Model-assisted measurements of suspension-feeding flow velocities

Benthic jellyfish dominate water mixing in mangrove ecosystems

Water mixing is a critical mechanism in marine habitats that governs many important processes, including nutrient transport. Physical mechanisms, such as winds or tides, are primarily responsible for mixing effects in shallow coastal systems, but the sheltered habitats adjacent to mangroves experience very low turbulence and vertical mixing. The significance of biogenic mixing in pelagic habitats has been investigated but remains unclear. In this study, we show that the upside-down jellyfish Cassiopea sp. plays a significant role with respect to biogenic contributions to water column mixing within its shallow natural habitat ([Formula: see text] m deep). The mixing contribution was determined by high-resolution flow velocimetry methods in both the laboratory and the natural environment. We demonstrate that Cassiopea sp. continuously pump water from the benthos upward in a vertical jet with flow velocities on the scale of centimeters per second. The volumetric flow rate was calculated to be 212 L⋅h^-1 for average-sized animals (8.6 cm bell diameter), which translates to turnover of the entire water column every 15 min for a median population density (29 animals per m²). In addition, we found Cassiopea sp. are capable of releasing porewater into the water column at an average rate of 2.64 mL⋅h^-1 per individual. The release of nutrient-rich benthic porewater combined with strong contributions to water column mixing suggests a role for Cassiopea sp. as an ecosystem engineer in mangrove habitats.

Overcoming hydrodynamic challenges in suspension feeding by juvenile Mya arenaria clams

We present some of the few suspension-feeding measurements and to our knowledge the first velocity-field measurements for early post-settlement juvenile bivalve clams. We verify and extend our experimental results with numerical simulations. For 1.8-2.8 mm shell length Mya arenaria clams, pumping rates ranged 0.03-0.22 μl s-1, inhalant siphon Reynolds numbers (Re) ranged 0.16-0.79 and mean inhalant velocities ranged 0.8-3.2 mm s-1 Owing to the low Re at which they pump and the small diameters of their siphons, juvenile clams are subject to unique hydrodynamic challenges, including high siphon resistance and susceptibility to refiltration. At least three features of juvenile clam siphons differentiate them from those of adults-shorter inhalant siphon length, a more rapid increase in inhalant siphon diameter with shell length, and the presence of a prominent exhalant siphon extension. These features are probably adaptations to the challenges of suspension feeding at low Re.