Tuesday, August 5, 2008

Lovells Island observations - Ann

Field observations Monday Aug 4

Low tide: 7:55am
High tide: 2:13pm
Sunny and hot (80degreesF), low breeze, minimal wave action

Tide pool

We arrived at the tide pool at approximately 10:45am, and at the time the water in the tide pool got no deeper than approximately 2 feet. Sean and I waded further out than the rest of the people in the group did because we were curious to see what we would find in the more exposed areas of the pool. We went out beyond the left edge of the curve of the pool, and turned back when the water got too deep to see to the bottom.

The substrate throughout the area was rocky, with a mix of large (6-12”) and small (pebbles to a few inches across) stones. There was very little vegetation- we observed only small patches of seaweed that looked similar to the rockweed we saw in the Inner Harbor. However, we noticed that it didn’t look exactly the same, so we weren’t sure of the ID. Later consultation of my field guide confirmed that it is a type of rockweed, likely Fucus vesiculosus, the same type we saw in the Inner Harbor. This is the most common type of rockweed, and the only type with air bladders. The morphology can vary depending on the level of wave disturbance.

View into tide pool

One or more small dark snails (later identified as periwinkle snails) were evident on all but the smallest of stones, making these animals the most abundant living creature (at least that was visible to the naked eye). There were very few mussels, even along the rocky intertidal zone. There were also grey crabs approximately 2-3” wide hiding under the larger stones. We could see them when we turned over the stones, but they moved too fast to catch and inspect closely.

We noticed a number of seaweeds that we had not seen in the Inner Harbor. These seaweeds were floating free in the water, so likely washed in from deeper water. Because of this and their general appearance, I believe these are two species of kelp. Although it is difficult to make an accurate identification without the stems of the plants, I believe the flat one is a Hollow-stemmed kelp (Laminaria longicruris) and the helical one is the Common Southern Kelp (L. agardhii), both of which are found in this region of the world.

We walked out to the farthest left edge of the curve of the tidepool, which was the most exposed part of the area. Here we found two examples of the “orange stuff” that was so plentiful in the Inner Harbor (actually a tunicate, but I am still unsure of what type). Both samples we found were free-floating pieces of rockweed-like seaweed, which was engulfed by the “orange stuff.” One of these samples had a 1.5cm translucent body attached to it, which squirted water in a stream when gently squeezed. By holding this structure up to the light, we could see what looked like organs inside. I tentatively concluded that this was an animal of some sort, possibly a juvenile sea squirt (based on the fact that it squirted water). This was later confirmed by Mr. Berman.

Baby sea squirt (top center of sample)

I was very curious to see what kinds of amphipods, the bug-like creatures that were so plentiful on the docks, live on the islands. Unfortunately, given the collecting conditions (no place to put down specimens for close inspection) and lack of time, I did not get to look closely at our specimens for these animals. Next time I will bring a bucket!

Snail Trails

In the afternoon, we split up into groups to search for land snails. Having been told by the rangers that most of the snails they see are in the sumac trees, almost everybody scattered across the island to search for tree-dwelling gastropods. Shashana and I decided to stay and investigate the ancient Mayan ruins (actually remnants from the old military installation on the island) we were standing near. We did this because the area seemed like prime snail habitat – damp and cool. At first we saw lots of slugs, but only a few tiny black snails scattered amongst the leaf litter on the bottom of the concrete depression. We almost gave up, but decided to check the other depression, just in case. Jackpot! Although the ground within this depression looked just like the other one, when we looked into a deep crack that ran around the entire circumference about a foot from the top (see first picture below) we saw numerous large snails. These were gorgeously colored – either a dark purple/brown or a deep goldenrod yellow.

Interestingly, looking back on my pictures of these snails before we collected them, it appears that they all have a single dark stripe, which is in contrast to the variation in stripe patterns the class identified later. An interesting future project would be to explore whether live snails found in this type of habitat have a greater preponderance of individuals with a single stripe than those live snails found in the trees.

When we reconvened with the group, we discussed periwinkle variation, then closely examined the pool of land snail shells that had been collected by the group. The first task was to sort the shells into distinct groups, which turned out to be more difficult than it first appeared. A number of groups of students attempted to sort the shells, first by color variation and variation in stripes, then by number of stripes and background color, and then by the shape of the aperture. The latter approach did not work out when the group discovered that all of the apertures were identical. The other two approaches were stymied when it became apparent that there was no easy way to categorize the colors or the stripe patterns on these shells. There was a gradient of yellow into tan into brown into purple, and variation in stripe number from zero to many (in addition, on some individuals it was difficult to tell which part was the stripe and which part was the background color).

We discussed how many species of snails we thought there were, and most members of the class thought there was one species with a large amount of morphological variation within the species. While it is impossible to ascertain the answer to that question with the limited data we had access to, I am inclined to agree with that assessment given the obvious gradient of colormorphs that we observed. This morphological variation should give the snail a diverse set of options for niches in which it could succeed.

I was surprised that such a large (relative to a snail) island with so many different ecological niches would (apparently) have just one species of land snail. However, regardless of how many species are there, there is clearly quite extensive variation in the morphotypes that are exploiting all of those niches. If I were to explore the question further, I would investigate whether the color morphs vary with the habitat in which they are found. As I described above, if I were to design a new study, I would hypothesize that the animals with more stripes would be more common in trees, and animals with fewer stripes would be found in more uniform environments, such as on the concrete structures that dot the island.

1 comment:

Ann said...

After reading the wikipedia entries on brown-lipped and white-lipped snails, I went back to my photos to see which kind seemed to be on the island. Interestingly, there seems to be a mix of shells with brown lips and white lips, leading me to wonder whether there are 2 species of snails on the island after all. The wiki entries do say that there are morphs of each that have the opposite colored lip, so it is hard to say at this point.

The wiki entries also point out that the lighter colors tend to be found in lighter colored environments. Although this does not completely agree with my observations yesterday, it would be another interesting question for future groups to explore. The hypothesis would be that there are proportionately more light-colored snails in the trees & grass, and proportionately more dark colored ones in the dark areas such as the bunkers.