TENTACLE NUMBER IN CULTURED HYDRA VIRIDIS

¹ Department of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 15260

1. The distribution of tentacle numbers on Hydra viridis, cultured in the laboratory were studied in order to gather some appreciation of the normal range of tentacle number, and how that number is influenced by vegetative reproduction, and by contingencies which alter the size of a hydra.

2. Large variation among individuals selected at random was found to be the predominant statistical factor in the analysis of variance of tentacle number and cautions against drawing inferences from small differences in sample mean tentacle numbers. Tentacle number is normally distributed and stable for each individual from the day before detaching as a bud for periods as long as 2 weeks despite changes in feeding schedule and temperature.

3. The mean tentacle number on samples of buds produced over time is subject to change. Tentacle number on buds can be negatively correlated with tentacle number on parents and decreases both when the normal daily feeding schedule is changed to a 3-days a week schedule (whether in succession or with intervals between feedings), or increased to twice-daily feeding. Buds produced at higher temperatures also have lower tentacle numbers. Conditions which might raise the metabolic rate of the parent hydras, therefore, can reduce tentacle numbers on buds.

4. Hydras wounded through the gastric region regenerate the same number of tentacles despite differences in size. Regeneration yields tentacle numbers tending to gravitate toward a mean for the population independent of the original tentacle number. Both increases and decreases are recorded. Animals enlarged by grafting, however, tend to regenerate fewer tentacles than originally present even when double-heads regenerate as one. Regeneration, like budding, can alter the population's mean tentacle number according to prevailing conditions.