Distribution and habitat
Among Drupella species, D. rugosa was the first to catch attention because of its potential to cause damage to coral reefs (Moyer et al., 1982). Afterwards, D. cornus and D. fragum outbreaks were observed with mean densities higher than 3 indiv. m-2 (reviewed in Cumming, 2009). The density of > 3 D. rugosa m-2 at Koh Tao for both shallow and deep quadrats during an ongoing outbreak is in line with the documented outbreaks of their congeners. A number of indicators for problematic Drupella populations are also in line with an outbreak (Cumming, 2009), since 9% of all coral colonies were preyed on and large aggregations of juveniles (50-100 indiv.) were frequently encountered. For D. margariticola local mean densities of approximately 5 indiv. m-2 were found in Hong Kong (Morton and Blackmore, 2009), which was up to 1.3 indiv. m-2 at Koh Tao. Regarding the distribution of M. spinosa the highest mean local density in the present study measured was 0.5 indiv. m-2. Earlier reports on the occurrence and distribution of this species did not give quantitative data (Taylor, 1978; Yokochi, 2004). Concerning the density of populations of both D. margariticola and M. spinosa it is not known whether these have grown over previous years and it is not established in what density they occur under normal circumstances.
Since the 2010 mass bleaching event in the Gulf of Thailand, stony coral cover had declined and coral reef communities had changed (Chavanich et al., 2012; Yeemin et al., 2012). The reefs of Koh Tao were also affected by bleaching and other detrimental effects (Hoeksema and Matthews, 2011; Weterings, 2011; Hoeksema et al., 2013), and as such Drupella snails were able to thrive and more than double their relative abundance between 2009 and 2014 (unpublished data). Although some variation over sites occurs, a very high difference in occupation was noted among different habitats around Koh Tao. This was also described for Lizard Island (Great Barrier Reef), where D. rugosa occurs mostly on sheltered slopes, and D. cornus and D. fragum mostly occupy exposed reef crests (Cumming, 1999). Most reefs around Koh Tao are built on gentle slopes with moderate wave action, which appears ideal for D. rugosa. The pinnacles and rocky outcrops around Koh Tao (see Fig. 1) are exposed to stronger wave action and currents. Their steep slopes show low coral cover and therefore Drupella snails were rare.
The present results show a preference for high Acropora densities (see also Cumming, 2009), large colony sizes (see also Cumming, 2009; Schoepf et al., 2010) and a high cover of rubble and dead coral on which individuals dwell when not on a living colony. A high healthy stony coral cover was also preferred, although indications of correlations with unhealthy reef sites have been found before in Hong Kong (Morton et al., 2002) and Koh Tao (unpublished data). No single variable was able to appropriately explain Drupella distribution and the generalized model could only explain part of the variation in the data. A negative correlation of Drupella density with depth was found in earlier years for D. rugosa at Koh Tao (unpublished data), but this correlation was not significant in the present study. This is presumably due to the wide 3-m intervals (2-5 and 5-8 m) within which quadrats were laid out and the high densities found below 5 m depth at Taa Chaa. The drivers for the distribution of Drupella species remain unclear, although various aspects have also been discussed in previous studies, such as depth (Schoepf et al., 2010), habitat (Cumming, 1999), substrate cover and coral communities (e.g. Cumming, 1999; Morton and Blackmore, 2009; Schoepf et al., 2010), reef status, and anthropogenic reef use (e.g. McClanahan, 1994; Morton et al., 2002).
The distribution of D. margariticola and M. spinosa seemed less dependent on coral cover owing to their relatively high occupation of other substrata and their ability to feed on other prey. M. spinosa was more commonly found on sandy substrates and isolated outcrops than Drupella spp. Juveniles of D. rugosa were found more in shallow areas and juveniles of all species were more abundant on stony coral, although this was not significant for D. margariticola. In general there appeared to be different habitat and prey requirements for juvenile snails as compared to the adults.