How Bubbles Super-Power the Mantis Shrimp’s Punch

close up photograph of red, white, green and blue crustacean

Biological Strategy

Peacock mantis shrimp

AskNature Team

Image: Silke Baron / Flickr / Some rights reserved

Transform Mechanical Energy

Mechanical energy is made up of kinetic energy (the energy of an object in motion) and potential energy (stored energy). Organisms use mechanical energy in a variety of ways, including capturing prey, transporting seeds, and moving around.

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Protect From Animals

Animals–organisms that range from microscopic to larger than a bus–embody a wide variety of harms to living systems, including other animals. They threaten through predation, herbivory, defense, and parasitism, and they compete for resources such as water, nutrients, and space. Any given living organism commonly faces threats from a variety of animals, requiring strategies that effectively defend from each. Trout and other bony fish, for example, escape predators by having scales made of very thin, flake-like pieces of bone covered with slippery mucus. They also have behavioral strategies such as camouflage, fast swimming, and twisting and turning to achieve release from a predator’s grip.

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Capture, Absorb, or Filter Organisms

Many living systems must secure organisms for food. But just as one living system must capture its prey to survive, its prey must escape to survive. This results in capture and avoidance strategies that include trickery, speed, poisons, constructed traps, and more. For example, a carnivorous plant called the pitcher plant has leaves formed into a tube that collect water. Long, slippery hairs within the tube face downward. When insects enter the tube seeking nectar, they lose their footing and slide inside, unable to climb out and escape being eaten and digested by the plant.

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Crustaceans

Subphylum Crustacea (“crust”): Crabs, barnacles, shrimp, pillbugs

Some crustaceans, like pillpugs and woodlice, live on land, but most are found in the ocean. Their heads are distinctive among the arthropods: they have two pairs of antennae and three pairs of feeding appendages that help them hold and crunch food. Crustaceans also have a unique larval stage called the nauplius (named after the son of Poseidon) which has three pairs of legs and one eye. Most crustaceans are in class Malacostraca, which contains a wide range of species that live in mostly marine environments, like krill, lobster, and mantis shrimp.

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The appendage of the mantis shrimp strikes with a tremendous amount of force enhanced by cavitation bubbles.

Introduction

The mantis shrimp is a marine crustacean distinguished by its ability to deliver high-velocity powerful strikes that can break mollusk shells and even aquaria glass. It does this with its raptorial appendages–forelegs specialized for protection and feeding. There are many different species of mantis shrimp, and the morphology of its raptorial appendage classifies the mantis shrimp as either a spearer, smasher, or undifferentiated. While the rapid movement and initial force of the striking mechanism is due to power amplification, smasher and undifferentiated species can add extra force to their strike by producing cavitation bubbles.

diagram of the mantis shrimp's raptorial appendage

The mantis shrimp's deadly weapon is generally held close to the body and hidden somewhat by other appendages. Seen in isolation, its structure is and its potential for action are more apparent. Illustration by Holly Sullivan / sulscientific.com

The Strategy

The raptorial appendage is divided into four segments: the merus (closest to the body), carpus, propodus, and dactyl. The shape of the dactyl differentiates the shrimp as a spearer, smasher, or undifferentiated species. Spearing appendages are long, pointed segments that slice through the water and soft prey. Smasher and undifferentiated appendages, on the other hand, have blunt, bulbous “heels” on the dactyl used to deliver forceful blows in close range. This rapidly accelerating heel is what creates a cavitation bubble.

Illustration of the mantis shrimp's club strike and resulting cavitation bubble

As the mantis shrimp's club strikes, it moves water so quickly as to create a low pressure area that then forms a bubble and collapses in a burst of high-energy light and sound. Illustration by Holly Sullivan / sulscientific.com

Cavitation bubbles are created when an object moves through water at very high speeds and creates extreme velocity gradients in the flowing water. Under the right conditions, this produces a cavity or bubble in which the pressure is so low that the water vaporizes. When the bubbles collapse under higher surrounding pressures, they emit energy in the form of sound, light, and heat waves. The whole process occurs in a matter of milliseconds. Cavitation is a common phenomenon with boat propellers, which create a continuous stream of bubbles that collapse and erode the propeller metal over time.

In the case of smasher and undifferentiated appendages in mantis shrimp, their anatomy and striking speeds create the perfect conditions for a single cavitation bubble. When the bubble bursts, its strength is almost equal to that of the strike itself. A single cavitation bubble is proportional to the firing of a 22-caliber bullet. It’s the one-two punch of the deep sea. The spearer appendage does not create a cavitation bubble because of its sleek design.

This summary was contributed by Allie Miller.