Fish otoliths are among the strangest and most wonderful bits of vertebrate anatomy. They are strikingly sculptural, and their clean surfaces tend to display an alluring opalescent sheen. No one is absolutely sure about all their functions (which would seem to vary from species to species), but it is safe to say that they generally serve in a sensory system very much like the saccule/utricle: they sit atop a mat of sensitive hairs and their sloshing around gives the fish information about its movement in space. Fish that have to deal with complicated spatial environments (reefs, kelp beds) usually have bigger otoliths; those open water predators that stick to swimming fast in straight lines (tuna, billfish) tend to have relatively small ones. Otoliths also seem to play a role in underwater hearing in many species: because they are stone (and therefore of a different specific gravity than the rest of the fish), their vibrations in response to sound waves are out of phase with those of the animal’s body; these differences can be translated into acoustic information. (Interestingly, although hearing in mammals is now handled by a very different system, it has recently been shown that human beings can “hear” very high frequency sounds by means of their otolithic organs, which appear to retain some acoustic sensitivity, despite having been converted almost entirely into sensors for movement and orientation).

There are several thousand researchers around the world who spend their whole working day looking at fish otoliths. This has nothing to do with their physiological functions, however, and everything to do with their structure and the staggering amount of information they contain. In the first place, each species of fish has a unique otolith shape. Couple this with the fact that they are stone (and therefore comparatively resistant to decomposition), and their utility as a biological marker becomes clear. Interested in the food habits of bottlenose whales? Pump their stomachs and you will end up with relatively few bones but lots of otoliths. Find an otolith expert and he or she will be able to give you a menu. Similar work has been done to reconstruct the ecology of seabirds or to determine, using kitchen middens from archaeological sites, the diet of early coast-dwelling humans.

But the true wonder of these peculiar pearls lies within. Should you have occasion to tonsure a snapper or sea bass, slicing off the top of its skull just above the eyes, you might take a moment to remove the two largest otoliths (there are, as a rule, six in all, three on each side) from their velvet seats to the right and left of the brain stem. With the heel of a knife you should be able to snap one of them in two, and then, holding it to the light, you will discern a set of concentric bands. These are growth rings—annuli—which, properly counted, will give the age of your fish in years. This in itself is interesting, and enormously valuable to those who wish to understand the life cycles and population structure of commercial species. But about thirty years ago a curious geologist, tinkering with an otolith (it was a rock, after all), made the truly shocking discovery that those annual layers can be further resolved, microscopically, down to daily layers, layers that contain, in their chemical composition and size, information about the temperature and the salinity of the water through which the fish moved, the food that it ate, and various environmental contaminants it encountered. The result is a stratigraphy unprecedented in the organic world: the diligent student can peruse the otolith of a long-lived deep sea fish, and reconstruct not merely its age, but (and I am barely exaggerating) what it had for breakfast on 6 March 1964, or roughly where it was on the occasion of a particular nuclear test. Not for nothing have those who gather at the biennial “Otolith Olympics” (the insiders’ name for the scientific conference of dedicated otolith researchers) taken to calling the ear stone the “flight recorder” of the piscine world.

And that’s not all. Ever inclined to make an inscription, human beings have figured out how to write their own messages in the heart of the pearl. By sequentially altering the temperature of the water in which salmon fry are hatched and raised, researchers can lay a distinctive “batch label” into the chemical layers of the otolith—a kind of barcode, inscribed in stone, and indelibly preserved within the maturing adult fish (a puckish early student of this technique used it to write “hi mom” in binary inside his experimental animal). Later, when these free-swimming creatures are captured at sea, each can be traced unfailingly to its hatchery of origin. Some five billion Pacific salmon have now been marked in this way, their inner qibla reconfigured to refer to their point of origin, and thus the point to which they seek return.