Pii: S0025-3227(00)00040-2

Fries et al. (1999), hereafter referred to as FBW, present interesting experimental data showing that biogenic mounds, formed by the conveyor-belt feeding polychaete Cistenides (Pectinaria) gouldii, can be reshaped into asymmetric bedforms at bed shear stresses below the Shields threshold of sediment movement. The ®nal geometry of the current ripples was found to depend on the spatial density of the mounds (number of mounds per unit area), mound height and the imposed bed shear velocity. In a series of ̄ume experiments at different boundary conditions, FBW distinguish between beds with migrating ripples (MR), beds with stationary ripples (SR) and beds without ripples (NR). We believe, however, that such a distinction is ̄awed. Recent advances in the stability analysis of current ripples demonstrate the existence of predictable sequences of current-ripple geometry during evolution towards equilibrium bed morphology (Baas, 1994, 1999; Oost and Baas, 1994). This discussion illustrates how the principles of time-dependent bedform development can be applied to FBW's data. In addition, we discuss the physical parameters governing the generation of current ripples from bed defects, which de®ne the threshold conditions (in terms of mound height, spacing and ̄ow strength) for which biogenic mounds are expected to generate current ripples. Finally, we highlight the inappropriateness of the terms asubcriticalo and asupercriticalo for describing the threshold of sediment movement.