Swimming and defense - competing needs across ontogeny in armored fishes (Agonidae)

Biological armors are potent model systems for understanding the complex
series of competing demands on protective exoskeletons; after all, armored
organisms are the product of millions of years of refined engineering
under the harshest conditions. Fishes are no strangers to armor, with
various types of armor plating common to the 400 million years of
evolution in both jawed and jawless fishes. Here we focus on the poachers
(Agonidae), a family of armored fishes native to temperate waters of the
Pacific rim. We examined armor morphology, body stiffness, and swimming
performance in the Northern Spearnose Poacher (Agonopsis vulsa) over
ontogeny.  As juveniles, these fishes make frequent nocturnal
forays into the water column in search of food, while heavily armored
adults are bound to the benthos. Most armor dimensions and density
increase with body length, as does body stiffness. Juvenile poachers have
enlarged spines on their armor whereas adults invest more mineral in armor
plate bases. Adults are stiffer and accelerate faster than juveniles with
an anguilliform swimming mode. Subadults more closely approximate adults
more than smaller juveniles, with regards to both swimming and armor
mechanics. Poacher armor serves multiple functions over ontogeny, from
facilitating locomotion, slowing sinking, and providing defense.