By Jared Erickson

The battles between predators and prey across the natural world are punctuated by vicious physical adaptations on both sides. The preyed upon develop defenses for discouraging or even attacking back against their predators. The predators in turn have their own ways to overcome their prey and be sure it is them who are victorious. A multitude of examples can be found throughout the natural world. The duck billed platypus of Australia has specialized hind limb spurs that can be used to stab predators and inject them with a venom that inflicts excruciating pain, giving the predator a painful lesson instead of a delicious meal. The majestic lion that is known for ranging across the African savanna has a set of razor sharp retractable claws it uses to attack and capture a wide range of prey. The not so cuddly looking porcupine of North America is covered in a defensive mat of barbed quills that it can use to puncture and embed in the soft flesh of any predator who thought to make the porcupine its snack. But vicious and destructive predator and prey adaptations aren't the sole domain of the animal kingdom; the microbial world has developed its own.

Paramecium as seen through a microscope.

A single drop of pond water can contain a dizzying array of microbes engaged in life and death struggles as they try to survive. Examining this tiny liquid safari finds a myriad of residents including the tiny Paramecium and Didinium. A unicellular ciliate, Paramecium has a vicious method that it can use to deter and avoid its carnivorous unicellular predators. This method is through the use of a system of sharp protrusions known as trichocysts. The Didinium also lurks about in the water with the Paramecium. But instead of defense, the Didinium has developed a system for attacking and capturing its prey through the use of toxicysts. 

Paramecium live their entire lives swimming about in the water. The beating of the finger like cilia of the Paramecium propel them through the water. Individual Paramecium can vary greatly in size, but most are between 100 and 300 µm in total length. The Paramecium swim from place to place, gathering and feeding on the bacteria and algae that co-exist in the water. Also lurking about in the water are the predators of the Paramecium. In addition to the Didinium, carnivorous microbes such as Cephalodella and Eucypris stalk and attempt to eat the swimming Paramecium. If a predator is able to strike and capture a Paramecium,  it will engulf it whole. The predator will then drift about in the water as it slowly digests its Paramecium meal. 

Trichocysts seen beneath the surface of a
 Paramecium. tric: indicates a trichocysts location.

The Paramecium safely sequesters the trichocysts inside of their own specialized membrane until the Paramecium feels the need to use them. When that need arises, a rapid fusion event occurs between the membrane surrounding the trichocyst, and the Paramecium's cell membrane. This membrane fusion event violently thrusts the trichocysts outside of the cell, exposing their sharp spear-like tips in a fraction of a second. The trichocysts extend away from the cell, attached to the tip of an extension of the membrane, like a spear head on the end of a spear handle. Trichocysts are sharp and pointed cellular organelles. Shaped like a spearhead, the trichocysts have a broad base which tapers down into a tiny point. The trichocysts are 3 – 4 µm in total length. A single Paramecium can harbor thousands of these sharp trichocysts.

Electronmicrograph of the surface of a
Paramecium. The trichocysts are stored
beneath the surface of the Paramecium
at the locations indicated by the arrows.

The trichocysts stand as both the best and last line of defense of the Paramecium from the certain death inside of the predator. When the Paramecium feels the mouth of the predator, it is able to quickly discharge the trichocysts in that area. Watching this interaction under a microscope, the predator can be seen to momentarily recoil from the sharp sting of the trichocysts. This momentary hesitation on the part of the predator is all the Paramecium needs to make its escape, furiously swimming away by rapidly beating its cilia. 

While the trichocysts are a defense against predation, the Paramecium are not always so lucky as to escape. The carnivorous Didinium has developed a system that assists in overcoming the trichocyst defense. Looking at a Didinium it would be hard to tell that this funny acorn shaped unicellular microbe is one of the most fearsome hunters of Paramecium. Similar in overall size to many Paramecium cells, the Didinium is still capable of swallowing Paramecium whole. 

A. didinium. The cone shaped
proboscis can be seen at the top. 

What makes the Didinium particularly dangerous is hidden beneath the tapered cone shaped proboscis that is located at the anterior end of the cell. The proboscis is what the Didinium uses to strike at the Paramecium. When the proboscis makes contact with the surface of the Paramecium, the Didinium reveals its weapon lurking beneath the surface, the toxicyst. 

Toxicysts are similar to the trichocysts, however they are used for offense instead of defense. The toxicysts rapidly fire from inside the proboscis pierce the Paramecium and embed themselves firmly in place. Long strands attached to the toxicysts like ropes attached to a whaler's harpoon keep the Didinium firmly attached to the Paramecium. The Paramecium can struggle against the toxicysts and attempt to strike back using its trichocysts, but the Paramecium will remain firmly in the grasp of the Didinium. With no hope of escape the Paramecium is drawn in close. The Didinium can then begin to swallow its immobilized prey whole. 

A. Didinium (top) that has begun to eat a
Paramecium (bottom). The Didinium will eventually
swallow the entire Paramecium.

The predator and prey interactions between large animals are familiar to most people. From film strips and movies in school and Sunday nature programs on television, we have become familiar with the brutality of the animal world. The microbial world shows us that taking an expedition to far away lands isn't required to see carnivorous predators and dangerous prey interact. Down at your local park in a single drop of pond water, tiny microbes are locked in their own life and death battles. A simple microscope can allow you  to unlock your own microbial safari.

References

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