Saturday, July 26, 2014

Catherine Zhang -- Assignment 2: why the snail shells are near the shore?

So from the shore of Lovells Island we explored, we found that there is a piece of huge white stone by the sub-tidal pool. We found several colorful snail shells on the surface. These shells are totally empty, and the areas near the umbilicus are broken. But we couldn’t find any other similar snail shells on the shore. Interestingly, on the way back to the boat, I found there are lots of similar broken snails under the bushes and trees. And I also found a live snail on the back of a leaf. Here are the questions: what are these snail species? Why some of them ended up near the shore since there are lot more under the trees?
            To find out the answers, I went online to search for “Boston Harbor Lovells Island snail shell” and it took me back to several posts in this blog both 6 years and 4 years ago (again…). From previous student’s observation, these snail can be 3 types of snail species which are white-lipped snails (Helix albolabris) or brown-lipped snails (Cepaea nemoralis). With doubt, I looked up both snails and found out they do look like the ones I found on the island. Although Brown-lipped snails are native in Europe, they were introduced to North America. And White-lipped snails are also native in central Europe and Western Europe until they were introduced in Northeastern America. 

            Now the question is why these empty broken snail shells are near the shore? I personally think that is natural force. The wind and water cannot bring only 4 snail shells from other places to the shore since I saw much more shattered shells under the bushes which are far away from the shore (unless there is a storm which brought only 4 shells to the shore. That sounds impossible.). I think they are brought here by other species. But with observation, I realize that all the shells are opened on the umbilicus as same as the ones I found under the bushes. I guess its predators ate them and left them on the rock. To know what species may eat snails, I looked up “land snail predators” and it comes out species of invertebrates, amphibians, reptiles, small mammals and birds. But the area with snail shells is just by the shore, there is no plant. It means the snails didn’t crawl all the way from woods and eaten by other species, which means some species brought them there. It is impossible for invertebrates, reptiles and amphibians to drag them all the way down to the shore. So it must be small mammals or birds. From the article “Snail Predators”, it says mammals such as the mice, shrews and birds will eat snails. On the other hand, I learned there are no mammals (except rangers) on Boston Harbor islands. So now we finally focus on the birds. From my own knowledge, I know big birds save their times by just swollwing the whole snail. But since the snail shells are broken on the almost same side, I think the birds use their beak to break through the shells and drag out the snails. So I looked up “small birds eat snails”. In the article “What kinds of birds eat snails,” it says songbirds such as song thrushes can eat snails by smashing them on rocks. So far, I think thrushes brought the snails to the huge rock near the shore.

PS: Pictures below are all from posts which are 4 years ago and 6 years ago. From the snail picture 4 years and 6 years ago, there were a lot of live snails, but last time we went to the Lovells Island, we only found 3 live snails and rest of them are only shells. I just wonder if it is because of human distraction, weather, temperature (4 years and 6 years ago the snails were found half a month later than this year ), birds and other reasons.....

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