The measurement of sperm motility is critical when studying fertilization kinetics and chemotaxis. Analysis of motility has traditionally been carried out on cells in small fluid volumes on microscope slides. Several theoretical treatments suggest that drag forces significantly affect flagellar motion within 10 sperm body lengths of the slide surface. Understanding how sperm move in the absence of surface drag is crucial when considering natural locomotory patterns. To examine the effects of solid surfaces, motile sperm from sea urchins (Arbacia punctulata) were placed in a Plexiglas chamber (69 mmx45 mmx15.5 mm; length x width x height). A system was constructed to minimize convective flow by limiting temperature differences within the chamber to less than 0.1 degrees C. The movement of sperm was video-recorded at two levels: (3/4)100 micron (3 body lengths) and 5 mm (150 body lengths) below the chamber lid. When swimming speeds were measured using a computerized video motion-analysis system, a highly significant difference (P<0. 0001) between cells at the two depths was found. Cells nearest the lid swam at 174.6+/-5.9 micron s-1 (mean +/- s.e.m.), whereas those farther away slowed to only 111.1+/-9.9 micron s-1 (mean +/- s.e.m.). Swimming speed was also found to be significantly (P<0.01) affected by paternity, but not by sperm age. We conclude that viscous wall effects must be carefully considered in studies of sperm motility and chemotaxis. The analysis of sperm on a microscope slide may substantially exaggerate swimming speed.

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