Opercula are super neat

WTF are opercula, you ask? Opercula are bony structures that cover and protect the gills of teleost (bony) fish. Wikipedia’s disambiguation page informs me that the word is also used to describe various 'lid-like' structures in other plants and animals.

What’s super neat about opercula (at least fish opercula) is that they can often be used to determine an individual's age. The bones (including opercula) of many fish species contain annual growth rings (similar to the cross section of a tree trunk) that form because bone is deposited at different rates throughout the year, depending on the season. The rate of bone deposition more or less tracks growth rate, so any environmental factor that influences an individual's growth rate can also influence bone deposition rates.

Figure 1. Operculum of 12-year-old walleye (Sander vitreus) taken from a lake in Northwestern Ontario. From Cooley and Franzin (1995)

Opercula are part of the reason that I’m not jealous of biologists that study fish in tropical areas… tropical fish can be difficult to age via bony structures because tropical fish don't usually exhibit consistent variation in bone deposition rates over fixed time periods. There are exceptions to this rule due to climatic and hydrological cycles that occur in the tropics, but in general, fish that occur in temperate areas (i.e. areas with distinct seasons, like Canda!) are much easier to age.

What’s so great about knowing an animal’s age? Well, age is obviously important when we want to determine whether an animal is legally allowed to vote, buy porn, or rent a car for a reasonable price (I just want to be 25 damn it!). It is also essential for the determination of individual growth rates, population growth rates, and sustainable harvest rates.

And now you know.