In northwestern Gulf of Mexico bays, Spirorbis spirillum (Linnaeus), a sessile tube-building polychaete, is a successful colonizer on ephemeral blades of the seagrass Thalassia testudinum Banks ex Konig. Positive phototactic behavior of S. spirillum larvae in the
laboratory (median duration = 10.5 min) and presence of larvae in field traps above blades suggest that larvae move into the water column upon release from adult brood chambers. Clear, plastic mimics of seagrass blades, designed to alter water flow, increased settlement onto nearby natural
blades, also indicating that larvae are planktonic. Relative settlement at various stations within the seagrass bed was related to daily changes in the direction of wind-driven currents. Release of larvae from adult brood chambers was a daily event occurring near mid-morning. This behavior,
along with considerable variation among larvae in duration of positive phototaxis (from 0 to 55 min), appears to disperse larvae widely over the seagrass via wind-driven currents that vary greatly from day to day. Dispersal distances (estimated from current velocity and larval behavior, and
independently from spatial patterns of settlement) indicate larvae are typically transported hundreds of meters. Abundance of post-settlement individuals was extremely patchy over the seagrass bed and positively related to bottom depth in early fall and negatively related in late fall. Despite
a short period of planktonic behavior (minutes), the timing and mode of transport disperse larvae widely, accounting for major shifts in postsettlement population abundances over the study area (hundreds of meters to kilometers).
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