Growth rates of interface-feeding polychaetes: combined effects of flow speed and suspended food concentration

Abstract

Most spionid polychaetes switch from deposit feeding to suspension feeding as the current speed and the flux of suspended food particles increase. Previous experiments testing the effects of flow on the growth rates of facultative, interface feeders have been limited to a single concentration of suspended food. We performed an experiment in counter-rotating annular flumes set to one of 3 flow speeds and 2 concentrations of suspended microalgae. This design allowed the horizontal flux of algae (i.e. concentration multiplied by current speed) to be manipulated by 2 independent means. The relative growth rates of Polydora cornuta and Streblospio benedicti were measured during a series of 3 d runs in replicate flumes set to each of the 6 food-flow treatments. Sediment collected from the worms' field site was added to each flume as a food source for deposit-feeding worms. The 3 flow speeds (U 5mm = 3, 6, or 12 cm s -1 , where U 5mm = velocity measured 5 mm above bottom) were all slower than the sediment's critical erosion velocity (U 5mm = 13 cm s -1 ; shear velocity U * = 0.9 cm s -1 ). P. cornuta grew significantly faster as flow increased in the lower algal concentration, but not in the higher one. The higher algal concentration led to faster growth of P. cornuta at the 2 slower flows, but there was no effect of algal concentration at the fastest flow speed. Overall, the growth rate of P. cornuta increased as the horizontal flux of suspended food increased up to a value that led to a maximal growth rate, regardless of whether the flux of suspended food was manipulated by flow speed or by algal concentration. In contrast, S. benedicti did not show significant relationships between growth rate and either flow speed, algal concentration, or the overall flux of suspended food. The data demonstrate that the juvenile growth rates of some interface-feeding spionids, such as P. cornuta, can be tied strongly to hydrodynamic conditions that favor passive suspension feeding, while the growth rates of other interface-feeding spionids, such as S. benedicti, can be less sensitive to flow.