High Densities of Coral-feeding Gastropods Threaten Hong Kong's Coral Communities?

by Robyn Cumming

Drupella rugosa, drawn by Doris Englehardt (actual length ~25mm)

Diverse coral communities occupy many shallow subtidal areas around Hong Kong, and are important components of the natural communities in Hong Kong's new marine park at Hoi Ha Wan and marine reserve at Cape d'Aguilar. In recent surveys from the Swire Institute of Marine Science, high densities of the gastropod Drupella rugosa have been identified feeding on corals at Hoi Ha Wan. Species of Drupella are important predators of corals. They are perceived as a serious destructive agent like the Crown-of-thorns starfish Acanthaster planci (Moran, 1986; Birkeland & Lucas, 1990), because population outbreaks of Drupella in Western Australia and Japan during the 1980s and 1990s denuded reefs of live corals (Turner, 1994; Cumming, 1996).

Drupella feed exclusively on live coral tissue, stripping it from the calcium carbonate substrate and leaving distinctive white feeding scars. These are quickly settled by algae and after a few days become discoloured and less obvious. Drupella feed at night, and during the day are immobile, clustered in small groups on dead parts of live corals or other non-living substrata. Often, groups are found on top of or near feeding scars, providing evidence of recent feeding. Drupella is not found resting on live coral tissue; it avoids direct contact unless feeding, suggesting sensitivity to the sting of the corals' defence cells, the nematocysts.

Drupella populations are unusual in Hong Kong in three ways. First, only one species, D. rugosa has been identified; usually D. rugosa occurs with other Drupella species (Cumming, under review). Second, D. rugosa here preys on a variety of coral species that are avoided elsewhere. Drupella has strong feeding preferences for branching corals (Acropora spp. and Pocilloporidae), which dominate many Indo-Pacific reefs (Turner, 1994; Cumming, 1996). Such corals are rare in Hong Kong; here, most assemblages are dominated by massive and encrusting species. Rather than being restricted to the few areas with branching corals, D. rugosa here feeds on a variety of massive and encrusting genera: Favia, Favites, Goniastrea, Hydnophora, Leptastrea, Montipora, Mycedium, Platygyra, Plesiastrea and Porites. Third, D. rugosa is very abundant at Coral Beach, Hoi Ha Wan. Mean density was 36 m^-2 (five 1x1 m quadrats). In the most destructive Drupella population outbreak recorded to date, at Ningaloo Reef (a 260 km fringing reef in Western Australia), the highest recorded mean density was 19 m^-2 (Osborne, 1992; Cumming, under review). Coral Beach therefore has by far the highest density Drupella population ever recorded.

Do these high high numbers of Drupella pose a threat to Hong Kong's coral communities? In terms of numbers alone, this population would undoubtedly be defined as an outbreak, but our concept of an outbreak refers to relatively large temporal and spatial scales, and to unsustainable predation on corals. Potts (1981) defined an outbreak as "any large aggregation of many hundreds or thousands of individuals which persists at high densities for months or years and causes extensive mortality among corals over large areas of reef''. Currently, we do not know the spatial or temporal dimensions of the D. rugosa populations, nor whether extensive coral mortality is occurring. I am carrying out quantitative surveys at different sites around Hong Kong to determine the distribution and abundance of D. rugosa, recording D. rugosa density and size-frequency distributions, the size and species of prey corals, and the size and frequency of sub-lethal injuries (feeding scars).

To determine whether extensive coral mortality is occurring will require repeated monitoring of marked sites to record rates of coral mortality and community change over time. Denise McCorry of the Swire Institute has photographic records of Coral Beach over the last 1.5 years, so it will be possible to compare previous and current community dynamics.

The next step is to determine the ecological significance of sub-lethal injuries. These are expected to be common because in a previous study on the Great Barrier Reef, Drupella roamed amongst colonies, usually devouring only pats of each (Cumming, 1996). Sub-lethal injuries could reduce colony growth, increase susceptibility to further injury and whole-colony mortality, or reduce fertility. Such demographic responses could change the population and community dynamics of the corals. The demographic impact of sub-lethal injury must be investigated with experiments comparing the fates of injured and uninjured colonies.

We also need experimental studies of feeding rates and feeding choice. Does D. rugosa prey preferentially on certain species of corals in Hong Kong? Feeding preferences have significant repurcussions because of the potential to change the balance of species, making species relatively more or less common than they were previously. For instance, a very few of the preferred branching corals (Acropora) remain at Coral Beach. Preliminary surveys have revealed that many of these colonies were being fed upon by Drupella; these corals could thus disappear from the site, reducing its biodiversity.

We need behavioural studies to determine how far individuals move each night. Do they visit many colonies, devouring small parts of each, or do they spend days, weeks or months on a single colony? Many individuals are not found associated with live corals or feeding scars. Do individuals feed only sporadically or do they feed constantly but move away from prey corals after feeding?

Although feeding rates of D. rugosa have been measured previously for branching corals (Cumming, 1996), we do not know how quickly D. rugosa consumes its massive and encrusting prey in Hong Kong. Outdoor aquaria at the Swire Institute are an excellent facility for laboratory-based studies of feeding rates.

We need more people working on this problem. If you would like to help with volunteer SCUBA diving, please contact me at the Swire Institute of Marine Science (tel. 2809 2179).

References

Birkeland, C. & Lucas, J.S. (1990). Acanthaster planci: major management problem of coral reefs. CRC Press, Boca Raton, Florida.

Cumming, R.L. (1996). The corallivorous gastropods Drupella cornus, D. fragrum and D. rugosa: ecology and impact on coral communities at Lizard Island, Great Barrier Reef. Ph.D. Thesis, James Cook University of North Queensland, Townsville, Australia.

Cumming, R.L. (under review). Scales of variation in distribution and abundance of the corallivorous gastropods, Drupella spp., and the significance for coral reef management.

Moran, P.J. (1986). The Acanthaster phenomenon. Oceanography and Marine Biology Annual Review 24: 379-480.

Osborne, S. (1992). A preliminary summary of Drupella cornus distribution and abundance patterns following a survey of Ningaloo Reef in spring 1991. In: Turner, S. (ed.) Drupella corpus: a synopsis. CALM Occasional paper No. 3/92 (Proceedings of a workshop held at the Department of Conservation and Land Management, Como, Western Australia, November 1991, pp. 11-17.

Potts, D.C. (1981). Crown-of-thorns starfish - man-induced pest or natural phenomenon? In: Kitching, R.L. & Jones, R.E. (eds.) The Ecology of Pests. CSIRO, Melbourne, Australia, pp. 55-86.

Turner, S.J. (1994). The biology and population outbreaks of the corallivorous gastropod Drupella on Indo-Pacific reefs. Oceanography and Marine Biology Annual Review 32: 461 -530.

P.4-5

Back to Contents
Back to Porcupine Homepage
Go to Departmental Homepage