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Sean Berg - Marine & Estuarine Ecology

College of the Atlantic, Maine

Observations of Ctenophore Abundance Across the Tidal Cycle:
The Potential for Tidally Cued Vertical Migration

Previous unquantified observations suggested that the lobate ctenophore, Mnemiopsis leidyi, appears at the surface of the waters it inhabits at high tide and up to an hour thereafter. Such behavior, known as tidally cued vertical migration, would be advantageous for the ctenophore, which cannot propel itself horizontally against currents. By positioning itself vertically in the water column, M. leidyi could rely on advective transport in place of its limited locomotive abilities. To determine if M. leidyi exhibits tidally cued vertical migration, surface tows were completed at known points in the tidal cycle. The study site was the research pier at the Chesapeake Biological Laboratory in Solomons, Maryland. The tows were made using a 1/2 meter diameter, 202 micron mesh size net and occurred an hour before high or low tide and continued at twenty minute intervals until an hour after high or low tide. The volume of each tow ranged from approximately 18 cubic meters to 22 cubic meters. Analysis of the data revealed that there was no apparent peak in abundance at any of the observed points for either low or high tide, suggesting tidally cued vertical migration was not occurring. Additionally, a comparison of the active towing method used in this project and the passive towing method used by another research group performing a historical reconstruction of sea nettle abundance showed that average densities (ctenophores per cubic meter) were similar for both methods. This implies that active towing is an acceptable means of determining density of ctenophores in the water column and lends support to the validity of the finding of that tidally cued vertical migration is not observed in Chesapeake Bay for M. leidyi.