Wednesday, August 4, 2010
Yesterday we travelled to Lovell's Island, one of the more out-of-the-way Boston Harbor islands. We arrived just before 11:00 am, were greeted by Ranger Tim, and made our way to the end of the island to investigate some tide pools.
The most obvious difference between the environment on Lovell's compared to the Barking Crab is the relative lack of human habitation on the island. The pier, the ranger's yurt, some camp sites, paved trails, and the remains of abandoned WWII-era batteries constitute the bulk of the human presence on the island. Much of the vegetation, like the delicious wild blackberries, was introduced by people but has grown undisturbed on the island for a long time. This relatively low level of human activity is a drastic change from the floats at the Barking Crab, in an environment entirely dominated by humans.
Vegetation on the island consisted mostly of low trees and shrubs, including blackberries, sumac, and poplar. On the shoreline, the berm was sandy, but when we arrived at low tide the exposed intertidal zone was rocky with several large pools. The water was very clear and the temperature varied in each of the pools, presumably as a function of depth and isolation from the cooler seawater.
The first pool we examined was perhaps 1-2 feet deep at low tide, with warm water. The cobble bottom, beginning several feet before the edge of the water, was covered in periwinkles clinging to the rocks, and swarming with hermit crabs. There was at least one periwinkle or hermit crab for every 2 - 3 square inches of the bottom, and in terms of biomass these two definitely made up the bulk of the life in this tide pool.
There were tufts of a green seaweed that I can't identify, similar to sea lettuce but less broad, with ruffled or curled edges. Some of the rocks were encrusted with a very thin purple
substance. Other than the color and a very slightly slimier feel, the purple-covered rock surface was indistinguishable from the rest of the rock, and I believe this to be some sort of simple algae growth. Many of the rocks had limpets firmly attached to their wider surfaces.
One of the species that immediately drew my attention was a plant-like stalk, growing in small
clusters on some of the rocks. The stalks were about 1.5 - 2.5 inches
long, narrow at the bottom and wider at the top with two short tubular siphons, sort of greenish-brown. Upon first seeing one of these, in an excess of childlike enthusiasm, I regrettably plucked it from its rock and held it up in the air and was treated to a jet of water several inches long from one of its two siphons. Its skin was somewhat leathery and wrinkled like a raisin's. I believe this was a club tunicate, Styela clava.
In addition to the club tunicates, there were a variety of encrusting or sheath tunicates, forming a spectrum from a dark rich burgundy, to red, orange, and yellow. Many of these were simply a uniform, spongey, somewhat slimy mass, with no real distinction between them other then the shade of red. I think they were varieties of orange sheath tunicate. However, some of the yellower growths had numerous small raised holes or siphons growing out of them and may have in fact been crumb-of-bread sponges. Some of the rocks also had a purpleencrusting organism on them that had a similar look and feel as a sheath tunicate but was covered in an elaborate pattern of curved, dotted lines where the dots seemed to be small depressions or thin spots in the crust of the organism. I haven't been able to identify those.
The tide pool also had many small shore crabs in it. We spent some time chasing a larger one, several inches across, back and forth across the pool, but eventually it outsmarted us and escaped unexamined.
We moved on to a second, deeper pool, closer to the ocean. Here the water approached my
waist and was considerably colder. There was much less diversity and nothing new, just a subset of what lived in the shallow pool -- mainly barnacles, shore crabs, and periwinkles.
When we moved on to the land, Professor Berman pointed out a rock with the shells of several terrestrial snails on it. My hypothesis about how they got there was that a bird or other predator used the rock to access the snails' tasty insides.
As we moved away from the beach Professor Berman pointed out the abundance of terrestrial snails living in the trees, and we
spent some time searching for them. I found a beautiful yellow snail with a brown band on its shell. At Ranger Tim's insistence I named him Bartholomew. He was very shy at first, holed up in his shell, but when he finally emerged he was quite active and I believe he showed a lot of promise for the professional snail racing circuit. Although it can't be determined for certain without further testing, it looks like Bartholemew and his cohorts are Grove Snails, or brown-lipped snails (most of them we found had brown lips on their shells).
Notable differences between the common periwinkle and the grove snail:
- The grove snail has a sometimes bright, beautiful large shell with bands, as opposed to the periwinkle's drab, rough shell. I'm guessing exposure to the water and suspended sediment has something to do with this, as well as possibly the need to camouflage in a different environment.
- The periwinkle's foot is dominated by a "trapdoor" which Wikipedia tells me is an operculum. Indeed, without a great deal of patience and luck getting a periwinkle to emerge from its shell, one might get the impression that the operculum makes up the bulk of the organism. The grove snail doesn't have this.
- The grove snail is much, much more mobile and active.
My hypothesis about the relationship between them? The general trend seems to be that species (or, rather, families of species) emerge from the sea, and gastropods are probably no different. I think Bartholomew probably had a distant ancestor that was not unlike a periwinkle, perhaps more primitive, but millenia spent on land led to adaptations that result in the differences we see today.