Sea urchins are known for their spiny appearance, but they often exhibit a curious behavior in shallow waters: covering their bodies with materials from their environment. This action, scientifically termed “covering behavior” or “masking,” is a complex survival strategy used by many species. It involves actively collecting and securing fragments of shell, rock, algae, or other debris onto the dorsal surface of their spherical bodies. This deliberate collection of objects provides distinct biological advantages, allowing the sea urchin to thrive in harsh habitats.
The Physical Mechanism of Object Manipulation
The ability of a sea urchin to move and hold objects relies on specialized appendages located on its body, which is encased in a hard shell called a test. The primary tools for object manipulation are the hundreds of flexible, slender tube feet (podia), which extend through tiny pores in the test. These tube feet operate using a hydraulic water vascular system, allowing them to extend, retract, and firmly adhere to surfaces using suction-cup tips.
To initiate covering, the sea urchin uses its tube feet to grasp and lift available material from the substrate. The tube feet transport the object across the body surface toward the upper (aboral) side. Once positioned, the spines, which are attached by a ball-and-socket joint, help stabilize and secure the collected debris. This concerted action allows the urchin to maintain the cover even while moving slowly across the seafloor.
Adaptive Functions of Covering Behavior
The primary benefit of covering, especially in clear, shallow water, is protection from intense ultraviolet (UV) radiation. Direct sunlight can damage the sea urchin’s sensitive tissues, and the collected opaque materials serve as a physical shield against this harmful radiation. Exposure to UV light can actively stimulate a stronger covering response in some species.
The debris also aids in thermoregulation by buffering the urchin against temperature fluctuations common in shallow intertidal zones. Covering with insulating or light-reflecting materials helps maintain a stable internal body temperature.
Defense and Stability
In areas with strong currents or wave action, the added weight of dense materials acts as ballast. This increased density stabilizes the urchin, preventing it from being swept away or dislodged from the substrate.
Covering also provides camouflage by breaking up the urchin’s outline, making it harder for visual predators like sea otters or certain fish to spot. Furthermore, the cover offers protection against mechanical injury and abrasion from suspended particles.
Environmental Factors and Material Selection
The necessity and intensity of covering behavior are closely tied to the specific environmental conditions of the habitat. In environments with high solar exposure, such as clear, tropical tide pools, covering is more pronounced and involves selecting opaque materials. Conversely, in deeper, darker, or turbid waters, the need for a UV shield is reduced, and the behavior may be less frequent.
The specific materials chosen are determined by what is readily available in the immediate vicinity. Urchins commonly use:
- Small shells
- Pieces of coral rubble
- Rock fragments
- Blades of macroalgae and seagrass
Some species show an active selection preference, choosing heavier, wider, or more opaque items when available. Urchins in coastal areas have also been observed incorporating discarded human debris, such as plastic fragments, into their covering.