Using DNA Methylation to Determine Lobster Age
Until fairly recently, it was impossible to accurately determine the age of a lobster found in the wild. This is one of a number of marine species that exhibits negligible senescence, meaning few signs of aging across the majority of its lifespan. How long can a lobster live? That used to be quite unclear until it was found that it is possible to count growth rings in the eye stalks in order to age specimens caught in the wild. There is an ongoing process of aging in this species, despite their negligible senescence, as demonstrated here. Researchers have been able to correlate lobster age to changes in DNA methylation, indicating that gene expression is changing over time in this species, and it is thus just as possible to produce an epigenetic clock for aging in this species as it is in the case of mammals.
Lobsters are notoriously difficult to age. Nobody knows exactly how old they can get, and some experts have estimated they could live on the ocean floor for as long as a century or more. "Until now, a lobster's age has usually been estimated using its size - but this is inaccurate as individual lobsters grow at different rates. For a long time, it appeared that there was no accurate way to quantify a lobster's age. Some research suggested that you could tell a lobsters age by counting the rings in parts of their eyestalks and stomach - a little like counting tree rings. But you can't do that for a living lobster."
"Lobsters have hard, inelastic shells and so in order to grow they must shed their old shell and replace it with a new one. However, lobsters of the same age don't always grow and moult at the same time. For example, lobsters with more food or in warmer waters can grow more quickly, which makes it really hard to know how old lobsters actually are. It is crucial to be able to estimate how many lobsters of particular ages are present in a given area so that they can be sustainably harvested. We wanted to develop a new, non-lethal method of determining the age of European lobsters that could be of better use for lobster fisheries management. The European lobster was an ideal species to study because it is economically and ecologically very important."
The research team used a method that relies on quantifying epigenetic changes that accumulate with age within a lobster. Lobsters raised from eggs, so that the exact ages of individuals was known, allowed the researchers to calibrate their methods. "We identified a very strong relationship between age and epigenetic modifications, which allowed us to accurately estimate the ages of individual lobsters. Applying this method to wild lobsters predicted ages that generally aligned with minimum estimates of age based on size."
I suggest looking at heavy metal accumulation instead.
An interesting implication of negligible senescence addressed in the article is that it might (must?) result in a logarithmic shape (deceleration approaching a zero slope asymptote) for any true biomarker of aging after late stage developmental milestones. This is easy to understand in that methylation at a given loci can only hold values between 0% and 100%; a clock cannot "tick" past 100% (or 0%), and thus any clock for a truly negligibly senescent animal can't be linear.
From the article:
"Another potential cause of error in wild lobster age estimation is that rDNA methylation changes may be nonlinear with age and reach a plateau phase (irrespective of saturation)… Such trends of early acceleration followed by deceleration are best described by a logarithmic function with age… If a similar pattern occurs in European lobsters, a linear regression equation would result in an overestimation of age in older individuals, which does not appear to be the case in this study."
That the authors recognize this but in turn use (and show the accuracy / validation of) linear regression is interesting. It might suggests that the study subjects are in fact not negligibly senescent?