Monday, August 4, 2008

The Snails

Hello, everyone!

Today on this lovely warm day, we boated out to Lovell's Island in the Boston Harbor Islands National Park.Our first stop took us to the Northeastern side of the island to a large tidal pool. The pool was physically connected to the broader ocean, and was exposed to small waves of 2-6". Brave bare-footed or flipflop clad students traversed the slippery rocks a dozen or more yards into the tidal pool in search of the Periwinkle Snail. Fortunately for us, they were not terribly difficult to find.

The snails were all relatively small, I'm fairly certain that every specimen found was less than an inch in diameter and an inch high. I would estimate that the smallest periwinkle I found was 1/8th inch across. Their shells were dark brown when damp, sometimes mottled with a lighter brown color. It is important to note, however, that dry shells become a lighter sandy color, and very worn out aged dry shells could turn light gray. Shells had closable trapdoors that are used to close in water for the snail to breathe when it is stranded by the tide above water (useful for an intertidal zone creature).

Periwinkles could be found on or around the rocks that litered the floor of the tidal pool. Many were on rocks alone, however I would say that most appeared to share rocks with others in groups of 2+. On one rock, I saw around 8. Although they stuck a bit to the rocks, removing them only required a light tug, they barely put up a noticable resistance. Once extracted, however, getting them to emerge above water was rather challenging! I had little luck simply leaving them in the palm of my hand. After setting one on a moist rock resting upon one hand, and shading it with another (in an attempt to imitate their 'natural' habitat), I found them more likely to emerge. I do not know how valuable this method was, because I also overheard conversation that would imply that singing, humming, or poetry had also been found to be effective (perhaps my snail merely overheard someone elses story!). In spite of the difficulty of balancing snails on rocks, it got the job done for me whether it is silly personal supersticion or has some scientific merit.

After 3-4 minutes of sitting on the rock, the first 3 snails I tried this with emerged (it had to happen more than once because they kept falling off their rock!) and slowly crawled across the rock. I would estimate that it took them aproximately 10 secons to cross a 2.5" rock, though admittedly they were not travelling perfectly straight and may have been distracted and not moving their fastest.

The Periwinkles only emerged slightly at any point. The snail's flesh was dark brown to black and appeared slick and moist.When coming out of their shells, first extending two small feeler-like tentacles to probe the air and rock and wiggle slowly. If disturbed by physically poking or rock-shaking at this point, the snail withdrew rather quickly. If the snail is permitted peace long enough, its head will also emerge. I am not sure if it is possible for their heads to come out further, but in my examples, and those of classmates that I saw, the head only comes out maybe a 1-2mm, aproximately a semicircle to the shell. I did not see it come out further than this under water or above ground, and it appears that at this point it is able to move around. Once the head is out, shifting of the rock did not seem to disrupt the periwinkle as much and cause it to withdraw, however, poking of the shell did cause it to hide.

Back by popular demand, an MS-Perwinkle:
We found evidence of other types of Periwinkles as well, though we did not see them. Further out in the water, hermit crabs were found living inside darker, smoother, larger periwinkle shells. These shells might have at one point housed a variety of periwinkle that lives completely under water beyond the intertidal zone. Bruce additionally hinted at a third type of Periwinkle that lives in the spray zone, but we did not see these or anything to indicate they might be around.

We spent the next portion of the day looking at the Periwinkle's distant cousin, a variety of Landsnail also making Lovell's Island home. On the way to the tidal pool, we had already seen evidence of these critters in the form of numerous broken snail shells littering the pathside. Challenged with finding both living snails and these old snail shells, my group began at a higher altitude closer to the middle of the island. Here, we found numerous very small snails ( perhaps as small as a centimeter across), though we were more successful finding these down sloping side paths that ran off the hill. These tiny snails almost exclusively were found clinging to the underside of the leaves of Sumac Trees. Upon descending the path, we were more successful at finding both snail shells, and larger live snails. Larger snails might be found on the bark of Summac trees or the leaves. Some were found in blackberry bushes as well. Shells were most easily found almong the side of the paved path, by the edge or shaded by overhanging plants.

Categorizing snails and shells by specific features to identify either morphs of one species or to seperate species all together proved challening for our class as a whole. Some difficulty included identifying what "yellow" is, or the exact number of stripes on snail shells. In general, I would say the most common colors found of the dead snails of a 'random' sampling of 113 (narrowed to 100) were yellow, light brown, and darker brown, most having darker brown-black stripes and a few stiped with a lighter brown color. Shells either appeared to have one central stripe down the middle of the shell or several rings of stripes. Counting these stripes was another challenge because many of the shells were broken, so I really could not say how many stripes they had total. Color was also challenging to identify because brown shells, for example, appeared to turn a pinkish color with age. At first we thought this might be its own category alltogether. The one shell that really stood out was a solid yellow shell.

One group looked at the entrance to the shells and determined that all of the openings looked to be aproximately the same shape regardless of the shell color.

The live snails were also difficult to differentiate and brought a new factor to the equation: skin color. Snails appeared to either have brown or yellowy-white flesh. Skin color seemed to have little to do with shell color, as we found an example of a dark-shelled snail with light skin as well as yellow-skinned snails in yellow shells and brown skinned snails in brown shells. They looked a bit sticky/slimy/shiny visibly.

Here is a Landsnail:

To compare them to their seadwelling cousins, they did appear to emerge further from their shells than their Periwinkle cousins, as I attempted to demonstrate in my doodle. They had an additional set of visible tentacles mounted higher up on their head. Landsnails came in a far greater variety of colors and patterns, at least at first glance. Perhaps Periwinkle mottling patterns or shell shapes vary more greatly, but it was hard to tell at a quick glance. The periwinkles we looked at also had harder shells than their landsnail brethren as well as trap doors to contail water.

I am very tired after today's snailing and am going to go to sleep now, but I may come back and add more if I think of anything I forgot. Enjoy the break, everyone!

No comments: