Causes and consequences of stolon regression in a colonial hydroid

A cnidarian colony can be idealized as a group of feeding polyps connected by tube-like stolons. Morphological variation ranges from runner-like forms with sparse polyp and stolon development to sheet-like forms with dense polyp and stolon development. These forms have typically been considered in a foraging context, consistent with a focus on rates of polyp development relative to stolon elongation. At the same time, rates of stolon regression can affect this morphological variation; several aspects of regression were investigated in this context. More sheet-like forms were produced by periodic peroxide treatment, which induced high rates of stolon regression. Caspase inhibitors altered the effects of regression induced by peroxide or vitamin C. These inhibitors generally diminished physical regression and the abundance of associated reactive oxygen species. Caspase inhibitors also altered cellular ultrastructure, resulting in features suggestive of necrosis rather than apoptosis. At the same time, caspase inhibitors had little effect on reactive nitrogen species that are also associated with regression. Although regression is most easily triggered by pharmacological perturbations related to reactive oxygen species (e.g. peroxide or vitamin C), a variety of environmental effects, particularly restricted environments and an interaction between feeding and temperature, can also induce regression. Stolon regression may thus be a factor contributing to natural variation between runners and sheets.