Behavioural modification of local hydrodynamics by asteroids enhances reproductive success

The reproduction of apex species, such as sea stars, is important for sustaining many marine ecosystems. Many sea star species reproduce externally, introducing gametes in the turbulent benthic boundary layer. Sea stars often aggregate and adopt characteristic behaviour, such as arched posturing, while spawning. Here we quantify, for the first time, the hydrodynamic advantages of postural changes and the extent to which they enhance the efficiency of external reproduction. Hydrodynamic and fertilisation kinetic theoretical modelling were used to provide context and comparison. The arched posture was clearly important in the downstream advection of gametes. Digital particle image velocimetry, acoustic doppler velocimetry and dye release experiments indicated reduced wake and lower shear stresses downstream of arched sea stars, which increased downstream transport of gametes compared to those in the flat position. In all cases, sperm concentration decay rates of two orders-of-magnitude over distances < 20 cm were inferred from fluorometry, confirming the requirement for close aggregation. The level of turbulence and hence downstream gamete dilution was increased by greater current speeds and a rougher seabed. Both an arched posture and hydrodynamic conditions may improve external reproduction efficiency, with behavioural mechanisms providing the primary contribution.