Mackerels, tunas, and billfishes (suborder Scombroidei and Teleostei) provide an ideal taxonomic context in which to examine the evolution of endothermy. Multiple origins and diverse strategies for endothermy exist among these fish. Here a molecular phylogeny of the Scombroidei has been determined by direct sequencing of a portion of the mitochondrial cytochrome b gene. The distribution of endothermic species within this proposed genealogy indicates that the ability to warm the brain and retina arose independently in three lineages, each time in association with a movement into colder water. This suggests that the evolution of cranial endothermy in fish was selected in order to permit thermal niche expansion and not selected for increased aerobic capacity.

https://science.sciencemag.org/content/sci/260/5105/210.full.pdf

Evolution of endothermy in fish : mapping physiological traits on a molecular phylogeny

Protective materials and structures found in natural organisms may inspire new armors with improved resistance to penetration, flexibility, light weight, and other interesting properties such as transparency and breathability. All these attributes can be found in teleost fish scales, which are the most common types of scales in modern fish species. In this work, we have studied the structure and mechanics of fish scales from striped bass (Morone saxatilis). This scale is about 200–300 µm thick and consists of a hard outer bony layer supported by a softer cross-ply of collagen fibrils. Perforation tests with a sharp needle indicated that a single fish scale provides a high resistance to penetration which is superior to polystyrene and polycarbonate, two engineering polymers that are typically used for light transparent packaging or protective equipment. Under puncture, the scale undergoes a sequence of two distinct failure events: First, the outer bony layer cracks following a well defined cross-like pattern which generates four “flaps” of bony material. The deflection of the flaps by the needle is resisted by the collagen layer, which in biaxial tension acts as a retaining membrane. Remarkably this second stage of the penetration process is highly stable, so that an additional 50% penetration force is required to eventually puncture the collagen layer. The combination of a hard layer that can fail in a controlled fashion with a soft and extensible backing layer is the key to the resistance to penetration of individual scales.

/pdf/structure-and-mechanical-performance-of-a-modern-fish-scale-12s8802ewf.pdf

Structure and Mechanical Performance of a “Modern” Fish Scale

The functions of fish skin during swimming remain enigmatic. Does skin stiffen the body and alter the propagation of the axial undulatory wave? To address this question, we measured the skin's in situ flexural stiffness and in vivo mechanical role in the longnose gar Lepisosteus osseus. To measure flexural stiffness, dead gar were gripped and bent in a device that measured applied bending moment (N m) and the resulting midline curvature (m-1). From these values, the flexural stiffness of the body (EI in N m2) was calculated before and after sequential alterations of skin structure. Cutting of the dermis between two caudal scale rows significantly reduced the flexural stiffness of the body and increased the neutral zone of curvature, a region of bending without detectable stiffness. Neither bending property was significantly altered by the removal of a caudal scale row. These alterations in skin structure were also made in live gar and the kinematics of steady swimming was measured before and after each treatment. Cutting of the dermis between two caudal scale rows, performed under anesthesia, changed the swimming kinematics of the fish: tailbeat frequency (Hz) and propulsive wave speed (body lengths per second, L s-1) decreased, while the depth (in L) of the trailing edge of the tail increased. The decreases in tailbeat frequency and wave speed are consistent with predictions of the theory of forced, harmonic vibrations; wave speed, if equated with resonance frequency, is proportional to the square root of a structure's stiffness. While it did not significantly reduce the body's flexural stiffness, surgical removal of a caudal scale row resulted in increased tailbeat amplitude and the relative total hydrodynamic power. In an attempt to understand the specific function of the scale row, we propose a model in which a scale row resists medio-lateral force applied by a single myomere, thus functioning to enhance mechanical advantage for bending. Finally, surgical removal of a precaudal scale row did not significantly alter any of the kinematic variables. This lack of effect is associated with a lower midline curvature of the precaudal region during swimming compared with that of the caudal region. Overall, these results demonstrate a causal relationship between skin, the passive flexural stiffness it imparts to the body and the influence of body stiffness on the undulatory wave speed and cycle frequency at which gar choose to swim.

Functions of fish skin: flexural stiffness and steady swimming of longnose gar, Lepisosteus osseus

The skin of the bat wing in functionally unique among mammals: it serves as a major locomotor organ in addition to its protective and regulatory functions. We used tensile testing to investigate the mechanical capabilities of wing membrane skin, and compared stiffness, strength, load at failure, and energy absorption among specific wing regions and among a variety of bat taxa. We related these characteristics to the highly architectural fibrous supporting network of the wing membrane. We found that all material properties showed a strong anisotropy. In particular, wing membrane skin shows maximum stiffness and stregth parallel to the wing skeleton, and greatest extensibility parallel to the wing's trailing edge. We also found significant variation among wing regions. The uropatagium (tail membrane) supported the greatest load at failure, and the plagiopatagium (proximal wing membrane between laterl body wall and hand skeleton) is the weakest and most extensible part of the wing. We believe that the increased load bearing ability of the uropatagium relats to its key role in capture of insect prey, and that the great extensibility of the plagiopatagium promotes development of camber near the wing's centre of lift. In interspecific comparisons, energy absorpion and load to failure were greatest in Artibeus jamaicensis, the largest bat in our sample and the species with the highest wing loading, suggesting that wing loading may play a role in dictating the fuctional design of wing membranes.

Mechanical properties of bat wing membrane skin

Inspired by the overlapping scales found on teleost fish, a new composite architecture explores the mechanics of materials to accommodate both flexibility and protection. These biological structures consist of overlapping mineralized plates embedded in a compliant tissue to form a natural flexible armor which protects underlying soft tissue and vital organs. Here, the functional performance of such armors is investigated, in which the composition, spatial arrangement, and morphometry of the scales provide locally tailored functionality. Fabricated macroscale prototypes and finite element based micromechanical models are employed to measure mechanical response to blunt and penetrating indentation loading. Deformation mechanisms of scale bending, scale rotation, tissue shear, and tissue constraint were found to govern the ability of the composite to protect the underlying substrate. These deformation mechanisms, the resistance to deformation, and the resulting work of deformation can all be tailored by structural parameters including architectural arrangement (angle of the scales, degree of scale overlap), composition (volume fraction of the scales), morphometry (aspect ratio of the scales), and material properties (tissue modulus and scale modulus). In addition, this network of armor serves to distribute the load of a predatory attack over a large area to mitigate stress concentrations. Mechanical characterization of such layered, segmented structures is fundamental to developing design principles for engineered protective systems and composites.

http://web.mit.edu/cortiz/www/Browning2013.pdf

Mechanics of composite elasmoid fish scale assemblies and their bioinspired analogues

The skin of the Norfolk spot, Leiostomus xanthurus, is composed largely of a crossed-helical array of collagen fibers. Over most of the body of the fish these are oriented at angles of 45-80° with the long axis of the fish. The skin of the skipjack tuna, Katsuwonus pelamis, also contains a crossed-helical array of collagen fibers, although fewer fiber layers are present and fiber angles are generally in the range of 55-75°. Uniaxial stress-strain tests indicate that for both species skin is most extensible in the longitudinal direction. For the Norfolk spot, skin is stiffer in the direction of the fibers than in the circumferential direction, but for the skipjack tuna, the skin is of about the same stiffness in the circumferential direction as it is in the direction of the fibers. Biaxial stressing tests demonstrate that the skins of the spot and the skipjack do not behave as simple crossed-fiber systems, and are therefore incapable of transmitting forces down the lengths of these fishes or acting as "ext...

https://www.jstor.org/stable/info/1541920

The mechanics of fish skin: lack of an "external tendon" role in two teleosts

Over most of its length, the backbone of the blue marlin, like that of all other istiophorids, contains enlarged and flattened neural and hemal spines and zygapophyses, all of which span the intervertebral joints. These plates of bone restrict dorso-ventral bending of the backbone but their arrangement permits a high degree of lateral flexion. The spinesand zygapophyses also appear to be important in stabilizing the relatively large intervertebral joints against axial compression and lateral shearing during bending. Although bone is an elastic material, these overlapping structures are not arranged so as to contribute to elastic recoil of the backbone during normal swimming movements.

Backbone Mechanics of the Blue Marlin Makaira Nigricans (Pisces, Istiophoridae)

The skin of the Norfolk spot, Leiostomus xanthurus, is composed largely of a crossed-helical array of collagen fibers. Over most of the body of the fish these are oriented at angles of 45â€”80Â° with the long axis of the fish. The skin of the skipjack tuna, Katsuwonuspelamis, also contains a crossed-helical array of collagen fibers, although fewer fiber layers are present and fiber angles are generally in the range of 55@750@ Uniaxial stress-strain tests indicate that for both species skin is most extensible in the longitudinal direction. For the Norfolk spot, skin is stifferin the direction of the fibersthan in the circumferentialdirection, but for the skipjacktuna, the skin is of about the same stiffnessin the circumferentialdirection as it is in the direction of the fibers.Biaxialstressingtests demonstratethat the skins ofthe spot and the skipjack do not behave as simple crossed-fibersystems, and are thereforeincapable of trans mitting forces down the lengths of these fishes or acting as â€œ?�externalâ€ tendons.

John H. Hebrank

Papers

The mechanics of fish skin: lack of an "external tendon" role in two teleosts

Backbone Mechanics of the Blue Marlin Makaira Nigricans (Pisces, Istiophoridae)

â€œ¿EXTERNAL TENDONâ€ ROLE IN TWO TELEOSTS