Drupella species are known to be agents of large-scale disturbance to coral reefs, particularly in Western Australia and Japan, where population outbreaks have drastically reduced coral cover (Turner 1994; Cumming 1999). These snails also occur in other reefs, including the GBR. Drupella are perceived as a serious destructive agent to live corals, like the Crown-of-Thorns starfish, and ecological research on Drupella has focused on these high-density populations (Cumming 1999).

 

Saueracker (1997) described how the population of Drupella at Ningaloo reef, NW Australia, increased from 100-200 / km2 in the 1970s to 1-2 million / km2, and stated that it had destroyed 90% of corals in parts of the northern reef. Drupella has a thick shell that only large fishes could crack, so overfishing (predator removal hypothesis) may have allowed their population to increase. The fish population of Ningaloo has declined under fishing pressure. Since Ningaloo is isolated from human development, it is unlikely that extraneous human impact is responsible for the abnormal numbers.

 

Drupella spp. prefer Acropora, and some other corals seem to be unaffected or even benefit from reduced competition (Saueracker 1997). The pelagic larvae of Drupella settle on digitate corals such as Acropora and, when they are larger with a thick shell, graze openly on staghorn and plate corals. Drupella may actually assist in maintaining the diversity on some coral reefs because Acropora grows faster than other corals and can dominate the reef through shading (Saueracker 1997).

 

Cumming (1999) argues that even at lower densities, such corallivores still potentially affect the dynamics of whole coral reef communities because their prey, reef-building corals, provide the main structural framework of the reef. While this is in a strict sense true, many other organisms feed on living coral and such activity is part of the natural reef ecology. Cumming (1999) rightly argues that research is needed to identify the variation in Drupella density to distinguish normal from outbreak populations, and to quantify the impact of Drupella on coral reefs.

 

In Caribbean reefs another corallivorous snail, Coralliophila, has some impacts on Acropora (Miller 2001). Several members of this genus occur in Australian waters but do not appear to be a problem.



Copyright © Environment Australia, 2002
Department of Environment and Heritage