In 1982, Jennie O’Hern and Howard Lenhoff published a paper (O’Hern and Lenhoff, 1982) describing their study on the relationship between the diameter of the hypostome and the number of tentacles in the Hydra head. O’Hern was an undergraduate student at UC Irvine in the Lenhoff lab who subsequently went to medical school at Johns Hopkins and practiced as an obstetrician-gynecologist. She died of cancer at the age of 38.

Using Hydra species and strains with various hypostome diameters, O’Hern and Lenhoff found that there is a linear relationship between the diameter of the hypostome and the number of tentacles. In their paper, they briefly described an experiment that was done in the lab of Hans Bode at UC Irvine which provided important experimental data regarding the establishment of tentacle number in the Hydra head. The results of this experiment have, as far as I can tell, never been formally published, but they were also described in a subsequent book chapter by Hans and Pat Bode (Bode and Bode, 1984). I recall Hans describing this experiment at various times as well. Here is how the experiment is described by O’Hern and Lenhoff (1982):

“… tentacles tend to fill up all available hypostomal space. This feature … has also been observed with hydra whose hypostomes were experimentally enlarged by inserting nylon fish line through their bodies. Such hydra with enlarged hypostomes developed additional tentacles.”

Bode and Bode (1984) described their experimental results as follows:

“This is demonstrated quite dramatically when decapitated animals are strung onto fishline of varying diameters to stretch the tissue in the circumferential dimension. Increased tentacle numbers up to fifteen were obtained, as compared to the normal six.”

In describing the experiment in conversations, Hans Bode suggested that the stretching of the hypostome created tissue between two existing tentacles in which lateral inhibition of tentacle formation produced by the existing tentacles had dropped to a level that allowed a new intercalary tentacle to be produced.

REFERENCES

Bode, P.M., Bode, H.R., 1984. Patterning in Hydra, in: Malacinski, G.M., Bryant, S.V. (Eds.), Pattern Formation, A Primer in Developmental Biology. Macmillan, New York, pp. 213-241.

O’Hern, J., Lenhoff, H.M., 1982. Relationship between size of hypostome and number of tentacles in Hydra. J. Exp. Zool. 221, 1-7.