[This diary was originally posted to Daily Kos on August 4, 2006. New Marine Life Series content will be back next Friday with the new year.]
So, what do clams have to do with tree rings?
Dendrochronology is the study of past events using growth rings in trees. In some ways it's actually more a study of ecological history than biology (or put better, a study of history using biology). When viewed in cross section the rings of a tree can be clearly seen and its past growth can be measured as new cells are laid down by the cambium tissue layer just beneath the bark.
The rings form because during times of rapid growth (ie: when the tree is photosynthesizing) the cells produced are fairly large and laid down pretty quickly. As winter approaches the cells are laid down more slowly and the cells themselves are smaller. In the spring, the cell production increases once again. The end result is the familiar rings we've all seen in a cut tree trunk or branch. Because a tree has one fast (summer) and one slow (winter) growing season, the age of a tree can be known by simply counting these rings. One ring equals one year.
Dendrochronologists can look at hundreds of years worth of rings on a tree and see trends in climate change, floods, droughts etc. I remember as a kid visiting the Bronx Zoo, where they have a huge trunk of a tree thousands of years old, and being totally awed by the little labels saying things like "forest fire occured this year", "civil war began here", and "this year Christ was born".
This is a pretty simple background on growth rings, but for more information there is a great website called The Ultimate Tree Ring Page.
Bivalve shells also form growth rings, and for basically the same reason as trees do; which is a change in growth rate at various times of year. Although this applies to nearly all bivalves, I'm going to use my local Hard Shell Clam (Mercenaria mercenaria) as the main example. I should also point out that the term used here, dendrochronology, isn't really appropriate in this case since you can't really tell a lot about history from an animal with a life span of ten or twenty years. But I liked the sound of the title.
Bivalves are mollusks with two shells, made mainly out of calcium carbonate that is extracted from the surrounding water by an organ called the mantle, which converts the minerals into its protective shell. The layers of calcium are added to the outer edges of both shells equally so that over time the shell increases in width, length and thickness.
Looking at the image above you can see many concentric lines, each representing a layer of shell that the mantle has produced. Like trees, during periods of rapid growth the shell is produced quickly resulting in many growth rings which are spread fairly wide apart. During times of slow growth the layers are produced slowly resulting in the rings being set very close together. Thus each growth spurt ends with a band that is unusually dark. The space between these dark rings represents one growing season. However, unlike trees, clams have two growing seasons per year. So to estimate the age of a clam two rings equal one year.
Bivalves are filter feeders that get all their nourishment by straining drifting plankton, most of it microscopic, from the surrounding water. The most rapid growth in a clam therefor corresponds to the times when plankton is most plentiful. This happens to occur in the spring and in the fall when plankton populations undergo what is referred to as a "bloom".
To keep this a readable length I'll leave the discussion of why plankton blooms occur when they do for another diary. [Update: That diary is here, written in December, 2006.] Until then, just trust me on this. Because plankton populations aren't as reliably consistent as the sun, unlike trees the clam's rings just give you a pretty good guestimate of its age. To get an exact age you need a cross-section of the shell, which is not a good method if you plan on keeping the animal alive.
I have some bivalves that I've kept in captivity for several years. We feed them newly hatched brine shrimp, liquified commercial filter feeder food and live phytoplankton. What's interesting is that because our captive clams are fed pretty much the same diet all year round, the growth rings become uniform throughout the year and you can no longer tell the age of the mollusk by looking at its shell.
The Hard Shell Clam is usually called a Quahog around here, made famous by a popular, but incredibly unfunny, local cartoonist. The name originates from the Narragansett Indian word for this animal, poquauhock. And for you fans of the show "The Family Guy", the town of Quahog, RI in which the cartoon series is set, is fictional.
There is a saying that the more important something is to a group of people the more words for that thing they will have. For example it is commonly believed that Inuit tribes have unusually large numbers of words to describe snow. This is, of course, an urban legend. But with the Hard Shell Clam, there is some truth to this adage. It is one of the most economically important seafood species in Southern New England (second only to lobsters, I believe). And here in Rhode Island we do have several different common names for this animal.
Among the science community this species is called "Northern Quahog", to distinguish if from the very similar "Southern Quahog", which replaces it south of Virginia. Those that are undersized for legal commercial use we just call "young clams", since they are insignificant as far as most people are concerned. Very small clams, just barely over the legal size limit are known as "littlenecks" and often eaten raw with tabasco sauce. Medium sized clams, favorites for dishes like clam zuppa (basically a red soup cooked with clams in their shell) are known as "cherrystones", and the big guys, normally chopped up to be used in New England clam chowder or deep-fried clam cakes, are known as "chowder clams".
To determine whether or not a quahog can be called a littleneck (the smallest legal size) the state issues all clam diggers a small metal ring. If the clam passes through the ring you have to release it.
Fun Fact: Wampum, once used as currency by Native Americans, was made from the purple-stained internal part of the quahog's shell.
Other diaries in this series can be found here.