The idea that a shark eventually stops growing is a common misconception, often drawn from the growth patterns observed in mammals. Unlike humans, dogs, or birds, which exhibit determinate growth and stop increasing in size after reaching sexual maturity, sharks follow a different trajectory. Their unique physiology allows them to continue adding mass throughout their entire lifespan.
The Nature of Shark Growth: Indeterminate Growth
Sharks, along with their relatives the rays and chimaeras (Chondrichthyes), display a biological trait known as indeterminate growth. This means they continue to grow in length and mass from birth until death. This continuous growth pattern is also shared by many bony fish and reptiles. While growth never truly stops, the rate at which the shark adds size slows significantly once it reaches sexual maturity.
A large, old shark is a testament to this growth strategy, as its size is directly linked to its age. The indeterminate pattern ensures that the largest individuals in a population are also the oldest, having survived for a long time in their environment. This contrasts sharply with the determinate growth seen in most mammals, where a fully mature adult will maintain a relatively stable maximum size. The size of an ancient shark reflects decades or even centuries of continuous, increasingly slow, development.
Reading the Rings: How Shark Age is Determined
Empirical evidence supporting continuous growth comes from the specialized method scientists use to determine a shark’s age. This is necessary because sharks lack the hard, bony skeletons of other fish; instead, their spine is composed of cartilage. This cartilage is reinforced by calcium salts in a process called calcification, resulting in hardened structures called vertebrae, which form growth bands or rings.
Scientists collect these vertebrae, typically from the cervical region, and prepare a thin cross-section of the centrum, the central spool-shaped part of the vertebra. These rings, which are analogous to the annual growth rings found in a tree trunk, appear as alternating opaque and translucent bands. Counting these pairs of bands allows researchers to estimate the sharkâs age in years. For certain long-lived species, like the Greenland shark, this process is validated using precise methods like bomb radiocarbon dating.
This analysis is complicated because the growth bands in older sharks can become extremely thin and difficult to distinguish, sometimes even fusing together. Furthermore, the deposition of these bands is linked more closely to the individual shark’s growth rate than to a strict annual cycle. For this reason, age estimations in very old specimens are considered approximations, though they still confirm a life history of continuous growth.
Factors Governing Growth Rate and Maximum Size
Although all sharks exhibit indeterminate growth, the speed at which they grow and the maximum size they ultimately achieve are governed by several factors. A primary determinant is the species’ genetics, which sets the potential for size and lifespan (e.g., the difference between a small, fast-growing dogfish and the massive, slow-growing whale shark). Environmental temperature also plays a significant role, as sharks are ectothermic, and their metabolism and growth rate are accelerated in warmer waters.
The availability and quality of prey are also major factors, with areas of high food abundance allowing for faster growth and greater energy storage. Species that can exploit high-calorie food sources, like marine mammals, can reach larger sizes than those restricted to smaller, less energy-dense prey. Sexual dimorphism is a widespread variable, with females of many large species, including the Great White Shark, growing significantly larger than males. This difference is driven by the female’s reproductive demands, requiring a greater body size to carry and nourish developing pups.
Overall size is a function of both growth rate and longevity. Conservation efforts, which reduce fishing pressure and allow sharks to live out their full natural lifespan, are important. By surviving for many decades, these animals continue to add small amounts of mass each year, eventually reaching their species’ impressive maximum size potential.