Hydrodynamic mediation of density-dependent growth and adult-juvenile interactions of a spionid polychaete

Abstract

We performed an experiment to test the effects of adult density on the growth rates of juvenile Polydora cornuta. Our experiment was performed in three identical counter-rotating annular flumes, each set to one of three different flow speeds for a period o f3d( U5mm 5 3, 6, or 12 cm s 21 , where U5mm 5 velocity measured 5 mm above bottom). We implanted replicate vials containing a premeasured juvenile P. cornuta and either 0, 2, or 5 adults into a 2-cm layer of sediment in the flume. The relative growth rates of the juveniles were determined by measuring each juvenile’s body volume before and after the 3-d deployment in the flumes. We repeated the experiment three times. The growth rates of juveniles were negatively affected by adult density at the two slower flows, but there was no evidence of negative density-dependent growth at the fastest flow. In the two slower flows, the mean relative growth rate of juveniles declined from 53 to 34% d 21 as the number of adults increased from 0 to 5. Fecal mounds accumulated near adults in the two slower flows, but mounds were absent in the fastest flow. The hydrodynamic mediation of the effects of density suggests that the strength of density-dependent and adult‐juvenile interactions in nature are likely to be mediated by flow. Spionids should be able to achieve higher population densities at places where or times when faster flows facilitate suspension feeding and reduce the effects of biogenic disturbance of the sediment‐water interface.