If you look along the coast of Maine, you will find many places bearing the name “Quahog.” These are the hard-shell clams that live in the intertidal along our shores. They have much thicker, stronger shells than their soft-shell compatriots and bear a purplish tint that has made them valuable as jewelry and as currency for trading. Their Latin name, Mercenaria mercenaria, meaning “wages,” reflects this value. Quahogs tend to live for about 12-20 years and grow no larger than five inches or so across. They can be dug up along the shore and are a highly valued shellfish species in the state, often labeled as cherrystones, topnecks, littlenecks or countnecks depending on their size.
While these coastal quahogs are impressive on their own, it is their relative, the ocean quahog, that is literally record-holding. Ocean quahogs (Arctica islandica) are not just arctic, as their name implies, but are native to waters from Newfoundland to North Carolina. They inhabit the seafloor at depths ranging from 25 feet up to water as deep as 1,300 feet. Aside from size and habitat, they look a little different from our coastal quahogs. They have a thin covering on their shell that makes them look darker, hence their common names of black clam or mahogany clam.
Ocean quahogs are a harvestable species and are collected in Maine waters along with the more common coastal species. Because they do not live in tidal waters, instead, spending their entire lives underneath the water, they are harvested in a different way. They cannot be hand-dug but are instead collected by dredge. A dredge is essentially a frame with an attached collecting bag that is dragged across the seafloor by a boat. The Maine ocean quahog fishery is divided into two components – the state fishery within three miles of shore and the federal fishery out beyond the state limit. The federal fishery is managed by the Mid Atlantic’s Surf Clam and Ocean Quahog program. This program allocates each state, including Maine, a bushel quota for the year. The state fishery is managed by the Maine Department of Marine Resources and is open access. The catch from state waters is included in the total quota allocated for Maine.
It is not the difference in where they live or how they are harvested, however, that makes them remarkable. It is their life history. Ocean quahogs have been found to live for over 500 years. The amazing thing about shellfish is that you can measure their age by counting the rings on their shells much in the same way you can count rings in a tree’s trunk. That means that it is possible to measure a quahog’s age with great accuracy.
Many aspects of these quahogs are similar to the quahogs that live close to our shore. But the difference in life expectancy certainly separates them. There are many theories about why ocean quahogs may live so long including the fact that their metabolism slows way down in the deep cold waters where they live. This is somewhat similar to the diving animals I wrote about last week who slow down their metabolism in an effort to conserve oxygen so that they can dive deeper and stay underwater for longer periods of time.
A slower metabolic rate means that ocean quahogs don’t need to eat as often, which serves them well in the most northern parts of their range where there isn’t a lot of food for these filter-feeders in the darkest months where the sun doesn’t penetrate deeply enough to replenish their planktonic food supply. Another consequence of their slow metabolic rate is a slower growth rate. They don’t reach maturity until they are around 6 years old and don’t get to a harvestable size until they are nearly twenty. By comparison, our coastal quahogs reach their harvestable size of 2 inches at 4 or 5 years of age.
Evolution has a way of quietly instructing the rest of the natural world and that includes humans. I wonder, then, if those of us that live in the more frigid, northerly climes will, like the ocean quahog, be longer lived. Perhaps if we were better adapted to live in our climate without the critical aids of heat and shelter, that would be the case. Regardless, next time you find a clamshell, count the rings and learn the age of the creature it once contained – even if it isn’t the record-holding 500-year-old ocean quahog.
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