The animal kingdom is broadly divided into two major groups: vertebrates and invertebrates. This classification is based on internal structure, specifically the presence or absence of a backbone. Understanding where the shark fits requires looking closely at the anatomical criteria that define these two divisions of life.
The Definitive Answer: Sharks are Vertebrates
Sharks are classified as vertebrates, placing them in the subphylum Vertebrata alongside mammals, birds, reptiles, amphibians, and other fish. This classification is due to the presence of a vertebral column, or backbone. This segmented internal structure provides central support for the body plan. The vertebral column also functions to protect the dorsal hollow nerve cord, which develops into the brain and spinal cord.
What Defines a Vertebrate
The subphylum Vertebrata is defined by shared characteristics, starting with the notochord. The notochord is a flexible rod that provides skeletal support, at least during some stage of development. In most adult vertebrates, including sharks, this notochord is replaced by a segmented vertebral column composed of individual bony or cartilaginous elements called vertebrae. This internal skeleton, or endoskeleton, provides robust support for locomotion and protects the central nervous system.
Vertebrates also feature a distinct cranium, or skull, which encases and protects a large brain. They utilize a closed circulatory system driven by a muscular, chambered heart, which efficiently pumps blood through a network of vessels. This combination of a protective internal skeleton and a centralized nervous system distinguishes vertebrates from all other animal groups.
Sharks: The Cartilaginous Vertebrates
Sharks belong to the class Chondrichthyes, derived from the Greek words for “cartilage” and “fish.” Their skeleton is composed almost entirely of cartilage, not bone, contrasting with Osteichthyes, or bony fish. However, this does not disqualify the shark from being a vertebrate.
The cartilaginous skeleton fulfills the requirement of an internal, segmented support structure, as the spine is made of cartilaginous vertebrae. This material is lighter and more flexible than bone, aiding the shark in buoyancy and maneuverability. Cartilage is often reinforced by calcium salts, providing necessary strength without true bone tissue.
Distinguishing Features of Invertebrates
The vast majority of animal species, roughly 97%, are invertebrates. The defining characteristic of this group is the complete absence of a vertebral column or backbone. Lacking internal spinal support, invertebrates utilize diverse alternative methods for structural integrity and shape.
Many invertebrates, such as insects and crustaceans, rely on a rigid external covering called an exoskeleton for protection and support. Other groups, including worms and jellyfish, employ a hydrostatic skeleton, which uses the pressure of fluid contained within the body to maintain form and facilitate movement. Common examples of invertebrates include squid, jellyfish, insects, and snails.