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.KEY WORDS: Benthic boundary layer · Deposit feeding · Flume · Growth rate · Polydora · Spionid polychaete · Streblospio · Suspension feeding Resale or republication not permitted without written consent of the publisher
Chemical defenses are thought to be infrequent in most soft-sediment systems because organisms that live beneath the sediment rely more on avoidance or escape to reduce predation. However, selection for chemical deterrence might be strong among soft-sediment organisms that are sessile and expose at least part of their body above the surface. The phoronid Phoronopsis viridis is a tube-dwelling lophophorate that reaches high densities (26 500 m -2 ) on tidal flats in small bays in California, USA. We found that P. viridis is broadly unpalatable, and that this unpalatability is most apparent in the anterior section, including the lophophore, which is exposed to epibenthic predators as phoronids feed. Experimental removal of lophophores in the field increased the palatability of phoronids to predators; deterrence was regained after 12 d, when the lophophores had regenerated. Extracts of P. viridis deterred both fish and crab predators. Bioassay-guided fractionation suggested that the active compounds are relatively non-polar and volatile. Although we were unable to isolate the deterrent metabolite(s), we were able to rule out brominated phenols, a group of compounds commonly reported from infaunal organisms. One predator, juvenile Scorpaenichthys marmoratus, consumed all P. viridis extracts and whole P. viridis. However, over time, this predator rejected phoronids when fed a diet that included 2 to 3 phoronids d -1. The broadly effective defenses possessed by phoronids, which may be unusual among soft-sediment invertebrates, may play a key role in allowing them to reach high densities.
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 of 3 d (U 5mm ϭ 3, 6, or 12 cm s Ϫ1, where U 5mm ϭ 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 Ϫ1 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.
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