The origins of Hydra Medium
The first description of a defined medium for culturing Hydra in the laboratory was published by Loomis in 1953 (Loomis, 1953). This medium contained calcium chloride, sodium bicarbonate, and sodium chloride in deionized or distilled water. The story of how this medium was developed is an amusing one, and is told by Howard Lenhoff in the introduction to his Hydra research methods book (Lenhoff, 1983). Various versions of Hydra medium have been used over the years, and the importance of various components of the medium have been tested (Lenhoff, 1966; Lenhoff and Bovaird, 1960; Loomis, 1954). Calcium is the most critical component of the medium.
Some recipes for Hydra medium that you may encounter are odd. For example, a commonly used version, that we got from Hans Bode’s lab many years ago, has potassium nitrate as one of the components, even though there is no evidence that Hydra needs nitrate, and potassium has traditionally been supplied in Hydra medium in the form of KCl. This medium also contains both magnesium sulfate and magnesium chloride. It is unknown why magnesium is supplied in two forms in this medium and there is no evidence that Hydra needs sulfate in the medium. Plenty of chloride is supplied in the form of calcium chloride in this medium, so the magnesium chloride isn’t needed to provide chloride. The version of Hydra medium used in the Bosch lab contains potassium in the form of potassium carbonate. There is no obvious reason for using potassium carbonate instead of the traditional potassium chloride.
pH
You may see some recipes for Hydra Medium in which Tris is added as a buffer and the pH is adjusted by adding HCl. There is no need for this. The sodium bicarbonate in Hydra Medium buffers it to an appropriate pH (between 7.5 and 8) without the need for a synthetic buffer such as Tris or the addition of anything to adjust the pH. Convenient, huh?
How much Hydra Medium should you make at a time?
Typically Hydra medium is made up in 20 liter batches using several concentrated stock solutions. For the past several years, we have been making Hydra Medium in 100 liter batches. This has several advantages. First, it gets rid of the need to make up stock solutions of salts. We simply add the solid components individually to 100 liters of water, with mixing between the addition of each component with a large plastic paddle. In the past we have found that the spigots on Nalgene 20 liter carboys have a tendency to leak as they get older, and the screw-on lids of the carboys have a tendency to develop mold growth on their insides. We have not had these problems with the 100 liter (30 gallon) tanks. While 100 liters of Hydra Medium seems like a lot, a typical Hydra lab can go through 100 liters relatively quickly, especially if a significant number of large cultures in trays are being kept. In fact, we maintain two 100 liter tanks, so that when one is used up we have another one to go to immediately.
The 100 liter tanks we use come with a spigot and a cover. We bought them from Thermo Fisher Scientific. The catalog number is 11102-0030. The tank is manufactured by Nalgene and is made out of linear low-density polyethylene. The valve has two PTFE O-rings. We have had no leakage problems with this valve.
What kind of water should you use?
We used to use Nanopure water to make up our Hydra medium. Other labs use MilliQ water. Either is fine. We didn’t use house deionized water, because of uncertainty about the state of such water. When our department’s Nanopure system was down a couple of years ago, we had to switch to house deionized water. We noticed no difference in the health of our Hydra cultures, and we have continued to use house deionized water ever since.
Our recipe for Hydra Medium
Our current Hydra medium recipe (Hydra Medium 4.0) is shown below. It consists of only four components, and has the traditional amount of calcium chloride (1 mM). There is nothing special about the sources of the components. We buy them as reagent grade from Thermo Fisher. We chose the sulfate salt of magnesium instead of the chloride salt because it is much easier to weigh accurately. Magnesium chloride has a strong tendency to absorb moisture from the air in a process called deliquescence. This makes it difficult to weigh magnesium chloride accurately. We have used this medium successfully with Hydra vulgaris, Hydra oligactis, Hydra viridissima, Hydra canadensis, Hydra oxycnida, and Hydra hymanae.
| Component | Amount for 100 liters | Final concentration |
| Calcium chloride dihydrate | 14.7 grams | 1.0 millimolar |
| Magnesium sulfate, anhydrous | 3.97 grams | 0.33 millimolar |
| Sodium bicarbonate | 4.2 grams | 0.5 millimolar |
| Potassium chloride | 0.22 grams | 0.03 millimolar |
REFERENCES
Lenhoff, H.M., ed. (1983). Hydra: Research Methods (New York: Plenum Press).
Lenhoff, H.M., and Bovaird, J. (1960). The requirement of trace amounts of environmental sodium for the growth and development of Hydra. Exp. Cell Res. 20, 384-394.
Loomis, W.F. (1953). The cultivation of hydra under controlled conditions. Science 117, 565-566.
Loomis, W.F. (1954). Environmental factors controlling growth in hydra. J, Exp, Zool. 126, 223-234.
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