How on earth did thousands of clams end up in a vernal pool that shares no connection with other bodies of water? Furthermore, how did they colonize thousands of vernal pools across the United States? Those are good questions, and ones that almost everyone asks immediately after learning many vernal pools are populated by something called a “fingernail clam”.
Vernal pools, which are important breeding habitat for many amphibians, are filled with water by rain and snow melt, but completely dry out most years and are not connected to streams or permanent wetlands, yet somehow many of them are inhabited by tens of thousands of small freshwater clams no larger than a person’s pinky fingernail. When I asked the very same questions years ago I was told they can clamp down on bird feathers and amphibians which can then transport them to other pools (the words clam and clamp mean basically the same thing in old English), but I didn’t really believe it until I saw it. Back in college while taking a herpetology class at the University of Vermont, I vividly recall checking drift fence pitfall traps (a catch-and-release method of catching amphibians on rainy nights) next to vernal pools and finding several adult Jefferson Salamanders with fingernail clams literally attached to their toes. Furthermore, the salamanders were headed from the uplands to the vernal pools when we caught them, so these “toenail” clams had been attached to the salamanders underground and away from water for nearly a year! I later learned some species of fingernail clam can survive being ingested by ducks, which inevitably fly to another body of water where, one way or another, the clams then exit the ducks.
There are over 40 species of fingernail clam in North America, and some prefer vernal pool habitats. There isn’t really a way to tell the species apart without being something of a clam expert, which I am not, so most people are content just calling them by the same name and knowing they belong to one family (Sphaeriidae). They are small, with the larger species topping off at about half an inch, and they can be incredibly abundant, sometimes reaching population densities of 10,000/square meter! Fingernail clams only live for a few years at most, and do best in vernal pools with lots of leaf litter in calcium rich environments to help with shell growth. Like other clams, they are filter feeders, feeding mostly on algae and detritus suspended in the water. Remarkably, they can survive in oxygen deprived waters for up to 400 days and some species can survive prolonged periods of time without water by burying themselves up to 8-inches in the soil where they will remain moist (or by clamping down on salamander toes and being transported to an underground rodent tunnel). Unlike other freshwater clams which have parasitic larvae that live in the gills of fish, fingernail clams store their eggs and larvae in something colloquially known as a marsupial sac, which is a special compartment in their own gills, and then release their young as tiny fully-formed clams. For some reason, despite being among the smallest clams in the world, their eggs and larvae are bigger than those of much larger species.
When most people think of vernal pools, they think of amphibians. Spotted Salamanders are sort of like the charismatic megafauna of the vernal pool world and are an obvious testaments to the importance of vernal pool habitats, but it is important to remember there is actually a lot of biodiversity in those mini wetlands (fairy shrimp being another great example). While some conservation groups may use salamanders to tout vernal pool conservation, protecting those critical habitats helps far more species than most people realize. Yet, when people learn about fingernail clams and have a chance to see them in the heart of woodland environments, they are often as excited by the clams as they are finding the salamanders themselves. Salamanders may be the cute face of vernal pool conservation, but fingernail clams are just one of many more species to reap the benefits.