Sound scattering by the gelatinous zooplankters Aequorea victoria and Pleurobrachia bachei

Target strengths were determined for individual Aequorea victoria and Pleur obrachia bachei using separate 200, 420 and 1,000 kHz transducers in an enc losure deployed off the dock of the Friday Harbor Laboratories in northern Puget Sound, Washington, USA. The target strength for Aequorea, when averag ed over all animal sizes and all three transducer frequencies, was -72.6 dB . The target strength for Pleurobrachia was -75.9 dB when similarly average d. Average reduced target strengths for Aequorea and Pleurobrachia were -43 .3 and -33.1 dB, respectively. Differences in the sound scattering contrast s for Pleurobrachia and Aequorea are proposed to explain the larger reduced target strength of Pleurobrachia, relative to that of Aequorea. A new sound scattering model was developed to predict the backscattering cr oss-section of Aequorea. The model takes into account expected variations i n target strength due to changes in shape of the jellyfish as it propels it self through the water. The scattering model was tuned specifically for Aeq uorea in the present study, but has a general formulation that is widely ap plicable to any medusa-shaped gelatinous zooplankter. (C) 1998 Elsevier Sci ence Ltd. All rights reserved.