Whilst Amtrak's Auto Train was chugging on its way to Florida last Sunday, I read Houbrick's 1984 paper on the genus Cerithidea (American Malacological Bulletin, 2: 1-20). I was fascinated to learn that C. scalariformis and according to Houbrick, possibly all the other species in the genus have, not just 2, but 3 eyes. The 2 usual eyes are at the tips of the snail's tentacles, while the 3rd unpaired eye, the pallial eye, is at the edge of mantle situated near the corner of the aperture.
Drawing of retracted Cerithidea scalariformis showing the operculum and the pallial eye (pse). From Houbrick (1984).
A day later, I was even more fascinated when I spotted the pallial eye of a live C. scalariformis for the 1st time. The pallial eye is easy to notice when the snail lifts its operculum up and starts to come out of its shell with the aperture facing up. To see a crawling snail's pallial eye, you need to look at the snail from slightly below.
The pallial eye of Cerithidea scalariformis (arrow).
We know from the many examples of cave animals that when eyes are not needed they tend to disappear. The resources saved by not making eyes could then be diverted to other metabolic activities, including reproduction, giving the eyeless individuals an advantage over those who still make eyes in total darkness. So, if Cerithidea makes an extra eye, there must certainly be an evolutionary advantage to having one. Otherwise, mutants lacking it would have taken over a long time ago.
Cerithidea scalariformis emerging from its shell.
What purpose does Cerithidea's 3rd eye serve? According to Houbrick, the pallial eye may help detect predators. I would think that an extra eye would evolve if the snails had a very high probability of getting eaten and having an extra eye would make a big difference in avoiding predators. In the case of C. scalariformis, however, it is not apparent what those predators may be.
Could the pallial eye help detect mates? Or, does it have something to do with the semi-terrestrial lifestyle of this snail? Perhaps, the tentacular eyes are not suitable for terrestrial vision and the pallial eye evolved to take over their function on land.