Sharks may invoke a sense of dread as they stealthily glide through the water. Their dorsal fin pokes up through the water, showing a skin-like texture.
While sharks may appear as slicked-skinned creatures they are not.
Sharks have placoid scales, also known as dermal denticles. These scales are much like human teeth with pulp and strong enamel-like coated surfaces. If rubbed against the grain, they can cause lacerations. Sharks use their scales as armor, to move quickly, reduce drag, move with stealth, and for painful attacks.
This article discusses how sharks are classified as fish, and how scales help the shark to function and survive in the water.
Topics
Shark Family Characteristics
Sharks belong to the Chondrichthyes class and subclass family of Elasmobranchii.
Sharks are a type of fish that have skeletons made out of cartilage, and not bones.
Cartilaginous fish such as the shark have a softer, flexible, and light skeletal frame for buoyancy and ease of movement in the water.
Sharks are considered fish, and have the following common characteristics:
- Live in water (aquatic)
- Have gills throughout their lifespan to filter oxygen (respiration)
- Cold-blooded (ectothermic)
- Vertebrate (cartilage)
- Head, trunk, tail (no neck)
- Has scales (dermal denticles)
- Paired and unpaired fins (Most shark species have 8 fins: 2 dorsal, 2 pectoral, 2 pelvic, 1 anal, and 1 caudal)
- 2 nostrils (nares)
- Lateral line sense organs (sensitive to water movements and pressure)
- 10 pairs of cranial nerves (responsible for functions such as vision, taste, olfaction, balance, etc.)
- Mesonephric kidneys (excretory organ)
Other Shark Characteristics
Many fish have a swim bladder for buoyancy, however, sharks do not. To stay afloat, sharks have an oily liver, which has a lower density than water, in conjunction with a lighter cartilaginous frame.
Male sharks and other male elasmobranchs (such as skates and rays) have claspers, which is for inseminating a female.
Around 70% of sharks are viviparous, giving birth to live young. The others are oviparous, laying eggs externally.
Shark Scales
Many creatures have scales, both on land and in the water. They can be fish, arthropods, birds, reptiles, and even mammals.
Sharks are fish, and like other fish, they have scales. Under a microscope, these scales have pointed and bumpy, or sharp v-shaped scales pointing towards the tail.
If touching the outer layer of a shark, it would feel smooth if moving from head to tail. However if touched in the opposite direction, it will feel like sandpaper, possibly causing lacerations.
Dermal Denticles: Placoid Scales
Dermal denticles, also called placoid scales, are found on sharks, rays, and skates. These overlapping, spiny, toothlike scales are found only on aquatic cartilaginous creatures.
This type of scale grows out of the shark’s dermis (skin). The scales are embedded in the skin at the base with a spiny cusp.
Dermal denticles do not grow bigger as the shark grows. Instead, new scales form in between the older ones. Sharks can shed and regenerate scales, but again, they remain consistent in size.
Scale Composition
Placoid scales resemble the composition of human teeth.
Their inner cavity has a pulp with blood vessels, nerves, and connective tissues. The outside of the scale is covered with dentine enamel, called vitrodentine.
Why Sharks Have Scales: 4 Reasons
Sharks’ scales can vary in size depending on the location and species.
Smaller and thinner scales help with speed and drag reduction around the eyes and fins, for example. Larger, thicker ones aid in overall durable body protection.
Scales also aid the shark in gliding in water undetected. A shark can bump against a threat or prey and cause lacerations with its sharp scales.
1. Protection: Scales And Skin
Shark scales offer hard outer protection from other sharks, killer whales, and aquatic ectoparasites (parasites that live on the outside of an animal).
Scales protect the shark from scrapes or other external injuries.
These scales are connected to the shark’s skin, which helps it to retain heat and support muscles.
Females tend to have thicker skin, up to twice as thick as males for protection during mating.
This thick skin offers protection since the male will bite her to hold on during mating. Species vary in thickness, but a whale shark, for example, has 4-inch thick skin.
2. Speed And Drag Reduction
The scales of a shark can bristle up to angles of 60 degrees.
This movement reduces the amount of drag as they navigate the waters, allowing them to move swiftly. A study found that the average drag reduction was 6.91%, with top reductions as high as 8.25%.
Paired with a shark’s flexible skeletal structure, it can swim fast.
Sharks have been found to cruise at only 1.5 mph when looking for prey. Some species, such as the Mako Shark can swim in short bursts over 60 miles per hour (mph).
3. Stealth And Detection
Not only do the scales help sharks glide through the water, reducing drag, but they allow the shark to sense movement in the water.
As apex predators, this function can give a shark an advantage when trying to stay out of sight as it hunts for prey.
A study shows that sharks are aware of humans in the water, and can swim out of a person’s field of vision.
4. Attacks On Threats And Prey
While sharks may be feared for their powerful bite, their scales can cause painful lacerations too. If rough or forceful contact is made with a shark’s scales, moving against the grain, it can cut.
Sharks will bump or brush against a victim with a close pass of their body or a hit with their tail, inflicting lacerations and abrasions.
Sharks may bump their prey before attacking to assess its potential danger as fighting prey, or to incapacitate the prey first. Marine water has bacteria such as Vibrio spp. and Aeromonas spp., which can cause infection in open wounds.
In Conclusion
Shark scales offer benefits to the shark’s survival. Referred to as placoid scales, or dermal denticles, these scales are sharp tooth-like structures with a pulp and outer enamel-like layer.
Scales surround the shark’s body as an outer layer of protection. They can bustle in the water to allow for quick and stealth movements. Shark scales are also sharp and can bump prey or threats to cause lacerations.
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