14 September 2006

Zoning of the snails

hadazones1" />

The picture above shows the shore of a small island in the Aegean Sea off the western coast of Turkey. During a brief visit to this island last July, I made some very preliminary observations on the mollusks that live there.

I marked on the picture 4 distinct mollusk habitats or zones. Zone A is the broadly defined marine habitat proper that I will not discuss further. At the opposite end is the zone D, the terrestrial habitat proper. Regarding that zone, I will only mention that we found a few land snail shells on the island mostly under the small rocks and in small depressions in the ground filled with plant debris. What I was more interested in at that time as I am now are the 2 zones between A and D.

Zone B is a transition habitat. There are no discernible tides in the area. So the water level in zone B stays more or less the same except during rough weather and storms. But, even on calm days as in the picture, the water level constantly oscillates by about 10-15 cm.

The picture below shows those mollusks of zone B that I could spot while I was there and also in the photos later. The large snail (#1) is Monodonta turbinata. There are also limpets (#2) and chitons (#3) that I can’t identify further. I am sure there were other less visible, smaller mollusk species perhaps in the clumps of green algae that grow closer to the sea.


Zone B is a physically harsher habitat than zone A. Compared to the latter, the water temperature is probably more variable, there is more solar radiation and wave exposure and some risk of desiccation. As you can see in the picture, the limpets and chitons tend to stay in small depressions where they are probably protected from waves and drying and perhaps even predators. Most snails, on the other hand, always seem to be on the move, slowly gliding across the rocks.

Despite its physical shortcomings, one main advantage of living in zone B may be the relative scarcity of predators. There are no fish; crabs probably frequent the area, although I didn’t see any while I was there.

The harshest zone of them all is the zone C. It is totally dry except during rains and storms and in the summer the rock surface is not only dry, but also gets hot in the sun; very hot indeed. Also note that during rains, any marine snails in zone C are exposed to freshwater. Furthermore, there are no plants growing; it is basically barren rock probably with some microscopic algae on its surface.

Therefore, one wouldn’t expect to find any mollusks in zone C. In fact, from a distance it does appear lifeless. However, a closer look reveals not just a few, but many, many small snails attached to the rock surface. You can see 2 clusters in the picture below.


These snails, the shells of which were about 5 mm high, are littorinids (family Littorinidae). I am not good at identifying them, but I suspect these are Littorina neritoides. I noticed that like their more terrestrial distant relatives these snails also aggregate when they are aestivating. But I don’t know why.

Life at seashores, rocky or not, provide important clues to the evolution of terrestriality among gastropods. The littorinids, about which I have written before (here, here and here), have made it from zone A all the way up to zone C, but not yet to zone D.

A good introductory book on the subject of the biology of rocky shores is, well, The Biology of Rocky Shores by C. Little and J.A. Kitching (Oxford U. Press, 1996).

Note added 15 September: Henk Mienis has noted in an e-mail that Monodonta turbinata is now Osilinus turbinatus and Littorina neritoides is Melaraphe neritoides.


pascal said...

Looks to me like they're concentrated in the hollows - most studies of estivating snails comment on how they'll climb up to get away from the heat of the ground. These critters, being littoral, might be finding the low spots because there's less chance of being moved away from a suitable feeding location by waves or wind.

Slugs will congregate to reduce surface area and moisture loss, but I don't think that's a big reason here, since moisture loss through the shell is minimal.


Looking back, I realize there were so many other things I could have done while there, but my time was limited, because there was no shelter from the sun (this is a treeless island). Luckily I took several pictures & one of them, which I didn't post here, shows the littorinids clustered on a little rock protuberance.

But, see my post of 4 September about why snails may or may not climb.