Growth Bands in Lobsters, Crabs and Shrimp Reveal Age

Figure 1. Growth bands record the age of a 7-year old snow crab.

Figure 1. Growth bands record the age of a 7-year old snow crab.

Crustaceans such as crabs, lobsters and shrimp are tasty examples of Canadian seafood, and their fisheries are among the most valuable in Canada, worth more than $1 billion per year. Valuable fisheries deserve extensive research, and the science surrounding these delectable marine animals has increased markedly over the years. Perhaps surprisingly though, some of the most basic information on how long crustaceans live, and how fast they grow, has continued to elude scientists. Until now. A scientific research team from the University of New Brunswick at Saint John, the Université du Québec à Rimouski, and the Department of Fisheries and Oceans at both the Institut Maurice Lamontagne (Quebec) and the Bedford Institute of Oceanography (Nova Scotia) has finally discovered a method by which to determine the age of lobsters, crabs and shrimp.

Just as trees form yearly growth rings which can be counted to determine their age, all fish and bivalves (such as clams) form yearly growth bands on their bones and/or shells. It is the counts and measurements of these growth bands that underlies the stock assessment of fish stocks around the world, since they reveal not only the age of the fish, but how fast it has grown. But lobsters and other crustaceans do not have any bones. And unlike clams and oysters, crustaceans shed (molt) their shells frequently throughout their lives, and thus do not have any permanent structures in which growth bands could be formed and kept. Or at least, that was what was believed.

The university/DFO research team discovered that two small parts of the crustacean actually persist between molts, and thus potentially record their growth from year to year. Both the base of the eyestalk and a small grinding stone in the stomach (the gastrolith) did not get shed during a molt. When thin slices of the eyestalk and gastrolith were prepared and then examined with a microscope, growth bands were visible. The challenge then became one of confirming that the growth bands were actually formed yearly, rather than for example, as a result of molting.

The research team spent considerable time collecting crustaceans from areas where their age was known, or could be determined reasonably accurately from other information. When the growth bands in these known-age crustaceans was compared with their actual age, the researchers found that the growth bands were indeed formed at each moult. The team also exposed young lobsters to calcium-binding chemicals, and then allowed them to grow and molt for many months. Although the chemicals were lost from the lobster shell, they were retained within the eyestalk and gastrolith, clearly showing that these body parts were not lost during molting.

As a final test, the research team wanted to compare the number of growth bands with the actual age of an old, known-age lobster. Some aquaria keep lobsters for many years, so the team expected to be able to find their test lobster quite easily. But such was not to be. Although some lobsters had been kept in captivity for over 20 years, all had been captured from the wild when they were quite large, and of unknown age. So the total age of the lobster was also unknown. Rewards were offered for old, known-age lobsters, but none could be located anywhere in the world. At one point, a reward of $10,000 was offered for just one old, known-age lobster, but without success. That would have been a valuable lobster!

So how old do crustaceans get? The answer to that question is still being researched. However, the research team examined both lobster and snow crab that were more than 20 years old, while shrimp were much younger at 2-6 years old.

In general, fast growing fish stocks replace themselves more quickly than slow growing stocks, and thus allow for more to be caught by fishermen. Thus knowing the age of the fish in a stock is important to predicting fish quotas that are sustainable over the long term. Until this point, the age and growth of lobsters, crab and shrimp could only be approximated. With the discovery of a method to directly determine the age of crustaceans, new and more accurate stock assessment methods for crustaceans could soon become possible. As a result, fisheries biologists around the world may end up adopting this new method to enhance their own crustacean fisheries.

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