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

Microcosms permit controlled ecological experiments but the effects of smal l enclosures on predators, prey, and their interactions, compromise their r elevance to field populations. I examined the effects of Aurelia size on th e utility of impermeable mesocosms deployed in highly stable water columns. Aurelia of all sites (5-29 cm bell diameter) were observed adhered to wall s. Individuals larger than 10 cm adhered to walls in disproportionately hig h numbers, experienced a significant decline in mean size, and incurred phy sical damage at greater rates, than those less than 10 cm. These results de monstrate that, at least for impermeable walled mesocosms deployed in low t urbulence environments, larger enclosures will not necessarily (i) ameliora te wall effects for animals of the size typically manipulated in microcosms and (ii) make enclosures suitable for manipulating Aurelia larger than tho se normally enclosed in microcosms. The mechanism proposed to explain adhes ion suggests that enclosure walls alter the hydrodynamics of fluid flow aro und Aurelia, which may have implications for estimating the predation impac t of these animals since the hydrodynamics of feed and swimming have co-evo lved.