Morphology, swimming performance and propulsive mode of six co-occurring hydromedusae
Sean P. Colin1,* and
John H. Costello2
1 Department of Marine Sciences, University of Connecticut, 1084 Shennecossett Road, Groton, CT 06340, USA and
2 Biology Department, Providence College, Providence, RI 02918-0001, USA
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Fig. 1. Representative examples of the hydromedusae from Friday Harbor, WA, USA, selected for comparative study. Medusae are shown with their bells relaxed and are drawn to scale among species. Aequorea victoria and Mitrocoma cellularia can grow to be twice as large as depicted.
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Fig. 2. Representative kinematic profiles of individual prolate medusae. All graphs in the left-hand column refer to Aglantha digitale (2.03 cm height, 0.83 cm diameter), in the middle column to Sarsia sp. (0.91 cm height, 0.85 cm diameter) and in the right-hand column to Proboscidactyla flavicirrata (0.40 cm height, 0.56 cm diameter). Note the differences in the x and y axes. Re, Reynolds number; Fi, instantaneous fineness ratio.
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Fig. 3. Representative kinematic profiles of individual oblate medusae. All graphs in the left-hand column refer to Aequorea victoria (2.09 cm height, 5.00 cm diameter), in the middle column to Mitrocoma cellularia (2.03 cm height, 6.50 cm diameter) and the right-hand column to Phialidium gregarium (0.84 cm height, 2.14 cm diameter). Note the differences in the x and y axes. Re, Reynolds number; Fi, instantaneous fineness ratio.
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Fig. 4. Relationships between bell morphology and swimming performance for individuals representing six species of co-occurring hydromedusae from Friday Harbor, WA, USA. Performance variables (maximum velocity and maximum acceleration) and velar aperture ratios of individual medusae are described as a function of both bell diameter and fineness. Grey and black symbols denote prolate and oblate medusan forms, respectively. Re, Reynolds number.
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Fig. 5. Observed acceleration (Ao, filled symbols) and modeled acceleration (Am, open symbols) from the force balance equation (equation 6) of Sarsia sp. (A) and Phialidium gregarium (B) during the swimming profiles illustrated in Figs 2 and 3.
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Fig. 6. Means and standard deviations (N=3) of the maximum observed (Ao, open symbols) and modeled (Am, filled symbols) accelerations achieved during each pulse of the swim cycle for the six species of hydromedusae studied. Asterisks designate a significant difference between Am and Ao (ANOVA; *P<0.05).
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Fig. 7. Flow patterns around swimming hydromedusae. Arrows represent particle paths at the end of the effective phase of bell pulsation. Arrow lengths represent particle velocities. The bars to the right of each species’ name are a scaling reference and represent a particle velocity of 3 cm s–1. Medusae diameters (cm) were as follows: Aglantha digitale, 0.68; Sarsia sp., 0.8; Proboscidactyla flavicirrata, 0.54; Aequorea victoria, 4.32; Mitrocoma cellularia, 6.8; Phialidium gregarium, 2.2. Medusae are drawn at maximum contraction.
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Fig. 8. Schematic diagram illustrating the change in bell shape during bell contraction of swimming prolate (A) (e.g. Sarsia sp.) and oblate (B) (e.g. Phialidium gregarium) medusae and the region of maximum flow (arrows) around each medusa. Bell shapes are based on the measured fineness ratios of each medusa at minimum and maximum contraction.
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© The Company of Biologists Ltd 2002
