of Thorns affected reefs) at least 100 km of the reef tract. In this area coral cover had been reduced by a mean of about 86% on the back reef and lagoon coral dominated habitats since 1980, and by 47% in the single front reef slope site surveyed.
A number of subsequent surveys of the Ningaloo reef tract have suggested that the high density Drupella populations have been moving south, presumably by recruitment from existing populations rather than movement of adults. In late 1991 high densities of Drupella (16 per sq m) were only found in the extreme south of the Park, 150 km south of where peak densities were present in 1987 (Osborne 1992). Coral cover was low over most of the rest of the back reef from NW Cape to Coral Bay.
Drupella in Other West Australian Localities.
Since 1987 destructive Drupella aggregations have been recorded from a number of other reef areas in West Australia. These sites have ranged from the Abrolhos Reefs at 29°S (800 km south of Ningaloo) (Nardi 1992) to the Montebello Islands 200 km NE of NW Cape (Hilliard and Chalmer 1992). Hilliard and Chalmer (1992) reported that Drupella cornus was common on many reefs of the inshore north west shelf between Ningaloo and the Montebello Islands. There was some evidence that numbers were increasing and spreading between 1989 and 1991 in this area. Although D. rugosa has been recorded from Bundegi Reef in the Exmouth Gulf to the north of Ningaloo this species is apparently not common in the West Australian region.
Great Barrier Reef.
Initial surveys made between 1987 and 1990 on the Great Barrier Reef (GBR) showed that the most abundant species in this region was Drupella rugosa, although D. cornus is occasionally found (Ayling and Ayling 1992). Observations by aware observers have reported Drupella from the Swain Group of reefs and the Capricorn-Bunker Group at the southern end of the GBR (22-23°S) all the way to the Torres Strait (10°S).
Preliminary counts using 20 x 1 m visually searched transects on Norman Reef off Cairns (16°25'S) recorded mean densities of 1.1 per sq m. However, D. rugosa, being smaller and more secretive than D. cornus, is not easy to count visually and some preliminary destructive searches of sq m quadrats on Low Isles off Port Douglas revealed mean densities of almost 20 per sq m (Oxley 1988).
D. rugosa eats approximately the same range of coral species as did the Drupella at Ningaloo. This species preferred pocilloporids and most Acropora species but was occasionally found eating a few other corals including Porites colonies.
During a trip in July 1990 to look the extent of the damage caused by TC Ivor (Done et al. 1990) spot checks were made on a number of reefs between Lizard Island and Princess Charlotte Bay (14°S-14°30'S). On about 6 of these reefs Drupella damage was marked, especially on Davie Reef a small outer reef that had a high level of damage at all three sites visited. During January-March 1991 surveys were made of a wide variety of reef organisms on 50 reefs in the Cairns Section of the GBR Marine Park in a project funded by the GBRMPA. As part of this study the effect of Drupella grazing on coral communities was quantified.
On each reef three sites were surveyed on the front (windward) face and another three sites on the back (leeward) face. At each site four replicate 30 x 1 m transects were searched for coral colonies that had been damaged by Drupella grazing and the number recorded, along with simultaneous counts of the number of undamaged colonies. The results were converted to a percentage of the total corals that were damaged at each site and a mean percentage damage calculated for the entire reef (6 sites). Overall reef-wide damage figures ranged from an insignificant 0.4% to over 26% of coral colonies. The grand mean damage level for all 50 reefs was 6.6%. There was a significant S - N increase in the extent of Drupella damage on mid-shelf reefs of the Cairns Section, but this was not reflected on outer shelf reefs (table 2).
The maximum recorded damage at any site was on the front reef of Nymph Island (14°40'S), north of Lizard Island where 48.3% of corals were damaged. This survey suggested that these gastropods are present on at least some parts of the GBR in sufficient numbers to cause significant coral death.
Causes of Drupella Infestations.
There seems to be a perception, both in Australia, and in the world-wide context, that Drupella grazing is a new problem and anthropogenic effects must somehow be responsible. The emotive word 'outbreak' is being used to describe the high density populations that have been located.
However, preliminary studies of the basic biology of Drupella species have only recently commenced and the information available is not sufficient to be able to determine what causes high density, destructive aggregations to appear in some areas. Growth rates of D. cornus suggest that this species reaches a determinate adult size after 5-6 years with an expected further life expectance of 2 years (Black and Johnson 1992). Individuals begin to breed at an age of about 3 years with females laying egg capsules that release free swimming veliger larvae into the plankton after about 30 days. Studies in the laboratory suggest that females can produce about 60,000 larvae annually. These larvae apparently spend up to two months in the plankton before settling amongst the fingers of clump forming Acropora coral colonies (Turner 1992). The long larval life suggests that this species has the capability of wide dispersal; genetic similarity studies confirm that populations from the Abrolhos Islands to Dampier in WA (a distance of 1,200 km) are relatively similar (Johnson et al. 1992).
Unless observers are aware of the appearance and capabilities of Drupella it is easy to overlook their activities or attribute them to some other agency such as crown-of-thorns grazing, even when they are present at high densities. Although all reports of extensive coral damage caused by Drupella grazing have only occurred in the past 10 years it is possible that they were overlooked prior to this time. There has also been a marked increase in the number and range of scuba diving studies on coral reefs over that past 10 years giving a greater chance of turning up incidents of this type.
A number of mechanisms have been suggested to be responsible for the appearance of high density Drupella populations:
Jack Moyer, who reported damage over relatively small areas of reef in Japan and the Philippines, blamed siltation from coastal development for the supposed increase in Drupella numbers (Moyer et al. 1982). The WA populations could hardly be attributed to this cause as the entire coast is a desert with 200 mm annual rainfall and very low human use apart from fishing The highest density populations found to date on the GBR are remote from centres of population and from possible man-induced water quality changes.
In Ningaloo where there is a recreational fishery for the common local lethrinids it has been suggested that fishing pressure has encouraged the high density Drupella populations. Lethrinids are potential predators, at least of small Drupella - it is hypothesised that removal of the fish by fishing pressure has resulted in increased densities of Drupella. The available data suggest that high numbers of Drupella are not correlated with low numbers of their potential major predators: the fish families lethrinidae and large labrids. Densities of these families on NW Cape were many times greater than in any GBR sites (table 3) and yet destructive populations of Drupella are not widespread on the GBR.
Densities of lethrinids on the northern Cairns Section reefs were greater than those on the southern reefs - the opposite of what might be expected if these fish were affecting Drupella numbers. A plot of lethrinid density against the percentage of Drupella damaged corals on the 50 reefs surveyed in 1991 showed a positive correlation (figure 1), indicating that removal of lethrinids by fishing pressure is unlikely to have been responsible for the high densities of Drupella observed on the Lizard Island area reefs.
It is clear that the problem is not as simplistic as some previous studies have suggested. We need a lot more information on the biology and ecology of Drupella, especially some knowledge of what determines the success of particular cohorts of recruits, before the reasons for the apparent population fluctuations can be understood.
A very rough estimate of the numbers of Drupella within the Ningaloo Marine Park was made using aerial photos to estimate reef area and combining this with the measures of density from the 1987 survey. There were almost 500 million Drupella on the Ningaloo reef system, suggesting that even if it was deemed to be necessary, control of Drupella populations will not be a simple matter.
An attempt at control of Drupella populations by hand removal by divers was made on the Ningaloo Reef (Osborne and Williams 1992). Divers spent two days removing snails from a 25 m square experimental plot and numbers were compared with a nearby control plot of similar size after a period of 35 weeks. It was found that an experienced diver could remove 75% of the snails present from the experimental plot in about 3 days of intensive diving (assuming 6 hours in the water per day). Numbers in the experimental plot after 35 weeks were about 20% of those at the start of the experiment and were significantly lower than those in the control. However, it would require almost 50 diver-days to clear 75% of snails from a hectare of reef. Measurements from aerial photographs show that there is about 4,000 ha of back reef coral area in the Ningaloo Marine Park, requiring almost 200,000 diver-days, or over 500 diver-years, to clear of snails. Clearly, control using this method is not viable except for very small areas of particular interest or importance.
Ayling, A.M. and Ayling, A.L. 1987, 'A biological survey of selected sites on the Ningaloo fringing reefs', Unpublished report to the West Australian Department of Conservation and Land Management, 47 pp.
Ayling, A.M. and Ayling, A.L. 1992, 'Preliminary information on the effects of Drupella spp. grazing on the Great Barrier Reef', In S. Turner (Ed.), Drupella cornus: a Synopsis, Proceedings of a workshop held at the Department of Conservation and Land Management, Como, Western Australia 21-22 November 1991, CALM Occasional Paper No. 3/92, pp. 37-42.
Black, R. and Johnson, M.S. 1992, 'Growth rates of Drupella cornus', CALM Occasional Paper No. 3/92, pp. 51-54.
Done, T.J., Ayling, A.M. and Van Woesik, R. 1990, 'Broadscale survey of impacts of Cyclone Ivor on coral reefs', Unpublished report to the Great Barrier Reef Marine Park Authority, 39 pp.
Hadfield, M.G. 1976, 'Molluscs associated with living tropical corals', Micronesica, vol. 12, no. 1, pp. 133-148.
Hilliard, R.W. 1992, 'Incidence of Drupella on coral monitoring transects between Serrurier Island and Mermaid Sound', CALM Occasional Paper No. 3/92, pp. 19-36.
Johnson, M.S., Holborn, K. and Black, R. 1992, 'Population genetics of Drupella cornus', CALM Occasional Paper No. 3/92, pp. 71-76.
Loch, I. 1987, 'Whodunnit? - COT death versus the dastardly Drupella', Australian Shell News, No. 58, pp. 1-2.
Moran, P.J., Bradbury, R.H. and Reichelt, R.E, 1988, 'Distribution of recent outbreaks of the crown-of-thorns starfish (Acanthaster planci) along the Great Barrier Reef: 1985-1986', Coral Reefs, Vol. 7, pp. 125-137.
Moyer, J.T., Emerson, W.K. and Ross, M. 1982, 'Massive destruction of scleractinian corals by the muricid gastropod, Drupella, in Japan and the Philippines', The Nautilus, Vol. 96, pp. 69-82.
Nardi, K. 1992, 'The gametogenic cycle of Drupella cornus at Ningaloo and Abrolhos Reefs', CALM Occasional Paper No. 3/92, pp. 55-62.
Osborne, S. 1992, 'A preliminary summary of Drupella cornus distribution and abundance patterns following a survey of Ningaloo Reef in spring 1991', CALM Occasional Paper No. 3/92, pp. 11-18.
Osborne, S. and Williams, M.R. 1992, 'A preliminary summary of the effects of hand removal of Drupella cornus on Ningaloo Reef', CALM Occasional Paper No. 3/92, pp. 83-90.
Oxley, W.G. 1988, 'A sampling study of a corallivorous gastropod Drupella on inshore and midshelf reefs of the Great Barrier Reef', Honours thesis, James Cook University, Townsville, 83 pp.
Turner, S.J. 1992, 'The early life history of Drupella cornus', CALM Occasional Paper No. 3/92, pp. 63-70.
Wilson, B. 1992, 'Taxonomy of Drupella (Gastropoda, Muricidae)', CALM Occasional Paper No. 3/92, pp. 5-9.
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