JELLYFISH BLOOMS Kinematic properties of the jellyfish Aurelia sp. Tom Bajcar Æ Vlado Malac ˇic ˇ Æ Alenka Malej Æ Brane S ˇ irok Published online: 22 September 2008 Ó Springer Science+Business Media B.V. 2008 Abstract A new, relatively simple method for determining the kinematic properties of jellyfish is presented. The bell movement of the scyphomedusa (Aurelia sp.) during its pulsation cycle was analysed using computer-aided visualization. Sequences of video images of individual Aurelia in a large aquarium were taken using a standard video camera. The images were then processed to obtain time series of the relative positions of selected points on the surface of the medusa’s bell. The duration of the bell relaxation was longer than that of the bell contraction, thereby confirming published results. In addition, the area of the exumbrellar surface of Aurelia increased during bell relaxation by more than 1.3-times that of the exumbrellar surface area during the maximum con- traction of the bell. The volume change during the bell pulsation cycle was also measured using the same visualization method. Significant changes, of up to 50%, in the subumbrellar cavity volume were revealed while, in contrast, the volume between the exumbrellar and subumbrellar surfaces generally remained unchanged during the entire pulsation cycle of the bell. Comparison of the time series of the exumbrellar surface area and of the subumbrellar cavity volume indicated that the change of volume takes place before the change of the surface area of the bell. Keywords Scyphozoa  Computer-aided visualization  Bell pulsation Introduction Jellyfish is a key predator in many pelagic environ- ments worldwide. Accumulating evidence indicates an increase in the frequency and intensity of gelatinous plankton outbreaks (Brodeur et al., 1999; Hays et al., 2005; Attril et al., 2007) as well as expansion of the range of their distribution (Graham, 2001; Graham et al., 2003; Malej & Malej, 2004). When very abundant, medusae play an important trophic role and seriously affect zooplankton populations such as ichthyoplankton and, consequently, fisheries (Purcell & Arai, 2001). The mechanics of prey capture vary between medusae, which influences prey selection Guest editors: K. A. Pitt & J. E. Purcell Jellyfish Blooms: Causes, Consequences, and Recent Advances T. Bajcar (&)  B. S ˇ irok Faculty of Mechanical Engineering, University of Ljubljana, As ˇkerc ˇeva 6, 1000 Ljubljana, Slovenia e-mail: tom.bajcar@fs.uni-lj.si B. S ˇ irok e-mail: brane.sirok@fs.uni-lj.si V. Malac ˇic ˇ  A. Malej National Institute of Biology, Marine Biology Station Piran, Fornac ˇe 41, 6330 Piran, Slovenia V. Malac ˇic ˇ e-mail: malacic@mbss.org A. Malej e-mail: malej@mbss.org 123 Hydrobiologia (2009) 616:279–289 DOI 10.1007/s10750-008-9589-4