Hydrodynamic forces on larvae affect their settlement on coral reefs in turbulent, wave-driven flow

Abstract

This study investigates a key aspect of how ambient flow affects the recruitment of water-dispersed marine larvae onto benthic substrata: the sweeping away of larvae that have landed on surfaces. Through a combination of field and laboratory measurements, we studied how waves interact with a complex substratum to affect hydrodynamic forces encountered by microscopic larvae sitting at different positions on the bottom terrain. We used larvae of the nudibranch Phestilla sibogae settling onto coral reefs as the system to address this question. Laser Doppler anemometry was utilized within a laboratory flume to measure water velocities encountered 200 mm from coral surfaces by microscopic larvae sitting at different locations within a reef of the branching coral Porites compressa. Comparing wave-driven flow, based on conditions measured over P. compressa reefs in Hawaii, with unidirectional flow of the same mean velocity, we found that peak shear stresses along coral surfaces were ,15 times greater and hydrodynamic forces on larvae were ,10 times higher in wave conditions. Peak forces on larvae sitting on the reef top were 3-10 times greater than on larvae 5-10 cm below the reef top. Intermittent bursts of high velocity, which occurred more often at the reef top than within it, determined the settlement probability for larvae in different reef microhabitats. The faster and more strongly a larva sticks to a surface, the higher the probability of successful settlement. Using values for P. sibogae adhesive strength, we predict they can settle only on sheltered surfaces within reefs.

Turbulent shear spurs settlement in larval sea urchins

“…Other habitat properties also may be assessed by larvae once they contact the seafloor. For example, larvae often prefer certain substratum textures and topographies, light intensities, or local water velocities (9)(10)(11)(12)(13).…”

Hydrodynamic sensing and behavior by oyster larvae in turbulence and waves

“…Turbulence affects many well-studied aspects of larval life cycles, including external fertilization, predation and settlement fluxes (Abelson and Denny, 1997;Kiørboe, 2011;Crimaldi and Zimmer, 2014); in contrast, waves have better-known influences on benthic organisms (Denny, 1988). Accelerations have been explored as a sensory cue only in the context of organism-induced vibrations (Lang, 1980;Heuch and Karlsen, 1997;Kiørboe et al, 1999), and wave motions have only recently been investigated as a habitat characteristic for larvae outside the surf zone (Reidenbach et al, 2009;Koehl et al, 2013). Both deformations and accelerations are largest in the surf zone, but these signals have unique joint distributions in other seascapes: in sheltered inlets and estuaries, large deformations can occur with small accelerations, whereas on the continental shelf and open ocean, small deformations can occur with large accelerations (H.L.F.…”

Causes of decoupling between larval supply and settlement and consequences for understanding recruitment and population connectivity

“…In these conditions, larval settlement probability will be highest if larvae settle 10 quickly and adhere. Roughness elements enhance instantaneous turbulent flows that will result in 11 decreased probability of settlement on surfaces such as corals (Reidenbach et al, 2009). 12 Variability in roughness, however, creates microhabitats with reduced shear stress.…”