Adult Philophthalmus parasites are hermaphroditic and infect the eye cavity of vertebrates. Eggs are released in the tears and a larval stage called a miracidium hatches in the water. These miricidia burrow into the first intermediate host, which like for most trematodes is a snail. Within the snail, the parasite develops into another larval stage, called a redia. The redia eventually produce a third larval stage, the cercariae, which are released into the water. The cercariae encyst on hard substrates, forming flask-shaped cysts called metacercariae. These encysted parasites are able to infect new vertebrates when ingested.
Neal and Poulin (2012 ) reported that a species of Philophthalmus infecting mud snails (Zeacumantus subcarinatus) in New Zealand forms metacercariae on the shells of snails in its environment in preference to other hard substrate like the shells of cockles, rocks, and macro-algae. Additionally, they found that metacercarial cyst formation was density dependent, with more new cysts being formed on snail shells that already had cysts than on shells that did not previously have cysts. They propose that the density dependence of this behavior may help the parasite by increasing the number of potential mates in the future vertebrate host.
A recent blog also described the findings of this paper.
References and Useful LinksEdit
Neal, A. T. and Poulin, R. (2012) Substratum preference of Philophthalmus sp. cercariae for cyst formation under natural and experimental conditions. Journal of Parasitology 98: 293-298.