Sharks are a captivating part of marine life. A common question arises: are they mammals or fish? The answer is clear: sharks are definitively fish. While they possess some characteristics that might lead to confusion, their fundamental biological makeup places them firmly within the class of fish.
Defining Fish: Key Characteristics
Fish are aquatic vertebrates. A defining trait of most fish is being cold-blooded, or ectothermic, meaning their body temperature is largely influenced by their surrounding environment. Sharks, like most fish, are ectothermic, with their internal temperature matching that of the water around them.
Respiration in fish occurs through gills, specialized organs that extract oxygen from water. Sharks utilize gills to absorb dissolved oxygen as water passes over them. Fish also rely on fins for movement, stability, and steering. Sharks possess multiple fins for propulsion and navigation.
While most fish lay eggs, some, like sharks, can give birth to live young. The skin of most fish is covered in scales, and while shark scales are unique, they still possess a protective outer covering.
Defining Mammals: Key Characteristics
Mammals are vertebrates distinguished by several shared features that sharks do not possess. Unlike fish, mammals are warm-blooded, or endothermic, meaning they maintain a consistent internal body temperature regardless of external conditions. Mammals also breathe air using lungs.
Mammals have hair or fur at some point in their lives. They nourish their young with milk produced by mammary glands, and they typically give birth to live young. Mammals possess a four-chambered heart. Sharks lack these mammalian characteristics.
Shark Anatomy: Unique Adaptations
Sharks exhibit several unique anatomical features that differentiate them from most bony fish. Unlike most fish whose skeletons are made of bone, sharks and other cartilaginous fish have skeletons composed primarily of cartilage. This material is lighter and more flexible than bone, contributing to their agility and energy efficiency in water.
Sharks also possess multiple gill slits, typically five to seven pairs, directly exposed along the side of their head, rather than a single operculum (gill cover) found in most bony fish. Their skin is covered in placoid scales, also known as dermal denticles, which are tooth-like structures that give their skin a rough, sandpaper-like texture. These denticles reduce drag and aid in efficient movement through water.
Most bony fish use a gas-filled swim bladder for buoyancy control, but sharks lack this organ. Instead, sharks rely on a large, oil-rich liver containing squalene, which is less dense than water, and the dynamic lift generated by their fins to maintain their position in the water column. While many fish release eggs externally, sharks display a range of reproductive strategies, including laying eggs in tough cases (oviparity), internal hatching followed by live birth (ovoviviparity), and true live birth (viviparity). Despite some sharks giving live birth, they do not nurse their young with milk, further confirming their classification as fish.