Welcome to Snails to Whales, Bruce Berman's Boston Harbor blog focused on both the little and the big things that make Boston Harbor such an extraordinary place to live, work and play.
It is also a place for my Boston University students and my colleagues at Save the Harbor / Save the Bay to share their work and experiences.
Tuesday, August 3, 2010
If animals don't want me to pick them, they shouldn't look like plants. -Anonymous
First, some pictures from Lovells and the Barking Crab Seaweed and tunicate on rock (Lovells) Tunicate on seaweed
Club tunicate (Lovells) Tiny fish (Barking Crab) Red-eyed Amphipod (Barking Crab)
The Barking Crab A sea cucumber! Just kidding. Inside of sea grape (Barking Crab)
Intact sea grape (Barking Crab)
Lovells Island is different from the Barking Crab on many levels. Instead of a bar or restaurant, there's a yurt occupied by two rangers. The island's roads are not filled with cars or pedestrians. Compared to Boston, there is minimal human impact. There are differences and similarities in the aquatic environments we observed at each location. Both locations were subject to regular changes. At the tide pools, depth, temperature, and the direction of water flow change with the tide. At the Barking Crab, depth doesn't vary (and temperature doesn't vary much, day-to-day), but the direction of water flow and salinity change with the tide. At the Barking Crab, most of the organisms we looked at were attached to floats under the dock (or living on or in something attached to the floats) and were therefore not exposed to very much sunlight. In the tide pools, most of the organisms were exposed to direct sunlight for most or all of the day, unless they chose to burrow or hide under rocks.
At the beach on Lovells Island, sand gave way to cobble in the tide pools. Manmade jetties were composed of large rocks. When we began our observations, the tide was low and rising. The water in the shallowest tide pools barely breached the trendy holes in my Crocs. We focused on two deeper tide pools. The first was perhaps calf-deep and as warm as bathwater when we began (10:45ish) and up to my mid-thigh when we stopped for lunch (12:30ish). The second tide pool, further from the high tide line, was mid-thigh and much cooler when we began, and I didn't venture back in to check the depth when we stopped for lunch. Both cooled dramatically as the tide came in.
The rocks above the low tide line were covered by Northern rock barnacles (S. balanoides) and common periwinkles (Littorina littorea). Periwinkles were typically on top of barnacles, indicating that they don't compete for space.
Common periwinkles and Northern rock barnacles were also prevalent on submerged rocks.
I captured several Asian shore crabs (Hemigrapsus sanuineus) and failed to capture two green crabs (Carcinus Maenas). I'm not sure if the crabs outsmarted me or if my own repressed memories held me back. I remember my dad saying "Just catch it from behind- he can't bite you that way!" I also remember my finger bleeding. For those readers looking for a very special experience, go grab a blue crab from behind. Hermit crabs, of which I saw plenty, pose slightly less risk (though there was an incident involving a terrestrial hermit crab and my eyelid, it was resolved peacefully).
I saw several club tunicates (Styela clava) attached to rocks. They were dark brown and olive colored. They felt leathery and bumpy and had two prominent apical bumps. The one that accidentally, through no fault of my own, became detached from the rock it was attached to, squirted salt water. I wonder how they (and other sea animals and plants) deal with salinity, which poses quite the osmotic issue.
Compared to the dock of the Barking Crab, there was an abundance of tunicates. They were bright orange, red, burgundy, and all colors in between. I was excited to see so many new species, until I read the Hitchhikers Guide more closely and found that the orange sheath tunicate (Ascidiella aspersa) can be bright orange, red, or purple. I also wonder if I saw a purple sponge (Haliclona permollis): I found a purplish encrusting animal and assumed it was a tunicate, but it had larger, more volcano-like holes and felt softer than the orange sheath tunicates. I saw something that looked like a colony of flat rusty colored circles ringed in yellow, but I can't figure out what it is.
There were several species of seaweed, to which my drawings do not do justice. There were also no skeleton shrimp of squilla. As Prof. Berman aptly pointed out, when you pull out a clump of seaweed, you tend to notice the relatively exciting, tiny animals. When you're chasing crabs and lobsters and watching hermit crabs engage in territorial battles, however, you don't even notice the seaweed.
And now: terrestrial snails (grove snails) vs. marine snails (common periwinkles).
The shells of the periwinkles were duller and less variably colored than the grove snails. Wikipedia suggests that the variater in colors is due in part to selective pressure from the song thrush, which hunts for grove snails by sight and goes for the ones that stand out most against their surroundings (and ignores the others). Depending on the habitat, different colors will be selectively eaten. The more uniform periwinkle shells suggest that either their surroundings are more uniform, or their prey is less discerning (probably both).
The grove snails were lighter than the periwinkles, and the periwinkles had rougher, thicker shells. I wonder if this has something to do with the different forces they have to counter to adhere to the surfaces they travel over- gravity for the grove snails, and buoyancy for the periwinkles. The grove snails had four antennae (a large pair and a smaller pair), while the periwinkles had only one pair. The muscle of the periwinkle looked dark, while that of the grove snail was light beige/yellow.
Once captured, grove snails were more exploratory and the periwinkles stayed in their shells. Each tactic seemed geared towards survival: we took the grove snails out of the shade and carried them directly in the sun, and it was probably in their best interest to move about and look for shade. The only grove snail that didn't explore formed a mucus barrier to protect against water loss. The periwinkles, which are used to and equipped for dry periods (during low tides), stayed in their shells and minimized risk of predation.
I would guess that the marine snails and terrestrial snails are related only distantly, and are the product of convergent evolution (like armadillos in North America and pangolins in Africa, only in this case, North America and Africa are the ocean and the land).
As for the five land snail shells on the beach near the big rock: I know, as many of us know, that many bird species drop shelled creatures on rocks to crakc the shells and expose the meat inside. I also know that some rocks are favored over others and so have more shells scattered about. But that rock? That close to our field trip destination? It all seems too perfect. I'm going to throw caution to the wind and formulate a radical hypothesis: Prof. Berman put them there.
That, or they were plotting to overthrow their oppressors but were discovered and "taken care of".