Pick from the Past
Natural History, May-June 1927

Hydras as Enemies of Young Fishes

Together with mention of the geographical work of the Roosevelt-Rondon
South American expedition in exploring the “River of Doubt”


N August, 1902, a sudden epidemic occurred among the black-spotted trout fry in the hatchery of the United States Fish Commission at Leadville, Colorado. Examination showed that the hatching troughs were divided into three compartments, and that the greatest mortality occurred in that division into which the water entered. It was somewhat less in the middle division, and practically negligible in the last. The water supply was seemingly pure, almost entirely devoid of sediment, and entirely normal in temperature. For some weeks the matter was quite a mystery. Then Prof. A. E. Beardsley of the department of biology of the Colorado State Normal School was called in to investigate the trouble.

Figure 1

An enlarged figure showing a young fish caught in the tentacles of a hydra.
The interior of the hatchery was dimly lighted, but Beardsley, acting on a hint from one of the men who had seen something in a trough when a ray of sunlight fell on it, arranged a set of mirrors, with which he directed a beam of sunlight into one of the troughs. There he discovered the culprits—great numbers of very white and very transparent hydras were found covering the walls of the trough, hydras so colorless that in the somewhat twilighted interior of the hatchery they were entirely invisible. A careful count was made of various square inches of Division One of several troughs. This gave an average of 131 hydras per square inch, or 20 plus per square centimeter.

The whole body of a hydra is thickly set with peculiar stinging or poison cells called nematocysts.

When I was a student assistant in the general biology course at Johns Hopkins University a good many years ago, there was a favorite pond in Druid Hill Park wherein I collected hydras. Here, on the wooden partitions which subdivided the pond into sections, they were found in great abundance, more thickly than anyone in the laboratory had ever seen them before—perhaps 6 or 8 to the square inch—but in nothing like the high concentration which Professor Beardsley found.

The Colorado hydras were 10 to 20 mm. in length, and 0.15 to 0.30 mm. in diameter—slender white organisms, fastened by the basal end to the walls or bottom of the trough, and having at the free end a mouthlike opening surrounded by 5 or 6 long, very slender, threadlike tentacles.

Figure 2a

A much magnified part of the tentacle of a hydra showing the poison cells arranged in groups of “batteries.”
The whole body of a hydra is thickly set with peculiar stinging or poison cells called nematocysts (thread cells), and on the tentacles these are concentrated into regular batteries ready to be "touched off" by the first passing object which comes in contact with and irritates these cells. Figure 2a shows in enlarged form a part of a tentacle with some of these "batteries," the projecting hairs being the partly discharged nematocysts.

Figure 2b

The greatly enlarged poison capsule of a hydra.
Fig. 2b shows a poison cell as it appears when normally at rest in the ectoderm or skin layer of cells in the body of the hydra. The thread cell or nematocyst consists of a hollow bag with a finger-like inpushing having spines at the base and terminating in an inverted, long, hollow, whiplash-like tube. The bag, the finger-like inpushing, and the hollow whiplash are filled with a virulent poison. The nematocyst or thread cell is embedded in a modified ectoderm or skin cell, which forms its “carriage,” and this has projecting on the surface a hair or “trigger.” The enclosing cell is called a cnidoblast (nettle bladder) and the “trigger” a cnidocil (nettle hair).

Figure 2c

A much magnified poison capsule which has been thrown into action.
This somewhat complicated but highly efficient apparatus works as follows: The baby trout comes wriggling along and touches one or a half dozen of the trigger hairs. This calls into play the inherent irritability and contractility of the protoplasm of the cnidoblast and it instantly contracts sharply, putting pressure on the contained nematocyst. This practically explodes, the finger and whiplash part of the nematocyst turn inside out, and the tip of the lash penetrates the tender body of the troutlet and discharges the poison with which it is filled. Such an everted thread cell is portrayed in Fig. 2c. Moreover, not only do those cnidoblasts whose triggers are touched, contract, “go off” as it were, throwing out the thread cells, but the stimulation is communicated through the very rudimentary nervous system of the hydra to the neighboring cnidoblasts, and whole batteries explode at once, covering the baby fish with thread cells and paralyzing it completely. Figure 1 of this article shows such a baby fish paralyzed by the thread cells, grasped by the twining tentacles, and on its way to the mouth of the hydra.

To make absolutely sure that the hydras were the only cause of the high mortality of the baby fishes, Beardsley filled a number of glasses with water from the hatchery pipes. Into some he put troutlets by themselves, in others fishes and large numbers of hydras from the troughs. In the first glasses there was only the normal mortality usual in hatching fish, but in the others there was a heavy death rate due to the activities of the hydras. In fact Beardsley found that 25 per cent of the baby trouts were killed by the hydras in less than 30 minutes, 60 percent in 45 minutes, 80 per cent in 60 minutes, and 100 per cent in 75 minutes. Examination with a lens showed hydras attached by their mouths to the surface of the fishes—in some cases as many as a dozen were so attached. Low mortality was shown among the fishes in the glasses filled with water from the trough without hydras, and the remaining fry in the clean hatching trough were in good health at the end of twenty-four hours.

Figure 3

The little fish has been swallowed tail first by the hydra. The body of the hydra has shortened greatly in order to expand sufficiently in width to engulf the small fish. Note how the tentacles have also shortened and thickened.
No other cause for this wholesale mortality being discovered, Beardsley correctly concluded that the hydras were the culprits. Search was then made for the origin of these hydras in all the sources of water supply. All were found free of hydras save one small pond along whose shallow borders aquatic vegetation was thickly clustered. Innumerable hydras were attached to the submerged parts of these stems and leaves, as well as to the sticks and stones lying on the bottom of the pond.

No other cause for this wholesale mortality being discovered, Beardsley correctly concluded that the hydras were the culprits.

Thinking that he had discovered a phenomenon not merely interesting but absolutely new, Beardsley wrote and published an article, “The Destruction of Trout Fry by Hydra,” in the Bulletin of the United States Fish Commission for 1902, Washington, 1904, Vol. 22, pp. 157160. That his discovery was extremely interesting is undoubted, but that it was not new will be seen later in this article.

In 1905, there appeared in Allgemeine Fischerei Zeitung a notice of Beardsley’s article signed “Dr. Pl.” This was seen by one A. Schuberg, who seems to have been a German trout grower. In a later issue of the same journal for 1905, Schuberg refers to Beardsley’s article and recounts his own experiences which antedated Beardsley’s studies. In a little pond well stocked with duckweed (Lemna) he was growing young trout 30 or 40 mm. long. A progressive destruction of these fish went on. He examined both fresh and preserved fish and found on their bodies, but especially on their fins, very many of the nematocysts or nettle threads described and figured above. Examination of the duckweed in the pond showed great numbers of the brown hydra (Hydra fusca). These were judged to be the authors of the mischief, and an attempt was made to clear the pond of both Lemna and hydras.

Just here an interesting bit of corroboratory evidence may be introduced from the neighboring and closely related class of animals, the Amphibia—the tadpoles of which are the fish stage of their evolutionary life history. In 1911 there appeared from the pen of William West a note, entitled “Hydra vulgaris and the tadpoles of Rana temporaria” (Naturalist, London, p. 301), in which he writes as follows:

“In our biological laboratory it is a common thing to watch Hydra catch species of Daphnia, Cypris, and “Cyclops.” I have even seen them gorged with the large red larva of Chironomus plumosus, the Hydra, when distended, having room for half of it! (I have a Scyllium canicula with the hinder part of a fish in its stomach and gullet, and the other half projecting from its mouth). This Spring I had a fine lot of Hydra vulgaris in several large aquaria, and as I had previously had some batches of frog’s eggs developing, I placed some of them, when about a fortnight old, in the various aquaria, some being three or more weeks old in later experiments. On looking a few hours later, I was astonished to see several of the tadpoles held fast to the sides of the aquarium, they kept now and then struggling to escape, and if any succeeded in doing so, which was seldom the case, they invariably succumbed eventually. These experiments were eagerly repeated by a number of students. . . . The tadpoles were paralyzed, were too large to be engulfed, and they finally sank to the bottom, and did not reappear. In all the other aquaria where Hydra was absent, the tadpoles lived.”

However, long before either Schuberg or Beardsley, 160 years in fact, Abraham Trembley, the Father of “Hydraology” (the study of hydras), had in 1744 described their method of catching little fishes. He left little for us to learn about the behavior of hydras. His account, entombed in his great monograph on the hydras, seems not to be known. It is well worthy of reproduction herein literatim et verbatim. He says:

“Having taken in the month of June, 1743, a considerable number of little fishes about four lines long [about four twelfths of an inch or eight millimeters long], the first use that I made of them was to see if the polyps would eat them.

“I placed several of them in vessels where I had some polyps. The experiment very soon apprised me of what I had surmised, that is that the vivacity and energy of these little fishes gave them power to offer a sharp resistance, but I ventured to flatter myself that the polyps would soon put an end to this by catching them. The Gardons [young roaches] (this is the species of fish to which I refer), the Gardons, in swimming about, soon encountered the tentacles of the polyps, and this then was the beginning of the combats which indeed were not all finished in the same fashion.

“When the fish would encounter only one arm of the polyp, it ordinarily happened that it disengaged itself by a lively jerk; and it would sometimes even break off the tentacle which endeavored to hold it captive and would carry this part off with it. However, the combat would end less happily for the little fish when it would be caught by several arms at once. The efforts which it would then make to set itself free would for the most part be useless, and would often bring it about that it would become even more closely entangled in the tentacles of its enemy. It could be easily seen that the polyp was making great efforts to hold fast to the fish. The arms which enveloped it on all sides would become very much swollen [and shorter], but they came to the fish a few at a time and only when it made great efforts. Then they were vigorously wrapped around the fish—in a word that which Ovid says of the marine polyp [i.e., poulpe, Octopus?] would perfectly apply to the fresh water polyps under study here. One would think that it is the latter of which the poet speaks when he says, ‘And thus under the water the polyp with its tentacles out-thrown from all sides holds its submerged prey.’

The combat would end less happily for the little fish when it would be caught by several arms at once.
“When I saw a polyp which had arrested a fish and had brought it to its mouth, I wondered whether it would be entirely possible for it to swallow the fish which was four lines long and proportionally thick and which would not bend itself to fit itself comfortably in the body of the polyp. The polyp, which had undertaken to do the swallowing, having been obliged to contract itself because of the shocks which the fish had given it in its struggles, was now not longer than 2 or 3 lines. In spite of all this the greater number of polyps which had caught Gardons had put an end to the swallowing. When a long-armed polyp had swallowed a fish, that narrow part of its stomach which forms the tail end would be compelled to expand and receive a part of the prey. A polyp which had swallowed a fish was difficult to recognize. Let us suppose, for example, that it had swallowed it tail first, one would then see the contracted tentacles around the head of the fish. This is that which would appear the better. The skin of the polyp would be stretched so tightly and applied so closely to that of the Gardon that one could distinctly see the fish through it, so that if one had not known it to be there one would have thought that he only saw a fish which had at the anterior extremity barbels some lines in length.

This little fish would then occupy the entire length of the body [cavity] of the polyp whose skin was then very thin, wherein in the meantime it was undergoing digestion. It did not remain alive more than a quarter of an hour. After it had been subjected to the action of the digestive juice and had been returned by way of the mouth of the polyp, it was actually recognizable but nevertheless very much disfigured. This is what I have seen a considerable number of times.”

Plate VII, Fig. 5, in Trembley’s book is supposed to show this, but the figure is so small and so dark that I have not been able to make anything out of it—even with the use of a magnifying glass. The same is true of the figure in the German version (1791) which I have examined. However, the accompanying excellent figures by Mr. William E. Belanske have been made under the present writer’s supervision to show, in Fig. 1, how the fishlet is caught, and in Fig. 3 how it has been swallowed tail first and is undergoing digestion. The purpose of the figures is to portray visibly what Trembley described 183 years ago. Furthermore, an effort has been made to keep the relative sizes of fish and hydra within the limits set by Beardsley and Trembley, though of course these and the other figures are much enlarged. From the above counts, particularly Trembley’s, one may quote the author of Ecclesiastes that there is nothing new under the sun.

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