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Larval Swimming Overpowers Turbulent Mixing and Facilitates Transmission of a Marine Parasite

Jonathan T. Fingerut, Cheryl Ann Zimmer and Richard K. Zimmer
Ecology
Vol. 84, No. 9 (Sep., 2003), pp. 2502-2515
Published by: Wiley
Stable URL: http://www.jstor.org/stable/3450153
Page Count: 14
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Larval Swimming Overpowers Turbulent Mixing and Facilitates Transmission of a Marine Parasite
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Abstract

Planktonic cercariae (parasite larvae) of digenetic flatworms (Himasthla rhigedana) encyst up to 100% of intermediate host populations. Toward explaining such high prevalence, larval behavior and passive-transport processes were evaluated experimentally for their roles in waterborne parasite transmission. Using a new application of laser and digital video imaging technologies, we quantified cercarial movements in still water and in simulated field flows. In still water, downward swimming in response to light, irrespective of intensity or source, and gravity brought larvae to the bottom three-times faster than gravitational sinking alone. A 33% elevation in temperature (18-24⚬ C) caused a 71% increase in swim speed. In flume flows characteristic of southern California salt marshes (u* = 0.2 cm/s, occurring >80% of the time), vertical larval distributions were highly bottom skewed. The mean downward swim speed (0.59 cm/s at 24⚬ C) was three times faster than turbulent fluctuations (w' = 0.23 cm/s), indicating that cercariae overpowered eddies to reach the bed. When flume flows simulated rare storm events (u* = 0.8 cm/s, w' = 0.95 cm/s, occurring ≤ 1% of the time), turbulence overwhelmed behavioral effects and homogeneously mixed larvae throughout the water column. Environmentally triggered downward swimming can thus quickly bring larvae to the bed, promoting contact with benthic intermediate hosts (snails and crabs) under most flow conditions. The efficacy of vertical swimming in typical marsh flows identifies larval behavior, not passive transport, as the principal mechanism responsible for dispersal to host habitat.

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