Ontogeny and potential function of poacher armor (Actinopterygii: Agonidae)

Abstract

Many vertebrates are armored over all or part of their body. The armor may serve several functional roles including defense, offense, visual display, and signal of experience/capability. Different roles imply different tradeoffs; for example, defensive armor usually trades resistance to attack for maneuverability. The poachers (Agonidae), 47 species of scorpaeniform fishes, are a useful system for understanding the evolution and function of armor due to their variety and extent of armoring. Using publically available CT-scan data from 27 species in 16 of 21 genera of poachers we compared the armor to axial skeletal in the mid body region. The ratio of average armor density to average skeleton density ranged from 0.77 to 1.17. From a defensive point of view, the total investment in mineralization (volume * average density) is more interesting. There was 10 times the material invested in the armor as in the endoskeleton in some small, smooth plated species, like Aspidophoroides olrikii. At the low end, some visually arresting species like Percis japonica, had ratios as low as 2:1. We categorized the extent and type (impact vs. abrasion) in 34 Agonopsis vulsa across all 35+ plates in the eight rows along the body. The ventral rows show abrasive damage along the entire length of the fish that gets worse with age. Impact damage to head and tail plates gets more severe and occurs at higher rates with age. The observed damage rates and the large investment in mineralization of the armor suggest that it is not just for show, but is a functional defensive structure. We cannot say what the armor is defense against, but the abrasive damage on the ventrum implies their benthic lifestyle involves rubbing on the substrate. The impact damage could result from predatory attacks or from intraspecific combat.

Swimming and defence: competing needs across ontogeny in armoured fishes (Agonidae)

“…Moreover, these changes in armour phenotype do not necessarily indicate trade-offs as simplistic as 'more armour equal slower fishes,' but rather that a fish can have dense, protective armour and still swim effectively too (at least at sprint speeds). Complex phenotypes like armour are often the product of competing functional demands [68] and in the poacher's case armour must defend, add stiffness, assist in camouflage, and likely act as a signal of mate quality [26].…”

“…However, it is unlikely that the role is significant given the thin material between plates and the modest vertebral column. For point of comparison, the mineral investment in the vertebral column is less than 20% the mineral investment in the armour itself [26].…”

“…There is a further role for armour in camouflage; in Hypsagonus, like seahorses, spines serve as substrate for epibionts which further muddle the profile of the animal against its habitat [62]. Whereas in other poachers, such spines might serve a role in adults for sexual display [26]. Crypsis is not 100% effective, poachers are consumed by other fishes, particularly larger demersal and pelagic flatfishes (Pleuronectiformes).…”

“…However, something that predators like lingcod and crabs have in common, relative to predators like halibut, is that they pierce or crush prey [66], rather than swallow it outright. The armour of adult poachers would appear particularly robust to such puncture-related attacks [26].…”

“…The poachers (Agonidae) are a useful system for looking at trade-offs in armour function. They are cottoid fishes, related to sculpins, greenlings and other formerly 'scorpaeniform' fishes, but differ from their relatives in that all agonids have imbricating bony armoured plates similar to seahorses and sticklebacks, but with more complete body coverage reminiscent of pipefishes and pipehorses ( [11,25,26]; figure 1). However, unlike armoured pipefishes or seahorses, poachers lack the extreme exoskeletal dexterity that seahorses use to grasp substrates [19,28].…”