Turtles are an order of reptiles distinguished by a bony shell that encases their body. The question of whether these animals live underwater is complex because the term “turtle” encompasses a vast and diverse group of species that have adapted to nearly every environment except the polar regions. While some species are almost entirely aquatic, others are strictly terrestrial. The differences in their lifestyles are reflected in their anatomy and behavior, making it necessary to examine each group separately.
Categorizing Turtles by Habitat
The approximately 360 living species of turtles can be broadly divided into three main ecological groups based on the environment they inhabit. Marine turtles are the most specialized for aquatic life, spending over 90% of their lives in the open ocean. These animals, such as the Loggerhead and Green Sea Turtle, rarely approach land, except when females return to nesting beaches.
A second, highly varied group is the freshwater or semi-aquatic turtles, which live in lakes, rivers, swamps, and ponds. Species like the Red-eared Slider are proficient swimmers but regularly leave the water to rest or warm themselves in the sun. The Diamondback Terrapin thrives in brackish coastal estuaries, tolerating a mix of fresh and saltwater environments.
The final group is the tortoises, which are exclusively terrestrial and inhabit deserts, grasslands, and forests. Tortoises, including the Galápagos Tortoise, possess heavy, dome-shaped shells and sturdy, elephant-like legs for walking on land. These animals cannot swim effectively and would quickly drown if placed in deep water.
Specialized Features for Underwater Living
Aquatic species possess specific anatomical features that allow them to function efficiently in a water-based environment. Sea turtles have evolved forelimbs into large, powerful flippers that propel them through the water with great hydrodynamic efficiency. In contrast, freshwater turtles utilize webbed feet and claws, which help them navigate both the water and the muddy banks of their habitats.
Their shells are also adapted for water, typically being flatter and more streamlined than the high-domed shells of their terrestrial relatives, which reduces drag. These reptiles have developed respiratory adaptations for extended dives. Sea turtles possess an efficient lung capacity that allows them to exchange a high percentage of gases with each breath, enabling them to hold their breath for hours during resting periods.
Some aquatic species have evolved a supplementary method of absorbing oxygen directly from the water, known as aquatic respiration. This adaptation is most pronounced in certain freshwater turtles, which absorb dissolved oxygen through vascularized tissues in the throat or around the cloaca. This ability is important when metabolism slows during periods of cold dormancy, allowing them to remain submerged for months.
When Aquatic Turtles Must Leave the Water (Or Surface)
Despite their adaptations for underwater life, all turtles are reptiles and must interact with the air and land to complete fundamental life processes. The most basic requirement is breathing, as they all possess lungs and must periodically surface to take in atmospheric oxygen. Even the deepest-diving species must return to the surface for a brief breath of air.
Semi-aquatic turtles also rely on land for thermoregulation, a necessity because they are ectotherms and cannot generate their own body heat. They regularly haul themselves out of the water onto logs or banks to bask in the sun, raising their body temperature to optimize digestion and metabolism.
The reproductive cycle for all turtle species is an entirely terrestrial event. Female turtles, whether marine or freshwater, must leave the water and crawl onto land to dig a nest and deposit their eggs. This universal requirement means no turtle species is fully independent of the land throughout its entire life cycle.