Many people are aware that trees can be dated by counting the layers of tree rings in a cross section. But, less folks know that fish, snails, mussels, and the teeth of many local mammal species build seasonal growth rings that are used to age individuals.
In most Northwoods trees, the center rings are the oldest and new rings grow each year just under the bark in the layer called the cambium. Each spring and summer, trees build strong cells with thick walls that stack up to carry water from the roots to the leaves. These cells compose the tree’s xylem and have sturdy cellulose skeleton walls that stay intact long after the cells die or stop functioning. These skeletal cells are what build the wood of the tree each year. Fast-growing periods often result in lighter wood tissue while the darker delineations are caused by slow growth periods in the winter when there are no leaves on the trees to photosynthesize and give energy for new cell growth.
While tree rings are hidden in cross-section under a layer of bark, snails and mussels common in our freshwater lakes prominently display their rings in the growth patterns of their shells. Unlike insects and crustaceans which molt their skeletons as they grow, snails and mussels continuously build their shells. The oldest part of a mussel shell is right at the hinge where the top and bottom shells attach. That joint, called the umbo, is usually a little thicker than the rest of the shell. It is the remaining skeleton from the mussel when it was young. New shell is built away from the hinge at the margin where the two shells join. Inside the mussel there is a special layer of tissue that builds this skeleton of calcium carbonate. Each summer when there is lots of food and warm temperatures, mussels build new shell to house their growing body. The new shell is smooth inside and often lighter color on the outside. In the winter, growth slows and forms a dark ring on the outside, and often rougher ring on the inside creating ‘annuli’ or annual rings inside and out that can be used to calculate the age of a mussel.
Snails have a similar tissue layer that builds new shell. The oldest part of their shell is right in the middle and the youngest section closest to where the head sticks out. While the rings are not always visible, if you shine a flashlight on a snail’s shell, you can often see thickened rings indicative of those cold winter periods.
While these external skeletal rings can be easily observed, internal skeletons of fishes and many mammals also build growth rings. Fish have special structures in their heads called otoliths that act like our inner ear bone to help keep fish balanced and oriented upright while swimming. When cut in half, otoliths have clear annuli that can be used to track, daily, monthly, and annual growth rates across fish of all ages. Otoliths are commonly used to calculate the age structure and reproductive dynamics of fish populations, information essential to managing our highly valued Wisconsin fisheries.
Similarly, resource managers and harvesters collect age information from harvested deer, bear, moose, and bobcats using their teeth. This is something I learned working at a deer check in station in southern Wisconsin when I was a teen. When hunters brought in their harvest, I searched for the farthest back molars and counted the visible rings on their teeth. Mammals continuously deposit cementum, a connective tissue that keeps teeth held in place. Spring and summer are faster growth periods for mammalian tissues creating lightly colored, thick layers of cementum on teeth that contrast with thin, darker layers built in winter that creates measurable annual rings.
In our region of the world where winters are real and conditions can be challenging for plants and animals; it is interesting to consider how all of these distantly related organisms represent their struggles annually in the rings of their skeletons.
