Sharks are ancient and diverse marine animals that have successfully navigated the world’s oceans for over 400 million years. This long evolutionary history has resulted in a wide array of species, ranging from the massive, filter-feeding whale shark to the small, deep-dwelling dogfish. Scientists employ a system of classification that groups similar species based on shared characteristics to manage this biological diversity. This process places every shark species into a specific biological category, the most distinguishing of which is the Order.
The Taxonomic Position of Sharks
Understanding the Order of sharks requires first establishing their place in the broader biological hierarchy. Sharks belong to the Class Chondrichthyes, a grouping that includes all fish whose skeletons are primarily composed of cartilage rather than bone. This lighter, more flexible material is reinforced with tiny calcium salts, offering both strength and agility in the water.
Within Chondrichthyes, sharks are placed in the Subclass Elasmobranchii, a group that also contains skates and rays. Elasmobranchs are distinguished by having five to seven pairs of gill slits that open individually to the exterior, unlike the single gill cover found in most bony fish. They also lack the buoyancy-regulating swim bladder and instead rely on a large, oil-filled liver and movement to maintain depth.
The Eight Extant Orders of Sharks
Modern science recognizes eight extant, or living, Orders of sharks, each defined by distinct anatomical features. The largest and most diverse group is Carcharhiniformes, commonly known as the ground sharks. This order includes well-known species such as the Tiger shark and the Great Hammerhead, characterized by two dorsal fins, five gill slits, and the presence of a nictitating membrane that protects the eye.
Another significant group is Lamniformes, the mackerel sharks, which are often large, fast-swimming predators like the Great White shark and the Shortfin Mako. These sharks share the ground sharks’ two dorsal fins and five gill slits but notably lack the protective nictitating membrane. The Orectolobiformes, or carpet sharks, are bottom-dwellers, exemplified by the Nurse shark and the filter-feeding Whale shark.
The Hexanchiformes, or cow and frilled sharks, represent the most primitive living order, distinguished by having six or seven pairs of gill slits. The Heterodontiformes, known as bullhead or horn sharks, are a small order of bottom-dwellers, with the Port Jackson shark being a common example. These sharks possess an anal fin and a spine on each of their two dorsal fins.
The remaining three orders lack an anal fin. These include the Squaliformes, or dogfish sharks, a large group of small, robust species like the Greenland shark. The Squatiniformes, or angel sharks, have flattened bodies and broad pectoral fins, giving them a ray-like appearance. Finally, the Pristiophoriformes, or sawsharks, are instantly recognizable by their long, toothed snout, which they use to slash and stun prey.
Physical Traits Defining the Orders
One of the primary differentiating traits used for classification is the presence or absence of an anal fin, a small fin located on the ventral side between the pelvic and caudal fins. This feature divides the eight orders into two distinct supergroups. Orders like the dogfish and angel sharks lack the fin, while ground sharks and mackerel sharks possess it.
The number of gill slits also serves as a clear marker for classification; while most sharks have five, the Hexanchiformes are uniquely defined by having six or seven. The dorsal fins provide another set of distinguishing features. For example, bullhead sharks (Heterodontiformes) are the only order to feature a sharp spine at the leading edge of both dorsal fins, a characteristic absent in ground sharks (Carcharhiniformes).
Scientists also rely on the presence of spiracles, small openings behind the eyes that allow some sharks to draw in oxygenated water while resting on the seabed. The mouth structure and position, such as whether the mouth is located at the very tip of the snout or set back behind the eyes, are other diagnostic tools. By methodically examining these specific morphological details, researchers can accurately place any shark species into its correct Order.