Particle-size selection by Pseudopolydora paucibranchiata (Polychaeta: Spionidae) in suspension feeding and in deposit feeding: influences of ontogeny and flow speed

Abstract

Pseudopolydora paucibranchiata Okuda suspension feeds and deposit feeds at the sediment-water interface, where it is exposed to a variety of particles differing in physical characteristics and nutritional value. In flume experiments (conducted in August 1994 and May 1995) with two sizes of either suspended or deposited beads, I measured particle-size selection separately in each feeding mode. The same influences of palp width and of ambient flow speed were observed in each mode. At velocities ≤0.74 cm s^-1 there were no relationships between palp width and the proportion of gut contents composed of large beads. At velocities ≥1.8 cm s^-1 worms with narrower palps ingested relatively fewer large beads (and more small beads) than did worms with wider palps. Palp width and body length were linearly related, and results were similar when analyzed with body length as the independent variable. As flow speed increased, selectivity changed in a worm-size-specific manner: worms with a palp width <ca. 150 μm (or body length <ca. 12 mm) ingested relatively fewer large beads (and more small beads) as velocity rose, while the proportion of large beads ingested by larger worms remained generally constant. These results are most likely explained by mechanics of particle capture. Furthermore, when deposit feeding, worms of all sizes were biased toward ingesting small beads. In contrast, when suspension feeding, worms were biased toward ingesting large beads, with the exception that worms with a palp width <125 μm (or body length <8 mm) were biased for small beads in flows ≥4.0 cm s^-1. Assuming that in the field (1) particle size is the principle criterion for selection, and (2) the amount of digestible food component in deposited and suspended particles, respectively, is related to particle surface area and volume, I hypothesize that changes in selectivity as velocity rises can cause juveniles to experience a decreasing profitability of suspension feeding and a simultaneously increasing profitability of deposit feeding. Juveniles could maintain a diet of high food value despite flow variations by adjusting the proportion of time they spend suspension feeding relative to deposit feeding.