Color change in the animal kingdom is often associated with creatures like chameleons or octopuses, which can shift their hue almost instantly. The concept of a spider doing the same might seem like science fiction, yet certain arachnid species possess a remarkable, albeit slower, ability to adapt their body color. While most of the 50,000 known spider species have fixed coloration, a select few can reversibly change their appearance to match their surroundings. This adaptation is a form of camouflage that is distinct from the rapid, nervous-system-controlled color shifts seen in other animals. The process involves a physiological mechanism that allows them to blend seamlessly into their environment.
Which Spiders Display Color Shifting
The ability to reversibly change color is most notably found within the family Thomisidae, commonly known as crab spiders. These spiders, which include species like the goldenrod crab spider (Misumena vatia), are ambush predators that do not spin webs to catch prey. Instead, they rely on stealth and camouflage while waiting on flowers or foliage.
The color transition typically occurs between white and yellow, allowing the spider to match the color of the flowers it uses as a hunting ground. For instance, a crab spider on a white daisy can turn yellow when it moves to a goldenrod flower. The females of these species are the ones that exhibit this color plasticity; the smaller males usually retain a fixed coloration.
This change is not an immediate event but a more gradual process known as morphological color change. Shifting from a white to a yellow hue can take several days, with some studies reporting a change period of approximately three to nine days. The reverse change, from yellow back to white, is often faster, sometimes taking less than a week.
The Biological Mechanism of Color Change
The color change in crab spiders is not driven by the movement of pigment within specialized cells. Instead, it is a metabolic process involving the synthesis and degradation of specific pigment compounds. The yellow color is primarily due to the production and storage of a yellow pigment, often a precursor of ommochromes, such as 3-OH-kynurenine.
When the spider is on a yellow flower, it begins to produce and secrete this yellow pigment into the outer cell layers of its body, specifically beneath the transparent outer cuticle. This pigment accumulates, gradually shifting the spider’s appearance from white to yellow. The white color is the default state, which is achieved when the yellow pigment is absent or resorbed.
To change back to white, the spider must break down and excrete the stored yellow pigment, which is why the process is not instantaneous. This degradation process takes time, as the spider’s cells actively resorb the pigment and then eliminate it from the body. Recent studies have classified the pigment-storing organelles in these spiders as lysosome-related organelles, which are cellular compartments capable of degrading their contents.
This mechanism is different from the physiological color change seen in some orb-weaving spiders, which can rapidly alter their appearance by moving guanine crystals within sub-dermal cells. The crab spider’s method requires energy and time for chemical synthesis and breakdown, linking the color change to its metabolic state.
The Function of Color Adaptation
The ability to change color serves a primary function of crypsis, which is camouflage that helps the spider blend into its background. By perfectly matching the hue of the flower, the crab spider becomes virtually invisible to both its prey and its own predators. This strategy allows the spider to be an effective ambush predator.
The spider positions itself on a flower and waits motionless for pollinating insects, such as bees or butterflies, to visit. Its camouflage prevents the prey from detecting the threat until the insect is close enough to be captured. This high degree of concealment increases the spider’s hunting success, allowing it to capture prey significantly larger than itself.
Beyond ambush predation, the camouflage may also offer protection from visual predators like birds, although this function is less frequently observed in the field. The reversible color change ensures the spider can maintain its cryptic advantage even as it moves between different colored flowers throughout its lifetime. The adaptation is an example of how a slower, metabolically controlled color shift can be effective for survival in a dynamic environment.