Crabs Have Teeth in Their Stomachs

A Surprising Secret Hidden Inside a Crab

    Crabs are already unusual animals walking sideways, living in rocky tide pools, and carrying hard shells that act like armor. But there is something even more surprising about them, something that many people never imagine: crabs have teeth inside their stomachs. While humans and many other animals chew their food with teeth in their mouths, crabs “chew” internally. This bizarre and fascinating adaptation reveals how diverse and creative nature can be when solving the challenges of survival.

Why Crabs Don’t Chew with Their Mouths

    Unlike mammals, crabs do not have jaws designed for chewing. Their mouthparts are used mostly for grabbing, tearing, and pushing food inward, not for grinding it. Because their external mouthparts are small and not suited for chewing tough materials like shell fragments, algae, worms, or small fish, crabs evolved a completely different system to process their meals. Instead of breaking food down at the beginning, they do it after the food has already entered the body.

The Gastric Mill: A Built-In Grinder

    Inside a crab’s stomach lies a structure called the gastric mill, a complex set of tiny, calcified “teeth” made of hard plates and ridges. These teeth do not resemble human teeth but function in a similar way. The gastric mill is located in the upper part of the stomach and is powered by strong muscles that move the teeth back and forth, grinding food just like a miniature internal blender. When a crab eats, the food is pushed into the stomach, where the gastric mill crushes it into smaller, digestible pieces. This system allows crabs to eat an incredible variety of food, from soft plants to hard-shelled animals.

How the Internal Teeth Work

    The movement inside the gastric mill is remarkably coordinated. Different plates scrape, rub, and press against each other in patterns that scientists describe as rhythmic and powerful. These motions are controlled by a set of nerves that keep the grinding motion consistent and efficient. In some species, the motion of the gastric mill can even be heard as a faint clicking sound, especially in larger crabs. By breaking down food internally, crabs maximize the nutrients they can extract and ensure that even tough pieces—like shell fragments or fibrous plants—can be digested.

An Adaptation that Fits Their Lifestyle

    Having teeth in the stomach is an evolutionary advantage for creatures like crabs, lobsters, and crayfish. Many of these animals live in environments full of hard, crunchy food sources: barnacles, snails, mussels, and even coral fragments. The gastric mill allows them to eat almost anything they can catch or find. Since crabs often scavenge, this internal chewing system helps them survive on a diverse diet, making them one of the most adaptable marine animals. It also allows them to keep their mouthparts small and protected, reducing the risk of injury during feeding or fighting.

More Than Just Chewing: A Window Into Crab Evolution

    The existence of stomach teeth tells scientists a lot about the evolution of crustaceans. Instead of developing jawbones like vertebrates, crustaceans evolved a digestive system that handles mechanical breakdown inside the body. This shows a different evolutionary path toward solving the same basic problem: how to process food. By studying the gastric mill, scientists learn more about crustacean anatomy, feeding behavior, and how these animals thrive in harsh environments. Some studies even explore how the gastric mill patterns are controlled by neural circuits, making crabs a model for understanding rhythmic biological movements.

A Hidden World Inside an Ordinary Creature

    What makes crabs especially fascinating is that such an extraordinary system remains hidden inside a creature that many people simply see as seafood or as small animals scuttling across the beach. Beneath their shells lies a remarkably efficient and complex “chewing machine” that works tirelessly, unseen. The idea of teeth in the stomach might sound strange, but it is also a powerful reminder that nature constantly surprises us. From diamond rain on planets to internal teeth in crustaceans, the world is filled with wonders waiting to be discovered.

References:

• Bliss, D. E., & Mantel, L. H. The Biology of Crustacea. Academic Press, 1983.

• Clements, K. D., et al. “Feeding Mechanisms and the Gastric Mill in Crustaceans.” Journal of Experimental Marine Biology and Ecology, 1999.

• Elner, R., & Campbell, A. “Feeding Behavior of Crabs and the Function of the Gastric Mill.” Canadian Journal of Zoology, 1981.

• Smithsonian Ocean Portal. “Crabs and Their Gastric Mill.” Smithsonian Institution, 2020.