Fish scale
The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as well as possible hydrodynamic advantages.
The scales comprise a non-growing "crown" composed of dentine, with a sometimes-ornamented enameloid upper surface and an aspidine base.
[6] Beyond that, there appear to be five types of bone growth, which may represent five natural groupings within the thelodonts—or a spectrum ranging between the end members meta- (or ortho-) dentine and mesodentine tissues.
[9] The morphology and histology of thelodonts provides the main tool for quantifying their diversity and distinguishing between species, although ultimately using such convergent traits is prone to errors.
It is a glassy, often multi-layered mineralized tissue that covers the scales, as well as the cranial bones and fin rays in some non-teleost ray-finned fishes,[15] such as gars, bichirs, and coelacanths.
[18] Ganoine is an ancient feature of ray-finned fishes, being found for example on the scales of stem group actinopteryigian Cheirolepis.
Native Americans and people of the Caribbean used the tough ganoid scales of the alligator gar for arrow heads, breastplates, and as shielding to cover plows.
Many teleost fish are covered with highly reflective scales which function as small mirrors and give the appearance of silvered glass.
At medium depths at sea, light comes from above, so a mirror oriented vertically makes animals such as fish invisible from the side.
A further complication for fish with bodies that are rounded in cross-section is that the mirrors would be ineffective if laid flat on the skin, as they would fail to reflect horizontally.
Placoid scales are structurally homologous with vertebrate teeth ("denticle" translates to "small tooth"), having a central pulp cavity supplied with blood vessels, surrounded by a conical layer of dentine, all of which sits on top of a rectangular basal plate that rests on the dermis.
[34] Both riblet shapes assist in creating a turbulent boundary layer forcing the laminar flow farther away from the sharks skin.
[35] Unlike bony fish, sharks have a complicated dermal corset made of flexible collagenous fibers arranged as a helical network surrounding their body.
[36] Depending on the position of these placoid scales on the body, they can be flexible and can be passively erected, allowing them to change their angle of attack.
[39] The crown and the neck of the denticles however play a key role and are responsible for creating the turbulent vortices and eddies found near the skin's surface.
During a recent research experiment biomimetic samples of shark denticles with a crescent like microstructure were tested in a water tank using a traction table as a slide.
The reason for this drag reduction was that the turbulent vortices became trapped between the denticles, creating a ‘cushion like’ barrier against the laminar flow.
The practical shapes were low profile and contained trapezoidal or semi-circular trough-like cross sections, and were less effective but nonetheless reduced drag by 6 or 7%.
[43] Frictional drag is a result of the interaction between the fluid against the shark's skin and can vary depending on how the boundary layer changes against the surface of the fish.
[38] Recent research has shown that there is a pre and post-breakdown regime in the near-wall boundary layer where the sublayer thickens at a declining rate and then abruptly undergoes a breakdown into turbulent vortices before finally collapsing.
Lifting vortices are what push the boundary layer out and away from the surface of the shark which results in reducing the overall drag experienced by the fish.
[44] The rough, sandpaper-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide leather, called shagreen.
Therefore, inexpensive and environmentally safe anti-fouling surfaces are in very high demand to increase the efficiency of shipping, fishing, and naval fleets, among other applications.
Dermal denticles are a promising area of research for this type of application due to the fact that sharks are among the only fish without build up or growth on their scales.
Studies by the U.S. Navy have shown that if a biomimetic material can be engineered, it could potentially lead to fuel cost savings for military vessels of up to 45%.
A large portion of the total drag on long objects with relatively flat sides usually comes from turbulence at the wall, so riblets will have an appreciable effect.
[49] Parametric modeling has been done on shark denticles with a wide range of design variations such as low and high-profile vortex generators.
Out of both the low and high-profile samples tested, the low-profile vortex generators outperformed the current smooth wing structures by 323%.
In the case of zebrafish, it takes 30 days after fertilization before the different layers needed to start forming the scales have differentiated and become organized.
It has been shown that during the process of cell differentiation and interaction, the level of ApoE transcription is high, which has led to the conclusion that this protein is important for the late development of scales.
