Pore Water Release and Mixing by Epibenthic Upside-down Jellyfish

Manikantam Goud Gaddam, Oklahoma State University, Mechanical and Aerospace Engineering, Stillwater, OK, United States and Arvind Santhanakrishnan, Oklahoma State University, Mechanical & Aerospace Engineering, Stillwater, United States
Abstract:
Organisms in sheltered marine environments have to often rely on self-generated flows for feeding and nutrient exchange. The epibenthic upside-down jellyfish, Cassiopea spp., is found in patchy aggregations in shallow, protected areas with low background flow conditions (e.g., mangrove swamps, seagrass beds). Previous studies on Cassiopea individuals have shown that bell pulsations can assist in suspension feeding and also release dye initially present underneath the substrate. However, the physical mechanisms underlying pore water release and mixing in the water column are not understood. The goal of this study was to characterize pore water release and mixing by Cassiopea medusae of bell diameters ranging from 2 cm to 6 cm in initially quiescent background flow conditions. Planar laser-induced fluorescence (PLIF) and 2D particle image velocimetry (PIV) measurements were conducted on Cassiopea individuals in a laboratory aquarium to visualize flow generated by bell pulsations and interaction with pore water released from the substrate. Fluorescent dye, initially located 2 cm below the substrate, was released into the water column by bell pulsations of all Cassiopea individuals tested in this study. Near-field PLIF measurements showed that during the power stroke of Cassiopea medusae, pore water was drawn through starting vortices generated near the bell margin. The oral arms broke down large-scale coherent structures into small-scale structures during the recovery stroke. Smaller individuals pulsated frequently and relied on interaction of starting vortices from multiple pulsing cycles to mix the released pore water with ambient water. In contrast, larger medusae pulsated less frequently but generated wider jets that aided in pore water mixing. These findings suggest that Cassiopea medusae can assist in benthic-pelagic coupling by acting as suction pumps.