From Smithsonian magazine April 1975
Text and photographs by
Andrew Skolnick

Hydra: versatile green predator

This common freshwater polyp can easily be taken for a plant,
but it acts like the mytholgical creature Hercules killed.
Contracted, the hydra looks like a tiny cactus. Its color
comes from symbiotic algae living in the hydra's tissues.

WE often borrow names from the gods, heroes, and monsters of ancient Greece in classifying and naming all we survey. Arachne, a maiden who was turned into a spider for challenging and arousing the envy of Athena in a weaving contest, gave us the name Arachnida for the class of arthropods which includes spiders. And you need know only its name to wish to avoid a Hercules beetle. Many such names are fitting, but few are as appropriate as hydra, the name of a freshwater coelenterate, a relative of the oceans' corals, sea anemones, and jellyfish.

One of the 12 tasks imposed on Hercules was to slay the Hydra of Lerna. It had nine serpent-like heads, one of which was immortal. So potent was its venom that even its smell was fatal to those who passed too close. Worst of all was that for every head Hercules cut off, two more grew in its place. He finally triumphed by cauterizing the wounds with a firebrand as he chopped away and by burying the immortal head under a rock.

Fully extended, its tentacles sweeping the water for its prey, the hydra reaches a height up to two inches.
With its tentacles, a hydra bud snares a Cyclops, a tiny, one-eyed crustacean, also named after an ancient Greek monster. The bell-shaped protrusion on the hydra's body (upper left) is a second hydra bud.
Long after Hercules' labors, in 1702, the Dutch inventor of the microscope, Anton von Leeuwenhoek, discovered the tiny freshwater polyp in his searches. In 1740, Abraham Trembley, a Swiss naturalist, first discovered the green species of this creature and described it scientifically. For a long time, he couldn't decide whether it was a plant or an animal. It was as green as a plant, yet it could move like an animal. To settle the question, he cut the polyp in half. It was known at the time that lizards could regenerate a tail and crayfish a claw, yet Trembley believed only a plant could regrow as much as half of itself. Tilting toward the animal theory, he was amazed to find the head half with a new bottom and the bottom half with a new and complete head. Continuing his regeneration experiments, he cut the polyp's head lengthwise in tow and the polyp grew two intact heads. He repeated the cuts until he had a seven-headed polyp and naturally enough compared his multiheaded creation to the mythological monster. In 1758, Linneaus, finding Trembley's comparison fitting, gave the name Hydra to this genus of freshwater polyp.

Hydras are commonly found hanging inconspicuously from vegetation or from the water's surface film in most streams, lakes, and ponds. From 3 to 50 millimeters in length, they search the water with their four to eight long, thin tentacles for crustaceans, insect larvae, worms, or any other animal small enough to be overcome and engulfed.

Some hydras are red, some are brown, but perhaps the most interesting is the gree species Chlorohydra viridissima shown on these pages. This hydra owes its color to algae (called zoochlorellae) that live within its inner cell layer in a relationship called symbiosis, which means, in effect, that both sides gain. The algae gain by having a place to live and reproduce. And thanks to the nutrition the hydra derives from the algae, it has remarkable regenerative powers, even when little or no other food source is to be found.

Tentacles curled, a hydra is ingesting its prey. Its mouth is at right.
With an air of surfeit, the hydra is at rest, digesting its meal.

Mistaking a hydra for a plant, as Trembley did, is easy; at first glance they are as immobile as any tine pond plant. After a few moments the hydra spontaneously contracts into a tiny spot, a mere fraction of what it was, and then slowly extends itself in a different direction. Patience will allow you to sell all of the varied forms of locomotion the hydra uses to get about. It may "walk" with almost imperceptible slowness along the substratum, clinging by its base with a sticky mucous. It may parachute to the bottom with outstretched tentacles, there to feed on organic debris. It may rise again like a hot air balloon by generating a gas bubble at its base. It can hang by its base from the depression of the water surface and walk or flow with the current. It can bend its tentacles and inch along by alternately attaching tentacles and base to the substratum. By moving its base entirely over, it can progress by somersaulting along.

Like other coelenterates, hydras are composed of two thin cell layers separated by a gelatinous substance called mesoglea. Like a rubber glove sealed at the bottom, the hydra encloses a digestive cavity extending from it base to the tips of its tentacles.

The whole organism is composed of just six basic kinds of cells. Nerve cells are laid out in a simple, two-dimensional network, primitive compared with the three-dimensional system of man and the earthworm. The digestive and gland cells are located in the inner cavity where food is broken down into minute particles. The particles are then absorbed by individual cells where the digestion of protein into amino acids takes place. There are the connective, supportive, and locomotive tissues, and there are sex cells, but the latter are created only when the hydra goes sexual under conditions not well understood.

Normally, hydras reproduce by budding. The buds begin as a pair of bumps 180 degrees apart on the growth region of the body column. They elongate, grow a mouth and tentacles, pinch themselves off, and inchworm or somersault away. Well-fed specimens often have a second or even third pair of buds above or below at a 90-degree angle to the first pair.

A mature bud has extended itself into a long stalk and is about to pinch itself off and depart.
One hydra bud somersaults away from its parent as another grows. Hydras often have four buds at a time.

In a manner of speaking, the hydra is as immortal an unaging as was its mythological namesake. Old cells migrate up the body and slough off at the ends of the tentacles, being replaced by new cells; the entire hydra is replaced by new cells every 45 days or so.

The source of most of these new cells is what are called interstitial cells. These cells can differentiate into almost any kind of cell that the hydra requires for its normal life functions; they are the sole source for nerve cells and glandular cells, for example. They are also the sole source of sex cells.

When autumn arrives, bringing conditions that threaten the survival of the hydra, it ceases to replace itself and goes sexual. For the hydra, the price of sex is generally death. The interstitial cells stop producing other kinds of cells and engage exclusively in producing eggs and sperm. (some hydras are hermaphroditic.) The fertilized ova develop into embryos with hard coatings that protect them when they separate from the parent and lie dormant in the mud. The next generation emerges in spring, thus achieving for a species a kind of immortality more familiar to man.

The mythical Hydra was deadly poisonous, and this zoological counterpart echoes its namesake in this regard as well. Along the tentacles are deadly batteries of cells called nematocysts, the most complex cellular organs in the animal kingdom. The hydra has four types. The largest contains a harpoon-like stylet with spines. When fired, the barb penetrates the prey and anchors it to the tentacle by a thread while injecting a virulent poison. Judging from its incredible potency and speed of paralysis and death, the poison is believed to be a nerve toxin. A second kind of nematocyst is smaller and contains a thicker and shorter corkscrew thread. When discharged, this type holds fast to the tiny hairs and bristles of the prey. A third type, which has a sticky bean-shaped object at the end of a smooth thread, serves during tentacle-assisted locomotion. Finally, a fourth kind differs in having spines along the thread and is believed to be used in defense against paramecia and other enemies.

Once the hydra's prey has been "harpooned," substances released from the prey's wounds initiate the feeding response. The portion of tentacle between the embedded nematocyst and the hydra's head contracts, bringing it to the mouth while the free tentacles make sweeping movements toward the mouth. The opening mouth then creeps over the prey with little or no help from the tentacles, much like a sock putting itself on a foot. With prey almost as large as itself, the hydra stretches so thin in getting its digestive gut around it that the prey animal appears to be covered with a thin transparent film; only the tentacles tell you that you are looking at a hydra.

Except for its size, this wetland creature with its many serpentine "heads," armed with harpoons and virulent poison, protected by incredible regenerative powers, and with its own kind of immortality, is very much like the fiend that Hercules killed. Indeed, imagining a creature to symbolize their fears of the dangers of the marshlands, the Greeks did well.

© 1975, 2001 Andrew A. Skolnick -- All rights reserved.