The Corallivorous Gastropod Drupella cornus: Dispersal, Species Delimitation, and Feeding Behaviour

Veera M Haslam

Gastropods of the species Drupella cornus (Rӧding 1798) are obligate corallivores known from tropical coral areas in the Indo-Pacific region. High, outbreak densities of these corallivores over sustainable density thresholds, have been shown to result in extensive coral losses. In the 1980s and 1990s, a D. cornus outbreak destroyed large areas of Ningaloo Reef (23 °-24 ° S), Western Australia, with the impact of the outbreak noted as far south as Houtman Abrolhos Islands (28 ° S). Multiple studies were conducted of the ecology of the species at the time, however, since the late 1990s, research efforts in this species have greatly reduced. In 2018, it was noted by the author that D. cornus has established feeding aggregations on the sub-tropical reefs of Rottnest Island (32 ° S), Western Australia, a higher latitude reef area that is considered a possible refuge for coral species in the light of increasing water temperatures. The aims of this study were to further understand whether the Drupella cornus snails can be found breeding on Rottnest Island and whether they can complete their life cycle in these cooler waters. The genetic structure of the D. cornus groups from the western coast of Australia and, further, between groups in the Indian Ocean, was also studied (using mitochondrial Cytochrome Oxidase I [COI]) to understand their dispersal patterns between these regions. This phylogeographic assessment highlighted that two species comprise the species ‘D. cornus’, although in the currently described six Drupella species, they are thought as one. Thus, a species delimitation study was also conducted. The final aim was also to evaluate the feeding impact and intensity, current densities, and aggregations of D. cornus at the backreef areas of Coral Bay, Ningaloo Reef, and to use these in developing effective monitoring protocols for D. cornus. Drupella cornus egg capsules were found in the cooler seawater temperatures of Rottnest Island during two La Niña-associated warmer summers, making it the highest latitude breeding population of D. cornus in the Indo-Pacific. Molecular evidence of high connectivity of D. cornus on the western coast of Australia was found, with occasional dispersal across the Indian Ocean. With additional sampling at the northern Great Barrier Reef, molecular and morphological evidence was found to support two cryptic species of D. cornus (separated predominantly between the Indian and Pacific Oceans), and field observations led to the discovery of additional distribution records for Drupella species. The findings of high connectivity, new species, and range extensions, reveal how elevated Drupella densities can have consequences in areas much larger than previously thought. Further assessment of the dispersal and ecological connections of Drupella is required, especially on critically important high-latitude reefs. The feeding intensity of D. cornus ‘Indian Ocean’ was found to be temperature-related in the tropical areas at Ningaloo Reef, with increased feeding correlated to increases in seawater temperatures in the late spring/early summer. Surveyed densities of D. cornus ‘Indian Ocean’ at the backreef areas of Coral Bay (Ningaloo Reef) were recorded at outbreak levels, resulting from high juvenile densities. Although outbreak behaviour was not observed, the high numbers of juveniles raise concern for future population trends, and careful monitoring is recommended of the ecological interactions of D. cornus ‘Indian Ocean’ at these reef areas. The challenges of effective monitoring of area-specific outbreak densities could be resolved by concentrating on species-specific behavioural monitoring of Drupella. Behavioural monitoring would require less detail and might provide faster recognition of unsustainable corallivore densities or other unbalanced coral-corallivore dynamics, and the methods requires assessment. Further research should also concentrate on understanding the role of increasing seawater temperatures on observed range extensions of Drupella, and on increased D. cornus ‘Indian Ocean’ juvenile densities and to evaluate whether there is a need for an oceanwide management plan for D. cornus.

The Corallivorous Gastropod Drupella cornus: Dispersal, Species Delimitation, and Feeding Behaviour

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