Pests – Drupella Snails

Pest in Paradise – Drupella Snails By Mike Jacques The Drupella snail is about as innocuous as you can get, but from little things, big populations grow. Drupella snails breed faster than spam mail. This wouldn’t be such an issue except that they like to gnaw on coral polyps. Between the mid 1980s and early 1990s, unusual environmental conditions caused high densities of Drupella cornus resulting in massive coral damage along at least 100 km of Ningaloo Reef in W.A., with coral mortality approaching 100% in some areas. Outbreaks of Drupella snails have also been reported in the Izu Islands in southern Japan, and in Eliat and the Gulf of Aqaba in the northern Red Sea. At the time, the detrimental effects of Drupella snails was compared to those of the notorious crown of thorns starfish. During population explosions there can be up to 175 individuals per square metre of reef. It doesn’t matter that you are small if you have strength of numbers. The field survey of D. cornus in the Gulf of Aqaba showed that number per square area was higher in industrial areas when compared with reserve areas. Data from a study of 8 Kenyan coral-reef lagoons showed that Drupella cornus populations have increased the greatest in heavily fished reefs and a transition reef (converted to a park in about 1990) but less pronounced in the unfished parks and a reserve (restricted fishing). The numbers of snails were larger where their predators were few rather than where their coral food was common. Drupella cornus can produce more than 150 thousand plankton one month after spawning and which swim to their coral prey. However, unlike the crown of thorns starfish, Drupella snails are not immune to the stinging cells – nematocysts – of live coral. They avoid contact with live coral tissue, preferring to perch on dead coral and feed on the live tissue by extending a proboscis. This means that corals that have been previously damaged by other predators, severe storms or climate change are more vulnerable to Drupella snails. Drupella particularly likes to feed on branching corals belonging to the family Acroporidae. Acropora corals provide the best combination of food and shelter. Stylophora was also popular, but they didn’t like massive corals such as Porites. Drupella snails can thus cause a reef to change its species composition even if it doesn’t kill it, displacing lots of other animals that need a special type of coral cover. Warm water makes them hungry and grazing rates increased by five times at 30ºC compared with 18ºC. The snails feed by rasping away the external coral tissue using the radula (a rough ‘tongue’), then feeding on the inside of the polyp using the proboscis. They leave only the dead white skeleton behind. Scientists believe that predation by Drupella is not as big a threat to corals as over-fishing, habitat destruction, and costal run-off, but they lie in wait to take advantage of a sick coral reef that isn’t at its best. Drupella snails are the vultures of the tropical reef. If our reefs aren’t in great shape the corals don’t have the strength to beat them off.