During the summer of 2011, I joined the Poulin lab at the University of Otago in Dunedin, New Zealand for 8 weeks of research funded by an NSF EAPSI (East Asia and the Pacific Summer Institute) grant. The project I undertook aimed to determine the substrate on which cercariae of a trematode in the genus Philophthalmus tended to form cysts.
Background and Life Cycle
Trematodes in the genus Philophthalmus have been found on all continents except Antarctica and adults inhabit the eye orbit of birds and occasionally mammals. Eggs, probably released in tears, hatch when they enter water and release a larval stage, the miricidium. Miricidia swim in search of a snail and inject another larval stage into the snail, the redia. Rediae produce either more rediae, which remain in the snail's tissue, or a third larval stage, the cercaria. Under the right environmental conditions, cercariae are released from the snail and encyst on a hard substrate, presumably a food item of the bird host. When a cyst is eaten by a bird, the metacercaria (another larval stage) escapes and makes its way to the bird's eye orbit, where it develops into an adult.
Snail Zeacumantus subcarinatus, Philophthalmus sp. cercaria, Philophthalmus sp. cysts, Philophthalmus cysts on snail shell
My project combined a survey of substrates collected in the field with experimental choice tests to determine where Philophthalmus cysts are found naturally and whether cercariae show a preference in substrate when forming cysts. Some of the substrates I included in surveys and choice tests were common species of snails, crabs, sea plants, cockles, and rocks.
Spotted topshell Diloma aethiops, smooth shore crab Cyclograpsus lavauxi, New Zealand cockle Austrovenus stutchburyi, mudflat whelk Cominella glandiformis
I collected all of the samples at Lower Portobello Bay on the Otago Peninsula, a short drive from the University of Otago. Lower Portobello Bay is a mudflat, covered mostly with eelgrass, rocks, cockle shells and mud. The species of snail that hosts Philophthalmus rediae, Zeacumantus subcarinatus (see above), is very common at this site and hundreds can be collected in a relatively short period of time. The parasite is also quite common, with around 15% of snails infected.
Lower Portobello Bay: from Mt. Cargill, at low tide, at high tide
The results of my summer's research indicate that Philophthalmus cercariae preferentially encyst on the shells of a number of snail species. This result is supported by evidence from both natural surveys of substrates at the field site as well as experimental choice tests conducted under controlled laboratory conditions. Additionally, when 2 of the same substrate were paired- 1 with cysts on it already and 1 without- cercariae tended to form more new cysts on substrates that already have cysts. This behavior is likely to lead to an aggregated distribution of cysts. This behavior may be beneficial to the parasite because it means that if a bird eats an infected shell, it is likely to end up with a number of parasites. Once these parasites develop into adults, they will have more potential mates than if they were eaten on a shell by themselves!