Meeting Abstract
Adult northern spearnose poachers (Agonopsis vulsa), like other poachers, are heavily armored, lack swimbladders, and live on the seafloor. However, as juveniles they spend a considerable amount of time in the water column and appear to be quite flexible. We investigated the scaling relationship of bone mineral density (BMD) and the volume of dermal armor plates, and measured the ability to resist compression changes through ontogeny. We hypothesized that (I) BMD and volume of dermal armor would scale isometrically with body size (0 and 3, respectively), and that (II) compressive load resistance would increase with body size. Fifteen Agonopsis vulsa (13 – 180mm SL) were collected and either micro CT imaged (N=9) or compression tested (N=6). Micro CT scans were performed with two objects of known densities of calcium phosphate hypoxyapatite (25% and 75%). BMD and volumes were measured on each fish and linearly regressed against body size. For compression testing, specimens were sectioned into 9 segments and compressed to 40% segment width (3 fish dorsoventrally, 3 fish laterally). We found that BMD scaled positively allometric with body size (Slope = 0.4) whereas volume of the armor scaled isometrically (Slope = 2.827). Dorsoventral compressions were inconclusive due to high variability in compressive forces, yet lateral compressions demonstrated scaling patterns and regions of increased compressive force resistance. Internal structures, such as the ribs, neural processes, and pterygoids appear to act as struts, resisting compression. CT scans of crushed segments reveal that armor plates appear to be locked in position, as the plates didn’t slide past each other during compression.
