ONTOGENIC 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 fo r a digestive constraint imposed by small body (and gut) size. Underst anding how and why particle-size selection varies ontogenetically can provide clues to the relative quality of forage juveniles obtain. Part icle selection by a tentacle or palp feeder can be viewed as the net r esult of particle contact, collection, and post-collection rejection. A geometrical model of particle contact that includes effects due to p alp size predicts that thinner palps and hence smaller worms are biase d toward contacting larger particles. The model's prediction was teste d using adhesive-coated palp mimics (monofilament line) of varying dia meters spanning the range of actual palps to contact and collect parti cles 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 p alp mimics collected a lower proportion of small beads (higher proport ion of large beads). Intraspecific variations in particle-size selecti on due to body,and palp size were determined for Pseudopolydora kempi japonica Imajima and Hartman, Palydora ligni Webster, Boccardia probas cidea Hattman, and Pygospio elegans Claparede by conducting laboratory feeding experiments with the same mixture of glass beads used with mi mics. P. kempi japonica results were consistent with the contact model and the palp-mimic experiment. B. proboscidea and P. elegans showed n o significant variation with body size. Polydora ligni showed a negati ve relationship between particle size and body (and palp) size; insuff icient 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 po st-contact sorting mechanisms that probably increase handling costs-pe rhaps 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 feedi ng is another likely means for juvenile spionids to increase diet qual ity and perhaps overcome the combined digestive and deposit-feeding co nstraints imposed by small gut and palp size.