Nutritional physiology and colony form in Podocoryna carnea (Cnidaria: Hydrozoa)

Abstract. We compared growth rates and final morphological states of the athecate colonial hydroid Podocoryna carnea in two nutritional environments: one varying the quantity of food provided at a fixed interval and the second varying the time between feedings of a fixed quantity. In both environments, replicate colonies were either fed uniformly, or fed on only one side and starved on the other. In addition, we fed colonies fluorescence-labeled cultures of Artemia salina and documented the subsequent distribution of label. We found that both the growth rates and the final morphological state varied logarithmically with food supply. Heterogeneous feeding had a marked effect on colony morphology, with a sharp boundary in polyp number, stolon density, and polyp size forming at the fed–unfed interface. The distribution of fluorescence was correlated with sites of colony growth. These results confirm and extend early work on the priority of growth zones in colonial hydroids, and present new challenges for understanding the relationship among energy metabolism, gastrovascular circulation, and colony form.