Spider crabs (family Majidae) are marine crustaceans characterized by their elongated legs and rounded bodies. A common question regarding their defensive capabilities is whether they bite or pinch when encountered. While “bite” is often used colloquially to describe a painful interaction, the biological reality of the spider crab’s defensive action is quite different.
Clarifying the Threat: Biting vs. Pinching
A spider crab cannot bite in the way a mammal or fish does; the action people fear is actually a pinch. A true bite requires specialized oral structures, such as mandibles or teeth, designed for piercing or cutting flesh. Spider crabs, like all crustaceans, lack this type of jaw assembly, executing defense solely through their powerful appendages.
When threatened, the spider crab deploys its claws, known as chelae, as its primary defense mechanism. This application of mechanical force is fundamentally a pinching action. The resulting injury is a compression or crushing wound, not one caused by slicing or puncturing teeth.
Anatomy and Function of Mouthparts
The structures that constitute the spider crab’s mouth are highly specialized for feeding, not for defense against large subjects. These oral appendages are located on the underside of the body, between the walking legs, and are designed to process the small food items that make up the crab’s diet.
The main oral structures include the mandibles, maxillae, and maxillipeds. Mandibles are heavily calcified and function to crush and grind food particles, often moving in a sideways motion. Maxillipeds are modified legs that manipulate food and filter water. Their placement and design make them incapable of delivering a defensive bite to a human.
Assessing Interaction: The Power of the Claws
The true defensive and offensive tool of the spider crab is its pair of large claws, or chelae, which are the terminal segments of the first pair of legs. These chelae operate as a lever system, with the fixed finger (pollex) and the movable finger (dactyl) closing together to generate a strong compressive force. The strength of this pinch is determined by the muscle mass within the claw and the mechanical advantage of the lever system.
Pinching force is strongly correlated with body size, meaning larger spider crabs can inflict a more forceful pinch. Many species show sexual dimorphism, where male crabs possess significantly larger and more powerful claws than the females. The immense size of species like the Japanese spider crab suggests a substantial potential pinching force, though most common varieties are much smaller.
A defensive pinch typically results in minor bruising or a superficial break in the skin. The crab’s goal is to startle and deter a perceived predator. For most smaller species, the force is insufficient to cause serious damage, but it will be painful.
In rare cases involving very large individuals, the force can cause a deeper laceration or puncture wound, especially if the claw tips are sharp. If you encounter a spider crab, the safest approach is to avoid direct contact. If a crab does pinch, remain calm and submerge the crab and the pinched area back into the water, which often encourages the crab to release its grip.