AbstractThe cnidocysts and the lytic, cyto- and neurotoxic potency of a large specimen of Cyanea capillata (L.) with 55 cm umbrella diameter were compared with those of a pooled C. capillata sample (average ø 14 cm) in order to investigate organismal developments at a cellular and biochemical level. Nematocysts of the type A-isorhiza in both fishing tentacles and oral arms and the O-isorhizas of oral arms were enlarged in the 55 cm specimen. Additionally, the number of nematocysts per battery in the fishing tentacles was increased. Increased gill cell toxicity and neurotoxic activity were demonstrated for the fishing tentacle venom of the 55 cm C. capillata in comparison with the smaller medusae. A two-fold higher haemolytic activity was detected for the venom of oral arms obtained from the large C. capillata compared with the oral arm venom prepared from the smaller medusae.