# Testing the Limits: Functional Strengths and Weaknesses of Poacher (Agonidae) Armor

## TL;DR

This study examines how different parts of the armored fish Bathyagonus alascanus handle various types of physical stress, revealing how armor function varies by location.

## Contribution

The study provides new insights into the functional diversity of dermal armor within a single fish species.

## Key findings

- Posterior armor is strong against crushing, likely protecting the fish from predators.
- Anterior armor resists puncture, possibly defending against frontal attacks.
- Dorsal spines erode from abrasion, while ventral plates remain smooth due to environmental wear.

## Abstract

Dermal armor serves a variety of functions across animal lineages including defense, offense, display, and prehension. Small differences in armor structure, plate size, or overlap may complement large differences in behavior or ecology. We characterized damage to an armored fish—the gray starsnout poacher (Bathyagonus alascanus) to probe whether there are differences in plate function within a single species. We quantified damage to poacher armor and skeleton under different force modes, including crushing, puncture, abrasion, and blunt impact, using micro-computed tomography, scanning electron microscopy, and material testing. Armor in the posterior region of the fish can withstand higher stress during crushing, suggesting they are well protected while fleeing from a crushing predator. It takes more work to puncture the anterior armor, perhaps poachers tend to face an animal threatening a puncturing attack. The dorsal plate spines are often eroded away from abrasion and/or blunt impact; we posit that the spineless ventral plates are smooth because strong sub-tidal currents cause collisions with a rocky substrate that would quickly destroy ventral spines if the plates were so equipped. The imbricated armor of B. alascanus has a diversity of performance against different threats, and this varies with location.