Image: Stephen Friedt
It is an hour after tea and from the row of deckchairs comes an occasional ripple of light applause. The home team is all out for a respectable score, and the visitors have just gone in to bat.
What could be more innocent and peacefully English than a village common? But behind the cricket pavilion is a small pond bordered with rushes, where not all is what it seems.
Earlier today, using our trusty grapnel, we took a few strands of weed from the middle of the pond and placed them in a large jar of water. By lunchtime a good many of the small creatures in the weed had made themselves evident.
The animal we were seeking was among them. Clinging to the glass were several threadlike specimens, two or three brown, but mostly green, of one of the strangest and most treacherous denizens of the underwater world, a tentacled relative of the Portuguese Man-o’-War, of the sea anemone and the coral, with a name from Greek mythology and a ferocious armoury of poison darts with which it spears its hapless prey.
Luckily for those who like to walk the common in safety, all this goes on in miniature. Not counting the tentacles, the biggest specimens are not much more than half an inch long.
Hydra – for that is the creature’s name – consists, besides its half-dozen or so tentacles, of a tube-like body and little else. The base of the tube is formed into a disc-shaped foot, with which it anchors itself to a stone or frond of weed. If all is well, the tentacles are extended and gently waved, and Hydra waits for chance or the current to bring it what they may.
In common with most of the coelenterates (the group of “hollow-bodied” animals to which it belongs) Hydra is equipped with stinging cells called nematocysts. In its most highly developed form a nematocyst consists of a capsule filled with poison and containing a long, hollow filament, at the end of which is a more or less elaborate arrangement of barbs.
Projecting from the mouth of the capsule is a short trigger, which, when activated by a passing prey-animal – often a water-flea – sets off the release mechanism. The capsule violently contracts and the barbed head of the filament is shot out like a harpoon, puncturing the body-wall of the prey and releasing the poison inside.
Besides these, so-called “penetrant” nematocysts, Hydra’s tentacles have “volvent” nematocysts also. Volvents are likewise used in feeding. Once the prey has been stunned or killed by the penetrants, the volvents are discharged. They have no harpoon or poison, but their filaments are designed to coil round bristles and other projections on the prey. Muscle cells in the tentacles then contract and the prey is brought inexorably to the mouth, which is no more than a hole at the apex of the body, in the middle of the ring of tentacles.
Digestion is a pretty straightforward business for Hydra. The cells lining the body cavity secrete enzymes which help to break down the food, and mechanical action (which occurs whenever Hydra moves about) does the rest. Nutrients are absorbed and dispersed either by simple diffusion or by cells which migrate about inside the animal, taking the food where it is needed. Unwanted material is egested through the mouth.
Nor is Hydra’s sex-life much more complicated. Usually it does not bother at all, and merely grows buds which, enlarging, eventually drop off and become a new individual. This enviably simple process is only suitable as long as the water remains warm, for Hydra cannot survive the winter in adult form.
In autumn, then, certain cells develop in the body wall which become testes. Lower down on the body an ovary also develops, but not at the same time, so that self-fertilization is rare. The covering on the testes finally ruptures, releasing sperms into the water which seek out a ripe egg. The resulting zygote develops into a cyst which sinks to the bottom of the pond and lies dormant till spring, when it soon hatches and grows into an adult Hydra.
The mythical Hydra had frightening powers of regeneration, growing two heads for every one that was cut off; and our local Hydra is not far behind. It can replace a mutilated, or even a completely amputated, tentacle. Even if the body is cut up into tiny pieces, each one will develop into a new individual.
This is made possible by a layer of “interstitial” cells, each of which is a basic and undifferentiated cell, waiting to turn into whatever is needed. The interstitial cells are the reservoir from which the nematocysts are replaced, and from them the sexual organs develop too; it is likely that a new Hydra could grow from a single interstitial cell.
The green Hydra gets its colour from the presence in its body wall of Chlorella, a single-celled plant that is a degenerate relative of the free-swimming algae present in almost any bit of stagnant water. Chlorella and Hydra exist in partnership: the plant absorbs carbon dioxide and minerals from the animal and gives back oxygen and excess sugars.
Having studied our specimens long enough, it is time to put them back. The cricket is still in progress; by now a number of those in the deckchairs seem to have fallen asleep. It would be a shame if their dreams were ever inhabited by monsters as fearful as those which, as we empty the jar into the pond, are now returning to their own murky world.