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A remarkable property of tetrapod bone is its ability to detect and remodel areas where damage has accumulated through prolonged use. This process, believed vital to the long-term health of bone, is considered to be initiated and orchestrated by osteocytes, cells within the bone matrix. It is therefore surprising that most extant fishes (neoteleosts) lack osteocytes, suggesting their bones are not constantly repaired, although many species exhibit long lives and high activity levels, factors that should induce considerable fatigue damage with time. Here, we show evidence for active and intense remodeling occurring in the anosteocytic, elongated rostral bones of billfishes (e.g., swordfish, marlins). Despite lacking osteocytes, this tissue exhibits a striking resemblance to the mature bone of large mammals, bearing structural features (overlapping secondary osteons) indicating intensive tissue repair, particularly in areas where high loads are expected. Billfish osteons are an order of magnitude smaller in diameter than mammalian osteons, however, implying that the nature of damage in this bone may be different. Whereas billfish bone material is as stiff as mammalian bone (unlike the bone of other fishes), it is able to withstand much greater strains (relative deformations) before failing. Our data show that fish bone can exhibit far more complex structure and physiology than previously known, and is apparently capable of localized repair even without the osteocytes believed essential for this process. These findings challenge the unique and primary role of osteocytes in bone remodeling, a basic tenet of bone biology, raising the possibility of an alternative mechanism driving this process.anosteocytic bone | osteon | bone toughness | damage-driven remodeling | fish skeleton

We quantified placoid scale morphology and flexibility in the shortfin mako Isurus oxyrinchus and the blacktip shark Carcharhinus limbatus. The shortfin mako shark has shorter scales than the blacktip shark. The majority of the shortfin mako shark scales have three longitudinal riblets with narrow spacing and shallow grooves. In comparison, the blacktip shark scales have five to seven longitudinal riblets with wider spacing and deeper grooves. Manual manipulation of the scales at 16 regions on the body and fins revealed a range of scale flexibility, from regions of nonerectable scales such as on the leading edge of the fins to highly erectable scales along the flank of the shortfin mako shark body. The flank scales of the shortfin mako shark can be erected to a greater angle than the flank scales of the blacktip shark. The shortfin mako shark has a region of highly flexible scales on the lateral flank that can be erected to at least 50°. The scales of the two species are anchored in the stratum laxum of the dermis. The attachment fibers of the scales in both species appear to be almost exclusively collagen, with elastin fibers visible in the stratum laxum of both species. The most erectable scales of the shortfin mako shark have long crowns and relatively short bases that are wider than long. The combination of a long crown length to short base length facilitates pivoting of the scales. Erection of flank scales and resulting drag reduction is hypothesized to be passively driven by localized flow patterns over the skin. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.

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In this study we investigated bite force and functional morphology of the feeding mechanism of the great barracuda Sphyraena barracuda through ontogeny. Theoretical estimates of bite force at two bite points were calculated for a size series of barracuda ranging from 18 to 130 cm TL (n = 27) using a threedimensional static equilibrium model. In addition, electromyography was used to determine the role of the adductor mandibulae subdivisions (A1, A2, A3) in jaw closure. All recorded subdivisions were active during jaw adduction, although onset times and activity durations differed among them. Bite force ranged from 1 to 93 N at the most anterior bite point, and from 3 to 258 N at the most posterior bite point. Mechanical advantage, in lever and posterior out lever, as well as the cross-sectional area of the majority of the adductor mandibulae subdivisions scaled with isometry; consequently bite force at both bite points also scaled with isometry. Bite force in S. barracuda increased in proportion to total length during ontogeny, which may be associated with a piscivorous diet throughout its life. When compared to other fishes, values of bite force in S. barracuda are among the lowest relative to its body size.

Perhaps the most striking feature of billfishes is the extreme elongation of the premaxillary bones forming their rostra. Surprisingly, the exact role of this structure in feeding is still controversial. The goal of this study is to investigate the use of the rostrum from a functional, biomechanical and morphological standpoint to ultimately infer its possible role during feeding. Using beam theory, experimental and theoretical loading tests were performed on the rostra from two morphologically different billfish, the blue marlin (Makaira nigricans) and the swordfish (Xiphias gladius). Two loading regimes were applied (dorsoventral and lateral) to simulate possible striking behaviors. Histological samples and material properties of the rostra were obtained along their lengths to further characterize structure and mechanical performance. Intraspecific results show similar stress distributions for most regions of the rostra, suggesting that this structure may be designed to withstand continuous loadings with no particular region of stress concentration. Although material stiffness increased distally, flexural stiffness increased proximally owing to higher second moment of area. The blue marlin rostrum was stiffer and resisted considerably higher loads for both loading planes compared with that of the swordfish. However, when a continuous load along the rostrum was considered, simulating the rostrum swinging through the water, swordfish exhibited lower stress and drag during lateral loading. Our combined results suggest that the swordfish rostrum is suited for lateral swiping to incapacitate their prey, whereas the blue marlin rostrum is better suited to strike prey from a wider variety of directions.

A B S T R A C TDrag reduction by marine organisms has undergone millions of years of natural selection, and from these organisms biomimetic studies can derive new technologies. The shortfin mako (Isurus oxyrinchus), considered to be one of the fastest and most agile marine predators, is known to have highly flexible scales on certain locations of its body. This scale flexibility is theorized to provide a passive, flow-actuated mechanism for controlling flow separation and thereby decreasing drag. Recent biological observations have found that the shortfin mako has highly flexible scales, bristling to angles in excess of 50°, particularly on the sides of the body downstream of the gills. High "contragility," which is explicitly defined here as the ability to change or move in a new or opposing direction while already in a turn, would occur if form drag were minimized. This would thus indicate the potential control of flow separation on body regions aft of the point of maximum girth or in regions of adverse pressure gradient. Thus results are consistent with the hypothesis that scale bristling controls flow separation. This scale flexibility appears to be a result of a reduction in the relative size of the base of the scales as well as a reorganization of the base shape as evidenced by histological examination of the skin and scales. Probable mechanisms leading to separation control are discussed.

Shark scales on fast-swimming sharks have been shown to be movable to angles in excess of 50°, and we hypothesize that this characteristic gives this shark skin a preferred flow direction. During the onset of separation, flow reversal is initiated close to the surface. However, the movable scales would be actuated by the reversed flow thereby causing a greater resistance to any further flow reversal and this mechanism would disrupt the process leading to eventual flow separation. Here we report for the first time experimental evidence of the separation control capability of real shark skin through water tunnel testing. Using skin samples from a shortfin mako Isurus oxyrinchus, we tested a pectoral fin and flank skin attached to a NACA 4412 hydrofoil and separation control was observed in the presence of movable shark scales under certain conditions in both cases. We hypothesize that the scales provide a passive, flow-actuated mechanism acting as a dynamic micro-roughness to control flow separation.