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).
(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
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).
(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.
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.