Pick from the Past
Natural History, May-June 1927
Hydras as Enemies of Young Fishes
|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 beforeperhaps 6 or 8 to the square inchbut 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 diameterslender 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.
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 fishesin 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.
|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. 157–160. 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 Beardsleys 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 Beardsleys article and recounts his own experiences which antedated Beardsleys 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 Amphibiathe 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 frogs 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 fishin 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.
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.
The combat would end less happily for the little fish when it would be caught by several arms at once.
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 Trembleys 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 iteven 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 writers 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 Trembleys, one may quote the author of Ecclesiastes that there is nothing new under the sun.