Horizontal swimming and gravitational sinking ofCapitellasp. i (Annelida: Polychaeta) Larvae: Implications for settlement

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Abstract
Horizontal swimming and gravitational sinking of the lecithotrophic larvae of Capitella sp. I were measured on laboratory-reared organisms during their planktonic life. Swimming was quantified using a video-computer system for motion analysis under both infrared and directed white light conditions. Gravitational fall velocities were measured on anesthetized larvae 10 a temperaturecontrolled particle settling chamber. Swimming and sinking measurements were made on larvae within approximately 12 hours of hatching and at about 24-hour intervals for the following two days. Mean swim speeds were between 2.5 and 3.3 mm/sec on the first day and decreased to 1.6 to 2.3 mm/sec on day 3. Mean fall velocities ranged between 0.8 and 1.0 mm/sec and did not change significantly over the pelagic period. The larvae displayed both photokinetic and phototactic behaviors, swimming slower and toward the white lights, especially early in their development. The capability of these organisms to select settlement sites by horizontal swimming among available habitats is limited to essentially still water, since water velocities occurring within millimeters of the seabed exceed these measured swim speeds for most realistic field flows. Vertical swimming up and down, coupled with passive transport and deposition, are more likely involved in the habitat selection process.