The ability of some animals to move across the surface of water is a remarkable example of specialized biological design exploiting fundamental physical laws. Rather than floating in the water, these creatures are supported by the water, using its unique properties to remain suspended at the air-liquid boundary. The mechanics of this movement vary dramatically depending on the animal’s size, mass, and speed, distinguishing between those that rely on static support and those that employ dynamic forces.
The Role of Water Surface Tension
Water molecules possess a strong mutual attraction due to hydrogen bonding, a phenomenon known as cohesion. At the surface of a body of water, these molecules are pulled inward and sideways by their neighbors, but not upward by air molecules. This inward and lateral pull creates a net tension. This tension causes the water’s surface to behave like a taut, elastic film or a flexible membrane. The force created by this tension is capable of supporting objects far denser than water itself.
For an object to remain on the surface, the downward force it exerts must be less than the upward force provided by surface tension. This upward force is maximized when the object resists wetting, meaning its surface is hydrophobic. When a lightweight object rests on the water, it creates a small indentation, and the surrounding surface tension forces resist this deformation, pushing the object back up.
Invertebrate Strategies for Surface Travel
Smaller organisms, such as the common water strider (Gerridae), rely almost entirely on surface tension for support. Their long, slender legs are designed to distribute their minimal weight over a wide area, reducing the pressure exerted at any single point. This distribution ensures the water’s elastic film is only depressed into dimples, rather than broken through.
The secret to their success lies in thousands of microscopic hairs, called microsetae, that densely cover their legs. These minute structures trap a layer of air, making the legs highly hydrophobic and preventing the water from adhering to them. This repulsion maximizes the upward force generated by surface tension around the indentation. Water striders use their middle pair of legs like oars, pushing against the surface film to propel themselves forward without ever penetrating the liquid below.
The fishing spider (Dolomedes) also uses its legs to stand and move on the water. Like the water strider, the spider’s legs are covered in non-wetting hairs, allowing it to leverage surface tension. The low mass-to-surface area ratio inherent to these invertebrates means the forces of surface tension are dominant over gravity. This static support mechanism allows them to rest motionless on the water as they wait for prey.
Vertebrate Adaptations for Aquatic Locomotion
For larger, heavier animals, such as the basilisk lizard (Basiliscus species), surface tension alone is insufficient to support their body mass. These vertebrates must employ a dynamic mechanism that generates a temporary upward force through sheer speed and momentum. Often called the “Jesus Christ lizard,” the basilisk runs bipedally across the water, reaching speeds of up to 1.6 meters per second.
The lizard’s movement is a rapid, three-part process: “slap, stroke, and recovery.” The lizard first slams its foot flat against the water with high velocity, a motion that accounts for a small portion of the upward support. The subsequent downward and backward stroke is the most significant phase, where the foot pushes water away forcefully.
The basilisk’s feet are specialized with lateral fringes that fan out, dramatically increasing the surface area during the stroke. This rapid, forceful motion creates a temporary air cavity—a pocket of air—around the foot that prevents the water from rushing in immediately. This action generates powerful hydrodynamic lift, which is the necessary force to keep the lizard above the surface. The final recovery involves pulling the foot straight up and out before the air cavity collapses, minimizing drag before the next step begins.