What you can do with an extra jaw - Darien Satterfield

After stalking an unsuspecting cuttlefish across the seafloor, this moray eel is finally ready to pounce. As the eel snags the mollusk in its long teeth, its prey struggles to escape. But before it can wriggle away, a second set of teeth lunge from the eel's throat, making short work of the captive cuttlefish.

This adaptation is called a pharyngeal jaw. And while it might seem more fitting for an alien parasite than a fish, it's actually one of the most common adaptations under the sea.

At some point millions of years ago, early fish evolved a jointed oral jaw from one of the bony arches that supported their frontal gills. This exterior, oral jaw was perfect for capturing or chewing prey, but these early fish had a problem. They had no limbs to manipulate food while eating, making it easy for their mobile prey to escape before they took the first bite.

Essentially, it was impossible for fish to capture and chew their prey at the same time with just one set of jaws. So to hold their food and eat it too, an even deeper gill arch evolved into a secondary set of jaws. Unlike oral jaws connected by a bone joint, these pharyngeal jaws were suspended in muscle, offering them a wide range of motion to turn, tug, and tear on food.

Some pharyngeal jaws actively chew, while others retract to pull chunks of food down the throat. Of course, the teeth a jaw has also factor in here. For example, an oral jaw with fangs is exceptional for catching and holding on to fast-moving prey.

But depending on what that prey is, a fish might want sharp pharyngeal teeth to tear through tissue, flat pharyngeal molars to grind plant matter, or a powerful pharyngeal bite to crush shelled prey. Fortunately, some fishes can change their secondary teeth to match a new diet over several years.

This flexibility allows fish to adapt and find food in a huge variety of aquatic environments. Some pharyngeal jaws have formed to eat food too hard for most fishes. California Sheephead use pointed teeth from their oral jaws to pry urchins off rocks before crushing their spiny exteriors in strong pharyngeal jaws that have fused with the bones of their skulls.

Other fish prioritize eating huge quantities of food. The pharyngeal jaws of grass carp have two bone components that move in unison to pull plant life into their stomachs, consuming over 18 kilograms of seaweed a day. Other pharyngeal jaws are adapted to suit incredibly specific circumstances.

Consider this species of pearlfish that lives inside sea cucumbers and feeds on their guts. Fish are the most diverse group of vertebrates with over 30,000 unique species, and much of that diversity is due to the hundreds of unique jaws separating otherwise identical species.

Perhaps the best example of this dental diversity can be seen in cichlids. This family of fish is primarily found in select African and South American lakes. Typically, large lakes like these would be occupied by a handful of distantly related species, each adapted to consume one of the lake's limited food sources.

But here, almost every corner of the ecosystem is occupied by a different species of cichlid. There are over 1,700 cichlid species, many almost identical save for their uniquely adapted jaws which have evolved to eat crustaceans, mollusks, worms, algae, plankton, and even the scales of other fish.

These oral adaptations are so essential for survival, that different species use their pharyngeal jaws to make unique mating sounds that allow female cichlids to identify males of their own species. This technique limits interspecies breeding, and ensures the parent fish will pass on their specialized jaws.

Scientists are still discovering all the ways this incredible skeletal mechanism functions. But given what we know already, it's fair to say that for most fish, two jaws are better than one.