Mangrove snails in the genus Littoraria are unusual for a couple of reasons. The first is that they spend their adult lives above the high tide line on leaves and trunks but nip back to the sea to lay their eggs. The other is that they come in a variety of colours — pink, yellow, orange or white, sometimes plain but more often decorated with dark dots and dashes.
Colour varies with location on the tree. Snails that hang around on the stems have rather sober shells. Not exactly sombre but certainly not this year's model. They're difficult to see against the grey, shaded background. Those that live on leaves are more likely to be colourful.
The variation is not confined to one species. It occurs across a range of mangrove-dwelling Littoraria. The parallels suggest that a common factor is working on maintaining the polymorphism. That factor is almost certainly predation.
The tropical intertidal zone is packed with predators that use vision to find their next meal. But crabs and birds are not the only animals to hunt by sight. Flies do it as well.The parasitoid Sarcophaga megafilosia deposits one or more larvae close to the snail. Alien schmalien. This is where the gross stuff happens.
A maggot pushes its way in between the operculum and the shell. Once it's in, it starts burrowing into the foot. The snail retracts as far up the spire as possible, trying to escape the predator, and clamps down the shell against the substrate. But it's too little, too late. Although the reaction prevents more maggots sneaking in, it also seals the first one inside. The snail is dead within the hour.
The shell remains sealed in place while the maggot feeds up on the snail's tissues. After three or four days, the maggot pupates and a little over a week later, the fly emerges and dislodges the shell.
Sarcophaga megafilosia picks its prey by sight. Snails that stand out against the background are in serious trouble when the flies come calling.
References
McKillup, SC, McKillup, RV & Pape, T. (2000) Flies that are parasitoids of a marine snail: larviposition behaviour and life cycles of Sarcophaga megafilosia and Sarcophaga meiofilosia. Hydrobiologia 439: 141 – 149.
McKillup, SC & McKillup, RV. (2002) Flies that attack polymorphic snails on coloured backgrounds: selection for crypsis by a sarcophagid parasitoid of Littoraria filosa. Biological Journal of the Linnean Society 77: 367 – 377.
McKillup, SC & McKillup, RV. (2004) The eastern Australian distributions of Sarcophaga megafilosia and S. meiofilosia, two flies that are parasitoids of littorinid snails. Journal of Molluscan Studies 70: 103 – 106.
