Sharks are ancient and diverse marine animals that have inhabited the oceans for hundreds of millions of years. They thrive in nearly every marine environment, from shallow coastal waters to the deep sea. Understanding their scientific classification helps illuminate the variety found within these creatures.
Organizing Life: Understanding Shark Classification
Biological classification, or taxonomy, organizes Earth’s diverse life. This system groups organisms by shared evolutionary history and physical characteristics, making relationships easier to study. An “order” is a classification level grouping related families. Sharks, like all living things, are organized this way, belonging to the class Chondrichthyes, which includes all cartilaginous fish.
This arrangement allows scientists to identify common traits and evolutionary pathways among shark groups. It helps researchers analyze their adaptations, habitats, and ecological roles, aiding comprehension of the vast biodiversity within the shark lineage.
The Nine Orders of Sharks
The world’s shark species are categorized into nine recognized orders, each defined by unique anatomical features.
- Carcharhiniformes, known as ground sharks, represent the largest order, including species like the tiger shark and hammerhead sharks.
- Lamniformes, or mackerel sharks, are often large, fast-swimming predators, such as the great white shark and mako shark.
- Orectolobiformes, called carpet sharks, typically have flattened bodies and often intricate patterns, exemplified by the whale shark and wobbegongs.
- Pristiophoriformes, or sawsharks, are distinctive for their long, saw-like snouts lined with teeth.
- Squaliformes, or dogfish sharks, are a diverse group found globally, including the spiny dogfish and Greenland shark.
- Squatiniformes, commonly known as angel sharks, possess flattened bodies and broad pectoral fins, resembling rays.
- Hexanchiformes, or cow and frilled sharks, are considered some of the most primitive living sharks, characterized by having more than five gill slits.
- Heterodontiformes, known as bullhead sharks, are a small order with prominent head ridges and spines on their dorsal fins.
- Myliobatiformes, while technically rays, are often discussed alongside sharks due to their shared cartilaginous fish ancestry within Elasmobranchii. This order includes stingrays and eagle rays, recognized by their flattened bodies and whip-like tails.
Key Traits Defining Shark Orders
Each shark order possesses distinct physical and behavioral characteristics.
Carcharhiniformes
These ground sharks typically have two dorsal fins, an anal fin, five gill slits, and a nictitating membrane covering their eyes for protection. Many species, like the bull shark, are active predators in warm temperate and tropical waters, often near coastal areas or the ocean floor.
Lamniformes
Mackerel sharks are known for their powerful, spindle-shaped bodies, large mouths, and often pointed snouts. Members like the thresher shark use their elongated caudal fins to stun prey. Some, including the great white shark, exhibit regional endothermy, maintaining a higher body temperature than surrounding water, aiding high-speed pursuits.
Orectolobiformes
Carpet sharks are generally bottom-dwellers with small mouths positioned in front of their eyes, two spineless dorsal fins, and often barbels near their nostrils for sensing prey. The wobbegong uses its flattened, camouflaged body to ambush prey from the seafloor. The enormous whale shark in this order is a filter feeder, consuming plankton and small fish.
Pristiophoriformes
Sawsharks are identified by their elongated, flattened snouts equipped with sharp, tooth-like projections on each side, which they use to slash prey. They also have long barbels on the underside of their snout, aiding sensory detection in deep-water habitats.
Squaliformes
Dogfish sharks are a varied group characterized by two dorsal fins, which may or may not have spines, and a lack of an anal fin. Many species, like the spiny dogfish, inhabit deep-sea environments and some can produce light through bioluminescence.
Squatiniformes
Angel sharks are unique for their extremely flattened bodies, broad pectoral and pelvic fins, and eyes positioned on top of their heads. These sharks bury themselves in sand or mud, acting as ambush predators, and lack an anal fin.
Hexanchiformes
Cow and frilled sharks are distinguished by having six or seven gill slits, unlike most sharks that have five. They possess only a single dorsal fin positioned far back on their bodies. Examples include the frilled shark, with its eel-like body, and the broadnose sevengill shark.
Heterodontiformes
Bullhead sharks have a distinctive bullhead-like snout, often with prominent ridges above their eyes, and a spine preceding each of their two dorsal fins. Their teeth are specialized, with sharp front teeth for grasping and flat, molar-like teeth in the back for crushing hard-shelled prey.
Myliobatiformes
This order, which includes stingrays and eagle rays, is defined by their dorsoventrally flattened bodies, large pectoral fins fused to their heads, and gill slits located on their ventral surface. Most species possess a venomous spine on their whip-like tail for defense.