Trees have rings, horses have teeth and even rocks have radiocarbon decay. But how can you tell an octopus's age?

This isn't a frivolous question. In fact, the future health of octopus populations depend on it.

To meaningfully study any animal population, scientists need to be able not only to count the individual numbers (already a challenge for octopuses), but also to determine animals' reproduction rates and ages. Fisheries scientists can track fish populations by studying the animals' otoliths. These hard calcium deposits, also called "ear stones," acquire growth rings based on different rates of growth over various seasons. These lines can be counted much like rings on a tree to give an estimate of the fish's age.

Octopuses, however, have no such bone. In fact, they are almost all soft-bodied; no bones, no "ear stones." So researchers have primarily relied on estimating age based on size. But body size can vary widely—even in the same species—based on water temperature, food availability and other factors.

A new paper suggests there might be a reliable way to ascertain an octopus's age for at least one common species.

Researchers in Mexico studied the local Octopus maya. They raised 32 of these octopuses in captivity and examined their beaks and stylets (both made from harder material) and their eye lenses at different ages.

Eye lenses did not seem to show any predictable deposit pattern. And beaks, when sliced through showed somewhat reliable predictions of an octopus's age in days (these, and most octopuses, are very short lived) for the younger octopods, but that reliability broke down as the octopuses got older.

The stylet, a hard component in the octopus's mantel, however, looked to be a good gauge. The stylet growth increments "were closely related to age in days," the researchers noted in their paper. The findings will be in the November issue of the Journal of Experimental Marine Biology and Ecology.

The stylet has been a common hope for determining an octopus's age, and had been suggested before for age assessments in the common octopus (Octopus vulgaris).

The choice of Octopus maya for this new research, however, could be extra helpful because it is a popular candidate for octopus aquaculture farming.

In the meantime, hopefully we won't have to keep guessing at these changeable invertebrates' age for much longer.

Illustration courtesy of Ivan Phillipsen