Limitations on the use of impermeable mesocosms for ecological experiments involving Aurelia sp. (Scyphozoa: Semaeostomeae)

Abstract

Microcosms permit controlled ecological experiments but the effects of small enclosures on predators, prey, and their interactions, compromise their relevance to field populations. I examined the effects of Aurelia size on the utility of impermeable mesocosms deployed in highly stable water columns. Aurelia of all sizes (5-29 cm bell diameter) were observed adhered to walls. Individuals larger than 10 cm adhered to walls in disproportionately high numbers, experienced a significant decline in mean size, and incurred physical damage at greater rates, than those less than 10 cm. These results demonstrate that, at least for impermeable walled mesocosms deployed in low turbulence environments, larger enclosures will not necessarily (i) ameliorate wall effects for animals of the size typically manipulated in microcosms and (ii) make enclosures suitable for manipulating Aurelia larger than those normally enclosed in microcosms. The mechanism proposed to explain adhesion suggests that enclosure walls alter the hydrodynamics of fluid flow around Aurelia, which may have implications for estimating the predation impact of these animals since the hydrodynamics of feeding and swimming have co-evolved.