TIDALLY TIMED BEHAVIOR: EFFECTS ON LARVAL DISTRIBUTIONS IN ESTUARIES

Abstract

Zoeal larvae of the xanthid crab Rhithropanopeus harrisii are retained in the upper regions of the Newport River estuary, North Carolina. This retention is effected by rhythmic vertical migrations which are similar for all zoeal stages, ranging above and below the depth of no net flow. Time series analysis of these migrations revealed that their dominant periodic components were usually those of the tidal cycle (period = 12.4 h) and the current flow cycle (period = 6.2 h). Under constant laboratory conditions, larvae taken directly from the estuary continued only the tidal vertical migration, reaching their greatest depth at the time of low tide. In contrast, larvae reared in the laboratory under a 12 h:12 hlight: dark cycle had only a weak circadian migration rhythm. The tidal rhythm is also expressed in larvae tested immediately after hatching from Newport River crabs, but the migration amplitude is greater for larvae which have spent at least one day under field conditions. Time of hatching may also play a part in setting the phase of this rhythm. Larvae are released near the time of local high tide; this timing is evidently adapted to take advantage of maximum salinity of the water and to help determine the region of the estuary in which they will subsequently develop. Tidal behavior and tidal rhythms could be widespread among estuarine zooplankton, particularly in regions where the tides exert strong dynamic effects on the physical structure of estuarine waters.