Last Sunday Stephanie Clark and I were down at the Potomac River. Yesterday, we explored the 10-Mile Creek area in Black Hill Regional Park in Boyds, Maryland. The sinistral Physa that we had found around the Potomac a week earlier were very abundant in the small shallow pools alongside the 10-Mile Creek.
The snail1 on the left was crawling on the bottom of the pool. You can see its long skinny tentacles out in front of the shell. These snails do not have gills, but obtain oxygen through a highly vascularized section of the mantle cavity, which is their “lung”. Hence, they, and their terrestrial relatives, are called “pulmonate” snails. The lung opens to the outside through a hole known as the “pneumostome” or simply, the breathing hole.
To breathe, Physa come to the surface of the water every now and then and bring the pneumostome in contact with air. So, if you sit by one of these pools and start paying attention to what’s inside, you will soon notice that there is a constant traffic of snails, sometimes two or three attached to each other’s shells, moving up and down between the bottom and the surface. It's an amusing sight. The picture on the right above shows one snail that was floating upside down at the surface.
Later during our hike through the woods, the trail passed over a tiny stream imperceptibly flowing down to the lake below us. We walked up the stream to its spring, which was nothing more than a muddy spot among the trees. While Stephanie took out her sieve to search for aquatic creatures, I took out my camera to go after a moth I had spotted. A few minutes later, she called out to me: “I’ve found bivalves!” In utter disbelief, I responded: “Bivalves? There is hardly any water here.” Expecting a large clam, I opened my hand and Stephanie put a speck on my finger. Sure enough, it was a bivalve.
This is a Pisidium (family Sphaeriidae). Some species grow bigger, but this one on the tip of my finger, possibly an adult, was barely 2 mm across.
1. The shells of these snails were about 10 mm long.