Graduation Year

2021

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Biology (Integrative Biology)

Major Professor

Brad J. Gemmell, Ph.D.

Committee Member

Stephen M. Deban, Ph.D.

Committee Member

Susan S. Bell, Ph.D.

Committee Member

Sean P. Colin, Ph.D.

Keywords

aggregation, jellyfish, mixing, diet, porewater

Abstract

Upside-down jellyfish, Cassiopea sp., can be locally abundant in shallow mangrove habitats throughout the tropics and subtropics. Unlike other jellyfish, Cassiopea sp. are epibenthic. Due to this unique lifestyle and their ability to achieve high population densities, an understanding is needed of their ecological impacts both individually and in aggregations, particularly given that Cassiopea sp. ranges are expanding poleward due to global climate change. We quantified the fluid flow produced by Cassiopea sp. feeding currents and found that an average-sized Cassiopea sp. can transport over 200 l/h of water in the vertical excurrent jet of their feeding current. Populations of Cassiopea sp. are thus capable of fully turning over the water column every 15 minutes, in a habitat that generally receives little environmental mixing. In addition, we examined the ability of Cassiopea sp. to liberate interstitial porewater by a suction-pumping mechanism. We found that individuals can release 2.6 ml/h of porewater and entrain it into their feeding current. Besides contributing to nutrient dynamics directly through release of nutrient-rich porewater and through water column mixing, Cassiopea sp. are also planktivores and capture large numbers of zooplankton. We demonstrate that Cassiopea sp. capture primarily epibenthic prey, with 50% of their diet consisting of harpacticoid copepods, at rates that could induce top-down regulation of zooplankton communities. This selective capture of epibenthic prey is likely mediated by the structure of the feeding current, which draws water horizontally along the benthos towards the jellyfish. These ecosystem effects are amplified by the fact that Cassiopea sp. occur in dense populations of up to almost 100 animals per m2. We found that Cassiopea sp. are able to actively move along the benthos and appear to organize themselves into local patterns that both maximize defensive benefits of aggregation and minimize competition with neighbors. Given their ability to form dense aggregations and affect local hydrodynamics, nutrient transport, and zooplankton communities, Cassiopea sp. should be considered important ecosystem engineers in mangrove ecosystems.

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