Evidence for Ammonia as a Natural Cue for Recruitment of Oyster Larvae to Oyster Beds in a Georgia Salt Marsh.

Competent veliger larvae of the oysters Crassostrea virginica and C. gigas exhibited settlement behavior when exposed to ammonia (NH3). The threshold for this response decreased with increasing larval age. The response of veligers to adult-conditioned seawater was correlated with the concentration of NH3 in the seawater. Although the concentrations of NH3 found in marsh water flowing over oyster beds on Sapelo Island, Georgia, were never high enough to elicit settlement behavior from oyster larvae, the concentrations found near the substrate were sufficient to induce settlement behavior in older larvae of C. virginica. In addition, dilution occurs during sampling in the field and may lead one to underestimate, by a factor of 1.7 to 3.5, the actual concentration of NH3 associated with surfaces. In conclusion, NH3 may be an important environmental cue triggering settlement behavior of larval oysters, which, along with other substrate cues, leads to cementation and metamorphosis.