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Hydra(Cnidaria), the basal metazoan most often studied incellular, molecular anddevelopmental biology, is difficult to measure because it is small, proteiform and aquatic. To facilitate broader organismal and ecological study of Hydra, we evaluated three methods, whereby a polyp's body column can be measured by means of photomicroscopy. The volume, cylindrical surface area and surface area corrected for changes in body shape were all highly repeatable methods (r.0.97) when shape varied little. However, shape changes altered volume and cylindrical surface area. Repeated corrected surface-area measurements of the same individuals in markedly different positions yielded standard deviations ,5% of the mean measured area. This easy, non-lethal means of individual size measurement explicitly accounts for the flexible morphology of a polyp's hydrostatic skeleton. It therefore allows for the elucidation of how growth and size vary overtime,age andfoodintake.Wefoundthat hydrachangedsizedramatically fromdaytoday,andthatalthoughthe food level influenced adult size, it had little effect on the early growth of recently detached buds. Finally, we discuss ecological and biological applications of this method. Additional keywords: food quantity, freshwater polyp, Hydra, individual size, photomicroscopy.

The consistent, non-destructive measurement of small proteiform aquatic animals, with application to the size and growth of hydra