Ontogenetic changes in particle-size selection by deposit-feeding spionid polychaetes: The influence of palp size on particle contact

Scaling arguments suggest that juveniles of species that deposit feed as adults must ingest relatively higher quality diets to compensate for a digestive constraint imposed by small body (and gut) size. Understanding how and why particle-size selection varies ontogenetically can provide clues to the relative quality of forage juveniles obtain. Particle selection by a tentacle or palp feeder can be viewed as the net result of particle contact, collection, and post-collection rejection. A geometrical model of particle contact that includes effects due to palp size predicts that thinner palps and hence smaller worms are biased toward contacting larger particles. The model's prediction was tested using adhesive-coated palp mimics (monofilament line) of varying diameters spanning the range of actual palps to contact and collect particles from a mixture of two sizes of glass beads (16 and 88 mu m dia). Results were consistent with the contact model's prediction: thinner palp mimics collected a lower proportion of small beads (higher proportion of large beads). Intraspecific variations in particle-size selection due to body,and palp size were determined for Pseudopolydora kempi japonica Imajima and Hartman, Palydora ligni Webster, Boccardia probascidea Hattman, and Pygospio elegans Claparede by conducting laboratory feeding experiments with the same mixture of glass beads used with mimics. P. kempi japonica results were consistent with the contact model and the palp-mimic experiment. B. proboscidea and P. elegans showed no significant variation with body size. Polydora ligni showed a negative relationship between particle size and body (and palp) size; insufficient mouth size of small juveniles is a likely explanation. Assuming that food value (per unit volume of ingested material of sedimentary detritus is negatively correlated with particle size, results suggest that deposit-feeding juveniles cannot increase diet quality without post-contact sorting mechanisms that probably increase handling costs-perhaps leading to macrophagous evaluation of individual particles based more on nutritional content than on mechanical properties like size. Increasing the time spent suspension feeding relative to deposit feeding is another likely means for juvenile spionids to increase diet quality and perhaps overcome the combined digestive and deposit-feeding constraints imposed by small gut and palp size.