The prey-capture mechanism of the hydromedusa Aequorea vitrina was studied by means of laboratory video-microscope observations. In stagnant water A. vitrina remains stationary with its very long (about 4x bell diameter) marginal tentacles motionless hanging down in the water, ready for ambush capture of prey organisms that collide with the tentacles. A. vitrina was found to be efficient at capturing brine shrimps ( Artemia salina ), less efficient at capturing rotifers ( Brachionus plicatilis ), and very inefficient at capturing copepods ( Acartia tonsa ). The initial hauling up of an extended marginal tentacle with an adhering prey is fast (>10 mm s -1 ). Both the bell margin and the mouth move towards each other so that the captured prey can be transferred from the tentacle to the elongated mouth-lips to be further transported into the mouth and stomach. It takes about 20 s from when an Artemia prey organism encounters a tentacle until it is transferred to the mouth-lips. The subsequent digestion in the stomach takes about 30 min. When A. vitrina encounters a jellyfish-prey (a small medusa of Aurelia aurita ), it starts to swim in order to adhere the relatively big prey to its mouth-lips. Then A. vitrina opens its mouth wide to swallow the captured medusa, a process which takes about 15 to 20 min. The subsequent digestion takes 2 to 3 h. Field observations of undisturbed A. vitrina made by snorkelling in the Limfjord (Denmark) revealed that the feeding behaviour was similar to that observed in the laboratory in stagnant water. It is concluded that A. vitrina is an ambush-predator, and not a cruising-predatory medusa as previously suggested.
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