Zombie snails on Australian beaches

Associate Professor Tom Cribb with two of his honours students.
Image: Gary Cranitch.


18 November 2010
The parasites studied by Tom Cribb can turn snails into zombies – and that may be the key to their abundance on coral reefs.
Tom, from the University of Queensland, is studying parasitic flatworms of the class Trematoda on the CReefs field expedition to Heron Island.
There are two subclasses of Trematoda: the Aspidogastrea, of which there are perhaps 100 species; and the Digenea, which may number up to 24,000 species.
"The digeneans are probably much more common than the aspidogastreans because of their complicated life cycles," Tom explains.
Digenean parasites have multi-host cycles. Most use a mollusc, usually a snail, as the first intermediate host. A larva will burrow into a snail, or a snail will eat a parasite egg.
"One tiny parasite will reproduce asexually until the snail is filled, completely overwhelmed with parasites. The snail becomes a zombie. It will never reproduce again; it's just being used to produce parasites," Tom says.
Larval parasites will then emerge from the snail to swim in the ocean until each penetrates another animal, which can be anything from a shrimp to a jellyfish or a fish. That animal is then eaten by a fish. The parasite will produce eggs inside the stomach or intestines of its final host, and the eggs will be expelled back into the ocean, eventually to infect a snail – and the cycle begins again.
This is bad for the snail, but good for the parasites!
"That extra round of prolific reproduction inside the mollusc seems to be related to the success of the digeneans," Tom says.
"The aspidogastreans don't have anything like that in their life cycle, which may be why they are far less common," he says.
Tom has also found that snails found on beach rock are often heavily infected with digenean larvae, while snails among coral have very low infection levels.
"The asexual reproduction seems to mean that the parasites don't need many infected snails to make the life cycle work," he says.
The mollusc-host stage of the digenean life cycle may also explain some of the distribution of the parasites on coral reefs.
Diversity of marine life tends to be richer in the northern waters, and yet some species of digeneans found here, on coral reefs around Heron Island on the southern Great Barrier Reef, are not found around Lizard Island, north of Cairns.
Tom speculates that this may be due to different species of snails between the two sites.