Lateral scan of the north spearnose poacher
Lateral scan of the north spearnose poacher.

For centuries, armor has protected warriors in combat, providing a defensive barrier and preventing damage to whatever is underneath its protective shell. It has seen many iterations as the years go on, always improving and allowing for more agility while decreasing in weight with the advancement of technologies allowing for better materials. That is exactly how armor on fish has changed over time, evolving from the clunky thick head shields seen on the earliest fishes to the scales we now see on modern fish. 

A new study from Friday Harbor Laboratories (FHL), led by FHL alumni researchers Cassandra Donatelli (now at the University of Ottawa) and Matthew Kolmann (now at the University of Michigan) in collaboration with researchers from Tufts University, examined how armor on the northern spearnose poacher fish changes throughout the course of its life, and how it balances defense needs with mobility needs. If you don’t know what a poacher is, don’t worry – they’re small, dragon-like fishes in the family Agonidae, found locally throughout the Salish Sea, up to Alaska and the Bering Straits, and all the way around the Arctic circle to Japan.

Lateral view of a north spearnose poacher's armor plate.
Lateral view of a north spearnose poacher’s armor plate.

Think of baby poachers like baby humans. Baby poachers have big hard spines on their armor, but the plates that make up the armor are still soft and flexible. Much like how a newborn human baby’s bones aren’t fully hardened, a juvenile poacher’s plates are softer and squishier than their adult counterparts. Without the protection that plates provide when fully mineralized, juvenile poachers spend their days hiding at the bottom of the ocean, swimming to the surface at night to feed on plankton. Maximizing the amount of energy needed to swim up to the surface of the water column, a poacher will curl into an S shape as it sinks, slowing down its descent, allowing it to leisurely feed all the way back down. The spines on a poachers’ plates may slow this rate of descent, allowing it to prolong its midnight mealtime. 

As poachers grow, so do their armor plates. The plates grow bigger and denser fairly quickly, leaving the poacher to figure out to maneuver in armor that is growing heavier and heavier. 

“The most unexpected thing we found in this study was that the morphology changed at certain stages pretty quickly,” says Donatelli. “There’s a big shift when they go from small to medium and that’s why their swimming needs to catch up.”

Think of a poacher at this stage of its life as a teenager: in a body that is very different from the one it was previously used to, trying to awkwardly figure out how to move around in the process. Once they have reached this stage in their lives, poachers are no longer able to bend their bodies, so they use their pectoral fins to swim around the ocean floor. The heavy armor also lends itself to more rigidity, which allows for a bigger push to cover more distance when they swim with their tail.

This is all very interesting but does studying this fish and its armor have useful applications outside of the lab and classroom?

Yes! We can learn a lot from these animals, utilizing the methods used by poachers to get around quickly in heavy armor and applying it to designs for technology, such as underwater remote operated vehicles (ROVs). ROVs must have an exterior strong enough to protect the sensitive equipment and cameras onboard from the pressure when roaming around on the ocean floor while maintaining enough flexibility to drive around obstacles. More agility also allows an ROV to explore parts of the ocean that were previously inaccessible, as well as causing less damage to the environment as it moves around.

“This is a rare example of how to build better armored protective structures that can still move,” says Kolmann. “It’s not just a trade-off between being protected and slow — the armor actually increased performance of the fish to make them accelerate faster. These are fish that no one cares about, they’re not a target of industrialized fishing, so this is a great example of a fish that minds its own business that inspires a huge amount of innovation.”

Kolmann and Donatelli hope to extend this study to other species of poachers, and eventually use what we know about armored fish today to make predictions about what extinct armored fish may have looked like, how they might have behaved and how they evolved into modern armored fish. This research was funded by the Seaver Institute and the National Science Foundation.