Meeting Abstract
Many fishes use large, modified fin spines to defend against predators. The material and structural properties of the spine must be capable of withstanding the forces associated with predatory encounters, as failure to do so can be costly. In filefishes (Monacanthidae) and triggerfishes (Balistidae), the large dorsal fin spine is modified into a formidable defensive structure that can be held in an erect position by specialized locking mechanism. The spine can either 1) increase the fish’s effective body depth to protect against gape-limited predators or 2) lock itself inside inaccessible crevices. In either case, the loads placed on the spine could cause it to bend or break. We use high-resolution CT scans of 19 species to describe morphological variation and quantify the mineralization and second moment of area of the spine. We found that triggerfish spines are completely solid, whereas filefish spines are hollow inside. Additionally, most filefishes had spines covered with small, laterally projecting spines, while triggerfish spines usually had small bristly spines at the tip. Both families had a second moment of area that starts high at the wide base and gradually decreases moving toward the tip. In filefishes, the presence of small spines produced substantial, intermittent rises in the second moment of area along length of the spine—producing an overall oscillating trend in the second moment of area not found in triggerfishes. Despite these differences in the second moment of area, mineralization was similar for all species—ranging from 40 to 60%. These data suggest that filefishes and triggerfishes modulate spine flexural stiffness primarily through changes in morphology, and not changes in the material composition of the spine.
